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1.
  • Belotcerkovtceva, Daria, et al. (författare)
  • High current limits in chemical vapor deposited graphene spintronic devices
  • 2023
  • Ingår i: Nano Reseach. - : Springer. - 1998-0124 .- 1998-0000. ; 16:4, s. 4233-4239
  • Tidskriftsartikel (refereegranskat)abstract
    • Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors, spin-torque oscillators, and potential spin-integrated circuits. However, despite the demonstrated high current densities in exfoliated graphene, the current-carrying capacity of large-scale chemical vapor deposited (CVD) graphene is not established. Particularly, the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection. In this work, we observe that despite structural imperfections, CVD graphene sustains remarkably highest currents of 5.2 × 108 A/cm2, up to two orders higher than previously reported values in multilayer CVD graphene, with the capacity primarily dependent upon the sheet resistance of graphene. Furthermore, we notice a reversible regime, up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm2, significantly high and durable over long time of operation with spin valve signals observed up to such high current densities. At the same time, the tunnel barrier resistance can be modified by the application of high currents. Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications. 
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2.
  • Belotcerkovtceva, Daria, et al. (författare)
  • Insights and Implications of Intricate Surface Charge Transfer and sp3-Defects in Graphene/Metal Oxide Interfaces
  • 2022
  • Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 14:31, s. 36209-36216
  • Tidskriftsartikel (refereegranskat)abstract
    • Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiOx, the AlOx/graphene interface shows the presence of appreciable sp3 defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp3 bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp3 hybridization at the AlOx/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlOx barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.
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3.
  • Belotcerkovtceva, Daria (författare)
  • Intricacies, Endurance, and Performance Enhancement in Graphene Devices : Towards 2D electronic and spintronic circuits
  • 2024
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Graphene, the atomically thin material of carbon atoms, first isolated experimentally in 2004, exhibits remarkable properties and holds potential for applications in quantum, electrical, and spin-based devices. The chemical vapor deposition (CVD) method enables graphene production on a large scale, merging its exceptional characteristics with scalability and high-quality implementation. Despite the extraordinary promise of CVD graphene with structural imperfections, the main challenge for graphene electronics and spintronics lies in achieving reliability at the device and circuit levels with scalable materials and interfaces. To address these, it is essential to understand the intricacies, endurance, and performance issues in graphene devices. In this thesis, to understand graphene interfaces in devices, we first explored a critical aspect of graphene's interaction with metal oxides, particularly titanium oxide (TiOx) and aluminum oxide (AlOx), and their implications for graphene-based nanoelectronic and spintronic devices. Investigating the electrical characteristics of graphene, both with and without oxides, uncovers the distinct behaviors of TiOx and AlOx when interfaced with graphene, highlighting the charge transfer-induced p-type doping and the formation of sp3 defects, traps, and impurities, especially at the AlOx/graphene interface. These findings bring new insights for graphene spintronic devices while opening possibilities for novel functionalities such as hybrid resistive switching devices. Advancing further towards van der Waals heterostructures in these studies, we could also observe the impact of monolayer MoS2 on graphene’s properties. Next, we explored how CVD graphene devices withstand high current stress to elucidate device durability and resilience. We examine the impact of extreme electric currents on channel structures and resistive tunnel barrier interfaces, focusing on their feasibility for high-capacity electronic and spintronic applications. Here, despite the polycrystalline nature of CVD graphene, we could observe the highest current density of 5.2×108 Acm-2 in graphene on Si/SiO2 substrates, elevating it further to 1.7×109 Acm-2 on diamond substrates, remarkably exceeding previous reports. Performing systematic cyclic electrical measurements, with a gradual increase in the applied high current, we could determine the limits of the reversible regime for safe device operation of both channels and contacts. This knowledge of high current limits and oxide interfaces with graphene leads to an innovative current-treated passive graphene (CTPG) system, where we passivated graphene with metal oxide and applied high current to enhance quality. This method addresses the challenge of interfacial defects and remarkably improves carrier mobility, thereby reducing Coulomb scattering while mitigating electromigration issues. The CTPG presents a scalable platform for stable nanoelectronic and spintronic circuits. The experiments and systems studied in this thesis open possibilities for the exploration of temperature-dependent charge and spin transport measurements via new heterostructures and interfaces with different material combinations.
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4.
  • Cedervall, Johan, et al. (författare)
  • Magnetic structure of the magnetocaloric compound AlFe2B2
  • 2016
  • Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 664, s. 784-791
  • Tidskriftsartikel (refereegranskat)abstract
    • The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic a-axis. The magnetic entropy change from 0 to 800 kA/m was found to be - 1.3 J/K kg at the magnetic transition temperature.
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5.
  • Chakravorty, Manotosh, et al. (författare)
  • Proposed Bose-Einstein condensation of magnons in nanostructured films of Gd at low temperature and its manifestations in electrical resistivity and magnetoresistance
  • 2017
  • Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 29:25
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we report the observation of a proposed Bose-Einstein condensation (BEC) of magnons in a temperature range of around 15-20 K in nanostructured films of Gd with grain sizes that are much larger than the size range where superparamagentism is expected. The observation was carried out using magnetic as well as high precision resistivity and magnetoresistance (MR) measurements performed to low temperatures. We observe that the experimental observations depend crucially on one parameter, namely softening of the spin wave stiffness parameter D at BEC and the resistivity as well as MR can be related quantitatively to magnetic measurements through the temperature variation of the constant D in the vicinity of the transition. This paper establishes that the BEC reported before in nanocrystalline Gd can be extended to a somewhat larger size range.
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6.
  • Chowdhury, Mouli Roy, et al. (författare)
  • Antiferromagnetic short-range order and cluster spin-glass state in diluted spinel ZnTiCoO4
  • 2022
  • Ingår i: Journal of Physics. - : IOP Publishing Ltd. - 0953-8984 .- 1361-648X. ; 34:27
  • Tidskriftsartikel (refereegranskat)abstract
    • The nature of magnetism in the doubly-diluted spinel ZnTiCoO4 = (Zn2+)( A ) [Ti4+Co2+]( B )O-4 is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (chi), ac susceptibilities (chi ' and chi ''), and heat capacity (C (p)) measurements. Whereas antiferromagnetic (AFM) Neel temperature T (N) = 13.9 K is determined from the peak in the partial differential (chi T)/ partial differential T vs T plot, the fit of the relaxation time tau (determined from the peak in the chi '' vs T data at different frequencies) to the Power law: tau = tau (0) [(T - T (SG))/T (SG)](-z nu ) yields the spin glass freezing temperature T (SG) = 12.9 K, z nu similar to 11.75, and tau (0) similar to 10(-12) s. Since the magnitudes of tau (0) and z nu depend on the magnitude of T (SG), a procedure is developed to find the optimum value of T (SG) = 12.9 K. A similar procedure is used to determine the optimum T (0) = 10.9 K in the Vogel-Fulcher law: tau = tau (0) exp[E (a)/k (B)(T - T (0))] yielding E (a)/k (B) = 95 K, and tau (0) = 1.6 x 10(-13) s. It is argued that the comparatively large magnitude of the Mydosh parameter omega = 0.026 and k (B) T (0)/E (a) = 0.115 (MUCH LESS-THAN1) suggests cluster spin-glass state in ZnTiCoO4 below T-SG. In the C (p) vs T data from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence of lambda-type anomaly near T (N) or T (SG) combined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence of T (SG) shows slight departure (phi similar to 4.0) from the non-mean-field Almeida-Thouless line T (SG)(H) = T (SG)(0) (1 - AH (2/phi )). Strong temperature dependence of magnetic viscosity S and coercivity H (C) without exchange bias, both tending to zero on approach to T (SG) from below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+ and Ti4+ ions leading to magnetic frustration. Magnetic phase diagram in the H-T plane is established using the high-field magnetization data M(H, T) for T < T (N) which reveals rapid decrease of T (SG) with increase in H whereas decrease in T (N) with increase in H is weaker, typical of AFM systems. For T > T (N), the data of chi vs T are fit to the modified Curie-Weiss law, chi = chi (0) + C/(T + theta), with chi (0) = 3.2 x 10(-4) emu mol(-1) Oe(-1) yielding theta = 4 K and C = 2.70 emu K mol(-1) Oe(-1). This magnitude of C yields effective magnetic moment = 4.65 mu (B) for Co2+, characteristic of Co2+ ions with some contribution from spin-orbit coupling. Molecular field theory with effective spin S = 3/2 of Co2+ is used to determine the nearest-neighbor exchange constant J (1)/k (B) = 2. 39 K AFM and next-nearest-neighbor exchange constant J (2)/k (B) = -0.66 K (ferromagnetic).
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7.
  • Datt, Gopal, et al. (författare)
  • Combined Bottom-Up and Top-Down Approach for Highly Ordered One-Dimensional Composite Nanostructures for Spin Insulatronics
  • 2021
  • Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:31, s. 37490-37499
  • Tidskriftsartikel (refereegranskat)abstract
    • Engineering magnetic proximity effects-based devices requires developing efficient magnetic insulators. In particular, insulators, where magnetic phases show dramatic changes in texture on the nanometric level, could allow us to tune the proximity-induced exchange splitting at such distances. In this paper, we report the fabrication and characterization of highly ordered two-dimensional arrays of LaFeO3 (LFO)-CoFe2O4 (CFO) biphasic magnetic nanowires, grown on silicon substrates using a unique combination of bottom-up and top-down synthesis approaches. The regularity of the patterns was confirmed using atomic force microscopy and scanning electron microscopy techniques, whereas magnetic force microscopy images established the magnetic homogeneity of the patterned nanowires and absence of any magnetic debris between the wires. Transmission electron microscopy shows a close spatial correlation between the LFO and CFO phases, indicating strong grain-to-grain interfacial coupling, intrinsically different from the usual core-shell structures. Magnetic hysteresis loops reveal the ferrimagnetic nature of the composites up to room temperature and the presence of a strong magnetic coupling between the two phases, and electrical transport measurements demonstrate the strong insulating behavior of the LFO-CFO composite, which is found to be governed by Mottvariable range hopping conduction mechanisms. A shift in the Raman modes in the composite sample compared to those of pure CFO suggests the existence of strain-mediated elastic coupling between the two phases in the composite sample. Our work offers ordered composite nanowires with strong interfacial coupling between the two phases that can be directly integrated for developing multiphase spin insulatronic devices and emergent magnetic interfaces.
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8.
  • Ghosh, Anirudha, et al. (författare)
  • Magnetic circular dichroism in the dd excitation in the van der Waals magnet CrI3 probed by resonant inelastic x-ray scattering
  • 2023
  • Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:11
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The dd excitation energies are similar on the two sides of the ferromagnetic transition temperature, T-C similar to 61 K, although MD in RIXS is predominant at 0.4 T magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intraorbital dd excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.
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9.
  • Ghosh, Sayandeep, et al. (författare)
  • Antiferromagnetism, spin-glass state, H-T phase diagram, and inverse magnetocaloric effect in Co2RuO4
  • 2020
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 32:48
  • Tidskriftsartikel (refereegranskat)abstract
    • Static and dynamic magnetic properties of normal spinel Co2RuO4= (Co2+)A[Co3+Ru3+](B)O-4 are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature rangeT= 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at T-N similar to 15.2 K, along with a spin-glass state at slightly lower temperature (T-SG) near 14.2 K. It is argued thatT(N)is mainly governed by the ordering of the spins of Co2+ ions occupying theA-site, whereas the exchange interaction between the Co2+ ions on theA-site and randomly distributed Ru(3+)on theB-site triggers the spin-glass phase, Co3+ ions on theB-site being in the low-spin non-magnetic state. Analysis of measurements ofM(H,T) for TT-N, analysis of the paramagnetic susceptibility (chi) vs.Tdata are fit to the modified Curie-Weiss law,chi=chi(0)+C/(T+theta), with chi(0)= 0.0015 emu mol(-1)Oe(-1)yielding theta= 53 K andC= 2.16 emu-K mol(-1)Oe(-1), the later yielding an effective magnetic moment mu(eff)= 4.16 mu(B)comparable to the expected value of mu(eff)= 4.24 mu(B)per Co2RuO4. Using T-N,theta and high temperature series for chi, dominant exchange constant J(1)/k(B)similar to 6 K between the Co(2+)on theA-sites is estimated. Analysis of the ac magnetic susceptibilities near T-SG yields the dynamical critical exponent z nu= 5.2 and microscopic spin relaxation time tau(0)similar to 1.16 x 10(-10)sec characteristic of cluster spin-glasses and the observed time-dependence ofM(t) is supportive of the spin-glass state. LargeM-Hloop asymmetry at low temperatures with giant exchange bias effect (H-EB similar to 1.8 kOe) and coercivity (H-C similar to 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed belowT(N)is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) forT>T(N)to inverse MCE forT
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10.
  • Haneef, Tahir, et al. (författare)
  • Recent progress in two dimensional Mxenes for photocatalysis : a critical review
  • 2023
  • Ingår i: 2D Materials. - : Institute of Physics (IOP). - 2053-1583. ; 10:1
  • Forskningsöversikt (refereegranskat)abstract
    • Transition metal carbides and nitrides, generally known as MXenes have emerged as an alternative to improve photocatalytic performance in renewable energy and environmental remediation applications because of their high surface area, tunable chemistry, and easily adjustable elemental compositions. MXenes have many interlayer groups, surface group operations, and a flexible layer spacing that makes them ideal catalysts. Over 30 different members of the MXenes family have been explored and successfully utilized as catalysts. Particularly, MXenes have achieved success as a photocatalyst for carbon dioxide reduction, nitrogen fixation, hydrogen evolution, and photochemical degradation. The structure of MXenes and the presence of hydrophilic functional groups on the surface results in excellent photocatalytic hydrogen evolution. In addition, MXenes' surface defects provide abundant CO2 adsorption sites. Moreover, their highly efficient catalytic oxidation activity is a result of their excellent two-dimensional nanomaterial structure and high-speed electron transport channels. This article comprehensively discusses the structure, synthesis techniques, photocatalytic applications (i.e. H-2 evolution, N-2 fixation, CO2 reduction, and degradation of pollutants), and recyclability of MXenes. This review also critically evaluates the MXene-based heterostructure and composites photocatalyst synthesis process and their performance for organic pollutant degradation. Finally, a prospect for further research is presented in environmental and energy sciences.
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11.
  • Höglin, Viktor, et al. (författare)
  • Irreversible structure change of the as prepared FeMnP1−xSix-structure on the initial cooling through the curie temperature
  • 2015
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 374, s. 455-458
  • Tidskriftsartikel (refereegranskat)abstract
    • FeMnP0.75Si0.25 experiences a first order para- to ferromagnetic transition at about 200 K. In common with some other alloy compositions crystallizing in the Fe2P structure, the magnetic transition of the as prepared alloy occurs at a lower temperature than on subsequent cooling events. This virgin effect is found to be accompanied by a magnetostrictively induced irreversible structure change that persists on succeeding cooling heating cycles. These findings provide means to understand and control the thermal hysteresis of the (Fe1-xMnx)(2)P1-ySiy alloy system which is a promising material class for use in magnetocaloric refrigerators.
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12.
  • Höglin, Viktor, et al. (författare)
  • Phase diagram, structures and magnetism of the FeMnP1-xSix-system
  • 2015
  • Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 5:11, s. 8278-8284
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic properties of the (Fe,Mn)(2)(P,Si)-system have been shown to be readily manipulated by small changes in composition. This study surveys the FeMnP1-xSix-system (0.00 <= x <= 1.00) reporting sample syntheses and investigations of crystallographic and magnetic properties using X-ray powder diffraction and magnetic measurements. Two single phase regions exist: the orthorhombic Co2P-type structure (x < 0.15) and the Fe2P-type structure (0.24 <= x < 0.50). Certain compositions have potential for use in magnetocaloric applications.
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13.
  • Höglin, Viktor, et al. (författare)
  • Phase diagram, structures and magnetism of the FeMnP1-xSix-system
  • 2015
  • Ingår i: RSC Advances. - 2046-2069. ; 5:11, s. 8278-8284
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic properties of the (Fe,Mn)2(P,Si)-system have been shown to be readily manipulated by small changes in composition. This study surveys the FeMnP1−xSix-system (0.00 ≤ x ≤ 1.00) reporting sample syntheses and investigations of crystallographic and magnetic properties using X-ray powder diffraction and magnetic measurements. Two single phase regions exist: the orthorhombic Co2P-type structure (x < 0.15) and the Fe2P-type structure (0.24 ≤ x < 0.50). Certain compositions have potential for use in magnetocaloric applications.
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14.
  • Iqbal, Jibran, et al. (författare)
  • A Hydrofluoric Acid-Free Green Synthesis of Magnetic M.Ti2CTx Nanostructures for the Sequestration of Cesium and Strontium Radionuclide
  • 2022
  • Ingår i: Nanomaterials. - : MDPI. - 2079-4991. ; 12:18
  • Tidskriftsartikel (refereegranskat)abstract
    • Highlights An environmentally friendly MAX phase etching methodology was established Sodium hydroxide produced magnetic layered M.Ti2CTx nanostructure M.Ti2CTx exhibited exceptional Sr2+ and Cs+ removal capacities of 376 and 142.88 mg/g Highly efficient magnetic nanostructures permitted selective radionuclide removal MAX phases are the parent materials used for the formation of MXenes, and are generally obtained by etching using the highly corrosive acid HF. To develop a more environmentally friendly approach for the synthesis of MXenes, in this work, titanium aluminum carbide MAX phase (Ti2AlC) was fabricated and etched using NaOH. Further, magnetic properties were induced during the etching process in a single-step etching process that led to the formation of a magnetic composite. By carefully controlling etching conditions such as etching agent concentration and time, different structures could be produced (denoted as M.Ti2CTx). Magnetic nanostructures with unique physico-chemical characteristics, including a large number of binding sites, were utilized to adsorb radionuclide Sr2+ and Cs+ cations from different matrices, including deionized, tap, and seawater. The produced adsorbents were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The synthesized materials were found to be very stable in the aqueous phase, compared with corrosive acid-etched MXenes, acquiring a distinctive structure with oxygen-containing functional moieties. Sr2+ and Cs+ removal efficiencies of M.Ti2CTx were assessed via conventional batch adsorption experiments. M.Ti2CTx-A(III) showed the highest adsorption performance among other M.Ti2CTx phases, with maximum adsorption capacities of 376.05 and 142.88 mg/g for Sr2+ and Cs+, respectively, which are among the highest adsorption capacities reported for comparable adsorbents such as graphene oxide and MXenes. Moreover, in seawater, the removal efficiencies for Sr2+ and Cs+ were greater than 93% and 31%, respectively. Analysis of the removal mechanism validates the electrostatic interactions between M.Ti2C-A(III) and radionuclides.
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15.
  • Ivanov, Sergey A., et al. (författare)
  • Composition dependence of the multifunctional properties of Nd-doped Bi4Ti3O12 ceramics
  • 2017
  • Ingår i: Journal of materials science. Materials in electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 28:11, s. 7692-7707
  • Tidskriftsartikel (refereegranskat)abstract
    • Sample preparation, evolution of the crystal structure with Nd content at room temperature, as well as dielectric and magnetic properties of polycrystalline Bi4-xNdxTi3O12 solid solutions (x = 0.0, 0.5, 1.0, 1.5, and 2.0) are reported. These solid solutions were structurally characterized by X-ray powder diffraction using Rietveld refinements. For x <= 1.0, the samples crystallize in an orthorhombic symmetry. The structural data could be refined using the polar orthorhombic space group B2cb. The orthorhombicity decreases with increasing Nd3+ concentration, and a paraelectric tetragonal structure (space group I4/mmm) is stabilized for x > 1. The ferroelectric Curie temperature was found to monotonously decrease with increasing Nd concentration. A polar-to-nonpolar phase transition takes place near x = 1, reflecting the existence of a morphotropic phase boundary between 1.0 < x < 1.5 at room temperature. All samples were found to be paramagnetic down to 5 K, however, the presence of significant antiferromagnetic interaction is inferred from Curie-Weiss analyses of the temperature dependence of the magnetic susceptibility of the doped samples. The effect of Nd3+ substitution on structure-property relationship is discussed and compared to that of other lanthanide cations.
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16.
  • Ivanov, Sergey A., et al. (författare)
  • Perovskite solid solutions La0.75Bi0.25Fe1-xCrxO3 : Preparation, structural, and magnetic properties
  • 2017
  • Ingår i: Journal of Solid State Chemistry. - : Elsevier. - 0022-4596 .- 1095-726X. ; 254, s. 166-177
  • Tidskriftsartikel (refereegranskat)abstract
    • Solid solutions of La0.75Bi0.25Fe1−xCrxO3 (x = 0.1, 0.25, 0.5, and 0.75) prepared by conventional solid state reaction have been studied by means of X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic measurements. The NPD and XRPD patterns indicate orthorhombic structure (space group Pnma) for all compositions in the whole temperature range investigated (4–900 K). The lattice parameters of La0.75Bi0.25Fe1−xCrxO3 were found to decrease with the Cr content. It was established that the Fe3+ and Cr3+ cations are randomly positioned at the B-site of the perovskite structure.All samples order antiferromagnetically below transition temperatures that decrease with increasing Cr content, from around 700 K for x = 0.1 to about 300 K for x = 0.75. The antiferromagnetic arrangement of the Fe3+/Cr3+ magnetic moments in the B-site is of G-type along the x-axis (Gx mode) with propagation vector k = (0,0,0) for all concentrations of Cr. Effects of the composition on several structural distortion parameters were investigated and an anomalous variation of the octahedral deformation with Cr content was found. Whilst the overall octahedral deformation varies irregularly with increasing Cr content, the octahedral tilting was found to decrease monotoneously.
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17.
  • Ivanov, Sergey, et al. (författare)
  • Cation ordering, ferrimagnetism and ferroelectric relaxor behavior in Pb(Fe1-xScx)(2/3)W1/3O3 solid solutions
  • 2019
  • Ingår i: European Physical Journal B. - : SPRINGER. - 1434-6028 .- 1434-6036. ; 92:8
  • Tidskriftsartikel (refereegranskat)abstract
    • Ceramic samples of the multiferroic perovskite Pb(Fe1-xScx)(2/3)W1/3O3 with 0 <= x <= 0.4 have been synthesized using a conventional solid-state reaction method, and investigated experimentally and theoretically using first-principle calculations. Rietveld analyses of joint synchrotron X-ray and neutron diffraction patterns show the formation of a pure crystalline phase with cubic (Fm3(_)m) structure with partial ordering in the B-sites. The replacement of Fe by Sc leads to the increase of the cation order between the B and B '' sites. As the non-magnetic Sc3+ ions replace the magnetic Fe3+ cations, the antiferromagnetic state of PbFe2/3W1/3O3 is turned into a ferrimagnetic state reflecting the different magnitude of the magnetic moments on the B ' and B '' sites. The materials remain ferroelectric relaxors with increasing Sc content. Results from experiments on annealed and quenched samples show that the cooling rate after high temperature annealing controls the degree of cationic order in Pb(Fe1-xScx)(2/3)W1/3O3 and possibly also in the undoped PbFe2/3W1/3O3.
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18.
  • Ivanov, Sergey, et al. (författare)
  • Modification of the structure and magnetic properties of ceramic La2CoMnO6 with Ru doping
  • 2018
  • Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 752, s. 420-430
  • Tidskriftsartikel (refereegranskat)abstract
    • Effects of Ru doping on the structure and magnetic properties of La2CoMn1-xRuxO6 ceramics with different compositions (x = 0, 0.1, 0.2, 0.3, 0.5, 0.75, 1.0) have been investigated using a variety of complementary techniques (X-ray powder diffraction and photoelectron spectroscopy, electron microscopy, and magnetometry). Phase-pure polycrystalline samples were prepared by the conventional solid-state reaction method including high pressure (3000 kg/cm(2) ) treatment of the original mixture of reagents. X-ray diffraction analyses of the samples accompanied with Rietveld refinement suggested a change in the crystal structure upon doping: for 0 <= x <= 0.3 the structure of the samples is predominantly disordered orthorhombic (s.g. Pnma) which transformed to ordered monoclinic (s.g. P2(1)/n) for 0.5 <= x <= 1. This suggests that Ru doping induces (partial) cation order in the B-sublattice. Magnetic measurements indicated a low temperature ferromagnetic phase, and a positive value of the Curie-Weiss temperature (theta cw) was derived for all samples of composition x <= 0.75. However, the Curie temperature (T-c) and theta cw decreased with increasing Ru doping. These changes in Tc upon doping can be correlated to the changes in the nearest neighbor and next nearest neighbor exchange interactions. The sample with x =1 orders antiferromagnetically at approximately 25 K.
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19.
  • Jena, Suchit Kumar, et al. (författare)
  • Slow spin dynamics of cluster spin-glass spinel Zn(Fe1-xRu (x))(2)O-4 : role of Jahn-Teller active spin-1/2 Cu2+ ions at B-sites
  • 2022
  • Ingår i: Journal of Physics. - : IOP Publishing Ltd. - 0953-8984 .- 1361-648X. ; 34:40
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRux)(2)O-4 by means of detailed dc-magnetization and ac-susceptibility studies combined with the heat capacity analysis. Two specific compositions (x= 0.5, 0.75) have been investigated in detail along with the substitution of Jahn-Teller (JT) active spin-1/2 Cu2+ ions at B-sites. Measurements based on the frequency and temperature dependence of ac-susceptibility (chi(ac)(f, T)) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel-Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperature T-SG (17.77 K (x= 0.5) and 14 K (x= 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida-Thouless (AT)-line approach (T-SG(H) = T-SG(0)(1 - AH(2/phi))), with an ideal value of phi = 3. Nevertheless, the analysis of temperature dependent high field dc-susceptibility, chi(hf) (2 kOe <= H-DC <= 20 kOe, T) provides evidence for Gabay-Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of Cu0.2Zn0.8FeRuO4 system, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature (T-FiM) and their volume fraction vanishes completely across similar to 6T(FiM). This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling (J(S)) between the spins which yields J(S) = -3.02 x 10(-5) eV for Cu0.2Zn0.8FeRuO4 representing the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cu2+ ions. However, the antiferromagnetic coupling is predominant in ZnFeRuO4 and Cu0.2Zn0.8Fe0.5Ru1.5O4 systems. Finally, we deduced the magnetic phase diagram in the H-DC - T plane using the characteristic parameters obtained from the field variations of both ac- and dc-magnetization measurements.
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20.
  • Jena, S. K., et al. (författare)
  • Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn0.8Cu0.2FeMnO4
  • 2023
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 35:37
  • Tidskriftsartikel (refereegranskat)abstract
    • Spinels (AB(2)O(4)) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn-Teller active Mn3+ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn-0.8(2+) Cu-0.2(2+)) A [Fe-0.4(2+) Fe-0.6(3+) Mn3+](B)O4-delta. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (chi' and chi''), dc susceptibility (chi), heat capacity C-p, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of chi vs. T fits the Curie-Weiss law: chi = C/(T - theta) from T = 250 K to 400 K with theta similar or equal to 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J/k(B) = 17 K, and C = 3.29 emu K mol(-1) Oe(-1) yielding an effective magnetic moment mu(eff) = 5.13 mu B resulting from the high-spin states of Cu2+ (A site) and Fe2+ (B site), while the B site trivalent ions Mn3+ and Fe3+ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu2+down arrow)(A)[Fe2+down arrow, Fe3+down arrow, Mn3+down arrow](B) leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(chi T)/ dT shows the onset of ferrimagnetism below similar to 100 K and peaks near 47 K and 24 K. The relaxation time t obtained from temperature and frequency dependence of chi'' when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperature T-SG (H) follows the equation: T-SG (H) = T-SG (0) [ 1- AH(2/phi)] with T-SG (0) = 46.6 K, A = 8.6 x 10(-3) Oe(-0.593) and phi = 3.37. The temperature dependence of hysteresis loops yields coercivity H-C similar to 3.8 kOe at 2 K without exchange-bias, but HC decreases with increase in T becoming zero above 24 K, the T-SG(H) for H = 800 Oe. Variations of C-p vs. T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, Cp varies as T-2; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization MTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at T-SG(0) = 46.6 K, followed by SL behavior below 9 K.
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21.
  • Joshi, D. C., et al. (författare)
  • Nature of magnetic ordering in nanocomposites of Zn1-pNipO and NiO
  • 2018
  • Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : ELSEVIER SCIENCE BV. - 1386-9477 .- 1873-1759. ; 103, s. 46-52
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed study of the nature of magnetic exchange interactions and anisotropy in Zn-1_pNipO/NiO (0 <= p <= 1) two-phase nanocomposites is reported. The exchange constants |J(1)| and |J(2)| have been determined using the molecular field approximation for the two sublattice model which yields |J(2)| similar to 11.26 meV and |J(1)| similar to 7.17meV for p = 1, consistent with type-II antiferromagnetic ordering (i.e. |J2|>>|J(1)|). A novel core-shell surface layer model has been employed to explain the nature of magnetic anisotropy in these nanocomposites which yields anisotropy constants K-surface= 2.42 erg/cm(2), K-bulk = 2.61 x 10(6) erg/cm(3) and K-Shell = 9.56 x 10(5) erg/cm(3) for shell thickness d(Sh) similar to 1.34 nm (p = 1).
  •  
22.
  • Joshi, Deep C., et al. (författare)
  • Room temperature ferrimagnetism in Yb-doped relaxor ferroelectric PbFe2/3W1/3O3
  • 2019
  • Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 115:7
  • Tidskriftsartikel (refereegranskat)abstract
    • We report ferrimagnetism and reentrant relaxor ferroelectricity near room temperature in a Yb-doped PbFe2/3W1/3O3 cubic perovskite. Structural analysis reveals the presence of a single cubic perovskite phase, with the m space group [lattice parameter: a=8.0112(3) angstrom], and partial B-site ordering. The B-site ordering yields uncompensated magnetic moments in the antiferromagnetic structure of PbFe2/3W1/3O3 and ferrimagnetism near room temperature. An excess moment of similar to 0.6 mu(B)/B-site may be estimated from magnetic hysteresis curves recorded up to 50kOe at 5K. The temperature dependent magnetodielectric study reveals a sequential phase transition from a long-range ferroelectric state (across 280K) to a short-range relaxor ferroelectric state (across 190K). The long-range ferroelectric ordering is found to be more affected by the application of external magnetic fields than the relaxor phase.
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23.
  • Kotnana, Ganesh, et al. (författare)
  • Novel mixed precursor approach to prepare multiferroic nanocomposites with enhanced interfacial coupling
  • 2020
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 511
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present work, we report the preparation of multiferroic PbZr0.52Ti0.48O3 (PZT)/CoFe2O4 (CFO) nanocomposites using a new synthesis technique that can maximize the surface area of contact, and hence, the interfacial coupling between the ferroelectric (PZT) and ferrimagnetic (CFO) phases. The samples have been characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the physical (magnetic and dielectric) properties have been investigated. XRD confirms the presence of the desired PZT and CFO phases in the samples without any undesired secondary phases. We also observe a reduction in the particle size of CFO in the nanocomposites as evidenced by a line broadening of the XRD reflections corresponding to the pure CFO phase. The nanocomposites show hysteresis loops and ferrimagnetic-like behaviors in their M vs H curves at room temperature, even for samples with very low fraction of the CFO phase. The coercivity of the nanocomposites is marginally larger compared to that of pure CFO, which can be due to the change in magnetic anisotropy of the CFO phase due to its reduced particle size in the nanocomposites. Room temperature polarization versus electric field measurements show a significant increase in the coercive field after the incorporation of CFO inside the PZT matrix. This work illustrates a simple, cost-effective synthesis technique that can be used to prepare nanocomposites of functional materials with desired room temperature functionalities and enhanced interfacial coupling between the two phases.
  •  
24.
  • Majumdar, Arnab, et al. (författare)
  • Interplay of charge density wave and multiband superconductivity in layered quasi-two-dimensional materials : The case of 2H-NbS2 and 2H-NbSe2
  • 2020
  • Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 4:8
  • Tidskriftsartikel (refereegranskat)abstract
    • Despite intense efforts on all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings, particularly charge density waves and superconductivity, are still attractive problems with several open questions. Here, in order to reveal how the superconducting gap evolves, we report on high quality complementary measurements of magneto-optical imaging, specific heat, magnetic susceptibility, resis-tivity measurements, Andreev spectroscopy, and London penetration depth lambda(ab)(T) measurements supplemented with theoretical calculations for 2H-NbSe2 and 2H-NbS2 single crystals. The temperature dependence of lambda(ab)(T) calculated from the lower critical field and Andreev spectroscopy can be well described by using a two-band model with s-wave-like gaps. The effect of pressure on the superconducting gap of both systems illustrates that both bands are practically affected. Upon compression, the Fermi surfaces do not change significantly, and the nesting remains almost unaffected compared to that at ambient condition. However, a strong bending in the upper critical fields (H-c2) curves is obtained under pressure and support the presence of a strong Pauli paramagnetic effect. In NbSe2, using a two-band model with s-wave-like gaps, the temperature dependence H-c2(T) can be properly described. In contrast to that, the behavior of H-c2 for NbS2 is ruled by the spin paramagnetic effect. The estimated values of the penetration depth at T = 0 K confirm that NbSe2 and NbS2 superconductors depart from a Uemura-style relationship between Tc with lambda(-2)(ab) (T ), the in-plane superconducting penetration depth.
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25.
  • Maltoni, piema929, et al. (författare)
  • Exploring the magnetic properties and magnetic coupling in SrFe12O19/ Co1-XZnXFe2O4 & nbsp;nanocomposites
  • 2021
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 535
  • Tidskriftsartikel (refereegranskat)abstract
    • Among hard/soft nanocomposites (NCs), ferrite-based materials are potentially promising for developing exchange-coupled systems, thus leading to enhanced magnetic properties. In this regard, we investigate the role of the synthesis approach in the development of SrFe12O19/CoFe2O4 (SFO/CFO) NCs, with special focus on tuning the magnetic features of the softer phase (CFO) by introducing Zn2+ in the spinel structure. X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and squid magnetometry were employed to clarify the relationship between morphology, size, and magnetic properties of the NCs, pointing out the feasibility of this method in obtaining successfully exchange-coupled systems. This work shows how optimizing the intrinsic magnetic properties of the CFO may be used to tune the extrinsic ones of the NCs. Despite the promising results in magnetic coupling, our study clearly confirms/strengthens that an enhancement of remanent magnetization is the most important factor for improving the magnetic performance.
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26.
  •  
27.
  • Maltoni, Pierfrancesco, et al. (författare)
  • Towards bi-magnetic nanocomposites as permanent magnets through the optimization of the synthesis and magnetic properties of SrFe12O19 nanocrystallites
  • 2021
  • Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 54:12
  • Tidskriftsartikel (refereegranskat)abstract
    • Sol-gel synthesis was used in order to obtain nanocrystallites of the SrFe12O19 (SFO) hexaferrite in an efficient and reliable way. By optimizing the initial synthetic conditions, we were able to control the size of the nanoparticles (NPs), at lower annealing temperature. The x-ray powder diffraction, transmission electron microscopy (TEM), and magnetic measurements have demonstrated a significant relation between the morphology, size, and magnetic properties of the nanoscale SFO, revealing a definite dependence on the crystallite size along the c-axis. The obtained NPs appear almost isotropic, in the form of platelets and exhibit similar magnetic performance, in terms of the energy product (BH)(MAX), thus, demonstrating the suitability of reducing the annealing temperature without any deterioration in the magnetic properties. Additionally, this work illustrates the feasibility of the sol-gel bottom-up approach to employ magnetic NPs as building-blocks for designing hard/soft exchange-coupled bi-magnetic nanocomposites, combining the high coercivity of a hard phase (SFO) and the high saturation magnetization of a soft phase (CoFe2O4); in this regard, we discuss the tunability of the magnetic anisotropy by symbiotically restricting the growth of both phases.
  •  
28.
  • Maltoni, Pierfrancesco, et al. (författare)
  • Tuning the Magnetic Properties of Hard-Soft SrFe12O19/CoFe2O4 Nanostructures via Composition/Interphase Coupling
  • 2021
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:10, s. 5927-5936
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetic nanocomposites (NCs) are extremely appealing for a wide range of energy-related technological applications, specifically as building blocks for next-generation permanent magnets. The design of such nanostructures requires precise chemical synthesis methods, which will permit the fine-tuning of the magnetic properties. Here we present an in-depth structural, morphological and magnetic characterization of ferrite-based nanostructures obtained through a bottom-up sol-gel approach. The combination of the high coercivity of a hard phase SrFe12O19 (SFO) and the high saturation magnetization of a soft phase, CoFe2O4 (CFO), allowed us to develop exchange-coupled bimagnetic NCs. A symbiotic effect is observed in a SFO/CFO nanocomposite, as the unique oriented growth of SFO prevents grain growth of the CFO, thus restricting the crystallite size of both. Through X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and magnetic measurements we clarify the relationship between the distribution and size of hard/soft particles, the optimization of interfaces and the obtained uniform magnetic response. This study allowed us to establish the potentiality of hard/soft SFO/CFO nanostructures in current permanent magnet technology.
  •  
29.
  • Mishra, Himanshu, et al. (författare)
  • Experimental advances in charge and spin transport in chemical vapor deposited graphene
  • 2021
  • Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 2515-7639. ; 4:4
  • Forskningsöversikt (refereegranskat)abstract
    • Despite structural and processing-induced imperfections, wafer-scale chemical vapor deposited (CVD) graphene today is commercially available and has emerged as a versatile form that can be readily transferred to desired substrates for various nanoelectronic and spintronic applications. In particular, over the past decade, significant advancements in CVD graphene synthesis methods and experiments realizing high-quality charge and spin transport have been achieved. These include growth of large-grain graphene, new processing methods, high-quality electrical transport with high-carrier mobility, micron-scale ballistic transport, observations of quantum and fractional quantum Hall effect, as well as the spintronic performance of extremely long spin communication over tens of micrometers at room temperature with robust spin diffusion lengths and spin lifetimes. In this short review, we discuss the progress in recent years in the synthesis of high-quality, large-scale CVD graphene and improvement of the electrical and spin transport performance, particularly towards achieving ballistic and long-distance spin transport that show exceptional promise for next-generation graphene electronic and spintronic applications.
  •  
30.
  • Montazeri, M., et al. (författare)
  • Enhancement of microwave absorption performance of cobalt ferrite by adding MXene (Ti3C2Tx) nanosheets
  • 2024
  • Ingår i: Materials Chemistry and Physics. - : Elsevier. - 0254-0584 .- 1879-3312. ; 322
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work, CoFe2O4 powders were combined with various amounts of MXene (Ti3C2Tx) through the solution combustion method for microwave absorption applications. Despite the absence of MXene reflections in X-ray diffraction patterns, the MXene sheets were observed in electron microscopy images. The CoFe2O4 nanoparticles were uniformly dispersed on the MXene nanosheets. The specific surface area increased from 57 to 83 m2g-1 by adding 20 wt% MXene. The saturation magnetization and coercivity decreased from 44 to 35 emug-1 and from 834 to 775 Oe, respectively. By adding the MXene, the minimum reflection loss and effective absorption bandwidth increased up to -38 dB and 5.6 GHz at a matching thickness of 1.8 mm, respectively, which was attributed to the enhancement of impedance matching by the decrease of permittivity.
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31.
  • Muscas, Giuseppe, et al. (författare)
  • Nanostructure-driven complex magnetic behavior of Sm2CoMnO6 double perovskite
  • 2022
  • Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 906
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetic double perovskite oxides have steadily emerged as an important class of functional materials. A clear understanding of the complex interactions that govern the magnetic behavior, and thereby, the functionality in these mixed valence compounds, however, remains elusive. In this study, we show that the complex nanostructure that forms in these compounds is at the root of their magnetic behavior. Using complementary experimental and micromagnetic simulation results, we have uncovered the complex nanostructure of polycrystalline Sm2CoMnO6, a typical double perovskite oxide, and established how the nanostructure drives its magnetic behavior. Our results show that Sm2CoMnO6 exhibits a Griffiths phase with the formation of ferromagnetic clusters above the ordering temperature. The isothermal magnetization curves show no sign of saturation, even at the highest measured field (9 T), and irreversibility in the entire magnetic field range. Despite a very clear indication of the presence of antiferromagnetic antisite defects, surprisingly, no antisite defect-induced exchange bias occurs. This is explained from the micro magnetic simulations that confirm the presence of ferromagnetic nanoclusters and nanosized, random, and uncorrelated antisite defects, resulting in no exchange bias. This work provides a clear understanding of the role of antisite defects, in particular, on how their structure can lead to the presence/absence of exchange bias. The fundamental insight offered in this work fills an important knowledge gap in the field and will be of immense value in realizing the true potential of double perovskite oxides for future technological applications. (C)& nbsp;2022 The Author(s). Published by Elsevier B.V.
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32.
  • Nag, Abhishek, et al. (författare)
  • Ba3MIr2O9 hexagonal perovskites in the light of spin-orbit coupling and local structural distortions
  • 2018
  • Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:6
  • Tidskriftsartikel (refereegranskat)abstract
    • Spin-orbit coupling (SOC) is found to be crucial for understanding themagnetic and electronic properties of 5d transitionmetal oxides. In 5d systems, with Ir5+ ions, where ideally a nonmagnetic J = 0 ground state is expected to be stabilized in the presence of strong SOC, often spontaneous moments are generated due to hopping induced superexchange. This effect is more pronounced when the Ir atoms are close by, as in systems with Ir2O9 dimers in 6H Ba3MIr2O9 compounds where magnetism is an outcome of complex Ir-O-Ir exchange paths, and is strongly influenced by the presence of local distortions. We find that subtle variations in the local structure of Ba3MIr2O9 (M = Mg, Sr, and Ca) lead to markedly differentmagnetic properties. While SOC plays a pivotal role in explaining the insulating ground states of these systems, it is seen that Ba3MgIr2O9, having a P6(3)/mmc symmetry, does not order down to low temperature despite having antiferromagnetic exchange interactions, while Ba3CaIr2O9 shows weak dimer-like features and stabilizes in C2/c' magnetic configuration with no net moment, and Ba3SrIr2O9 possesses a ground state corresponding to themagnetic space groupC2'/c' and exhibits ferromagnet-like features.
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33.
  • Nayak, S., et al. (författare)
  • Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4
  • 2016
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 120:16
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74 K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature T-N similar to 41 K and reentrant spin-glass behavior with glass transition temperature T-SG similar to 39 K. The Arrott plot (H/M versus M-2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32 K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (C-P T-1 versus T) below 15 K, and a zero-crossover of isothermal magnetic-entropychange (Delta S) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5K <= T <= 32 K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, which are responsible for very large magnitudes of the coercivity H-C similar to 20 kOe and bipolar-exchange bias H-EB similar to -20 kOe observed below 10 K.
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34.
  • Nayak, S., et al. (författare)
  • Magnetic compensation, field-dependent magnetization reversal, and complex magnetic ordering in Co2TiO4
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:21
  • Tidskriftsartikel (refereegranskat)abstract
    • The complex nature of magnetic ordering in the spinel Co2TiO4 is investigated by analyzing the temperature and magnetic field dependence of its magnetization (M), specific heat (C-p), and ac magnetic susceptibilities chi' and chi ''. X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be Co2TiO4 = [Co2+][Co3+ Ti3+]O-4. From analysis of the temperature dependence of the dc paramagnetic susceptibility, the magnetic moments mu(A) = 3.87 mu(B) and mu(B) = 5.19 mu B on the A and B sites are determined with mu(B) in turn yielding mu(Ti3+) = 1.73 mu(B) and mu(Co3+) = 4.89 mu(B). Analysis of the dc and ac susceptibilities combined with the weak anomalies observed in the C-p vs T data shows the existence of a quasi-long-range ferrimagnetic state below T-N similar to 47.8K and a compensation temperature T-comp similar to 32 K, the latter characterized by sign reversal of magnetization with its magnitude depending on the applied magnetic field and the cooling protocol. Analysis of the temperature dependence of M (field cooled) and M (zero field cooled) data and the hysteresis loop parameters is interpreted in terms of large spin clusters. These results in Co2TiO4, significantly different from those reported recently in isostructural Co2SnO4 = [Co2+][Co2+ Sn4+]O-4, warrant further investigations of its magnetic structure using neutron diffraction.
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35.
  • Padam, R., et al. (författare)
  • Magnetic phase diagram of Co(Cr1-xAlx)2O-4 (x=0.0-1.0)
  • 2017
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 122:7
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the role of Al substitution in the magnetic properties of spinel CoCr2O4 by means of temperature dependent dc and ac magnetization and heat capacity measurements. Various compositions (0.0 <= x <= 1.0) of polycrystalline Co(Cr1-xAlx)(2)O-4 samples have been prepared by sol-gel processing and their crystal structure was investigated by X-ray diffraction which was found to crystallize in the normal cubic spinel structure. For x <= 0.1, the system exhibits multiple magnetic orderings (long range ferrimagnetic ordering T-C, spin-spiral ordering T-S, and lock-in transition T-L), similar to that of the parent compound, CoCr2O4. However, all the compositions between x = 0.1 and 0.5 exhibit long range ferrimagnetic ordering below T-C and also a short range order at low temperature. Spin-glass like ordering was noticed between x = 0.6 and 0.8 due to the diluted B-site occupancy, whereas the end compound CoAl2O4 (x = 1.0) shows antiferromagnetic behavior. On the basis of these results, we propose a magnetic phase diagram for the Co(Cr1-xAlx)(2)O-4 series as a function of the Al content (x) and measuring temperature (T).
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36.
  • Pagliari, L., et al. (författare)
  • Strain heterogeneity and magnetoelastic behaviour of nanocrystalline half-doped La, Ca manganite, La0.5Ca0.5MnO3
  • 2014
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 26:43, s. 435303-
  • Tidskriftsartikel (refereegranskat)abstract
    • Elastic and anelastic properties of La0.5Ca0.5MnO3 determined by resonant ultrasound spectroscopy in the frequency range similar to 100-1200 kHz have been used to evaluate the role of grain size in determining the competition between ferromagnetism and Jahn-Teller/charge order of manganites which show colossal magneto resistance. At crystallite sizes of similar to 75 and similar to 135 nm the dominant feature is softening of the shear modulus as the charge order transition point, T-co (similar to 225 K), is approached from above and below, matching the form of softening seen previously in samples with 'bulk' properties. This is consistent with a bilinear dominant strain/order parameter coupling, which occurs between the tetragonal shear strain and the Jahn-Teller (Gamma(+)(3)) order parameter. At crystallite sizes of similar to 34 and similar to 42 nm the charge ordered phase is suppressed but there is still softening of the shear modulus, with a minimum near Tco. This indicates that some degree of pseudoproper ferroelastic behaviour is retained. The primary cause of the suppresion of the charge ordered structure in nanocrystalline samples is therefore considered to be due to suppression of macroscopic strain, even though MnO6 octahedra must develop some Jahn-Teller distortions on a local length scale. This mechanism for stabilizing ferromagnetism differs from imposition of either an external magnetic field or a homogeneous external strain field (from a substrate), and is likely to lead both to local strain heterogeneity within the nanocrystallites and to different tilting of octahedra within the orthorhombic structure. An additional first order transition occurs near 40K in all samples and appears to involve some very small strain contrast between two ferromagnetic structures.
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37.
  • Panda, Jaganandha, et al. (författare)
  • Ultimate Spin Currents in Commercial Chemical Vapor Deposited Graphene
  • 2020
  • Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 14:10, s. 12771-12780
  • Tidskriftsartikel (refereegranskat)abstract
    • Establishing ultimate spin current efficiency in graphene over industry-standard substrates can facilitate research and development exploration of spin current functions and spin sensing. At the same time, it can resolve core issues in spin relaxation physics while addressing the skepticism of graphene's practicality for planar spintronic applications. In this work, we reveal an exceptionally long spin communication capability of 45 mu m and highest to date spin diffusion length of 13.6 mu m in graphene on SiO2/Si at room temperature. Employing commercial chemical vapor deposited (CVD) graphene, we show how contact-induced surface charge l transfer doping and device doping contributions, as well as spin relaxation, can be quenched in extremely long spin channels and thereby enable unexpectedly long spin diffusion lengths in polycrystalline CVD graphene. Extensive experiments show enhanced spin transport and precession in multiple longest channels (36 and 45 mu m) that reveal the highest spin lifetime of similar to 2.5-3.5 ns in graphene over SiO2/Si, even under ambient conditions. Such performance, made possible due to our devices approaching the intrinsic spin-orbit coupling of similar to 20 mu eV in graphene, reveals the role of the D'yakonov-Perel' spin relaxation mechanism lin graphene channels as well as contact regions. Our record demonstration, fresh device engineering, and spin relaxation insights unlock the ultimate spin current capabilities of graphene on SiO2/Si, while the robust high performance of commercial CVD graphene can proliferate research and development of innovative spin sensors and spin computing circuits.
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38.
  • Pramanik, P., et al. (författare)
  • Cationic distribution, exchange interactions, and relaxation dynamics in Zn-diluted MnCo2O4 nanostructures
  • 2019
  • Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 125:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We report an experimental investigation of the electronic structure and magnetic properties of bulk and nanosized MnCo2O4 diluted with Zn. The cationic distribution for tetrahedral A-site dilution is (Co1-yA2+ZnyA2+)(A)[Mn3+Co3+](B)O-4 +/-delta, whereas B-site dilution results in (Co2+)(A)[Mn1-xB3+ZnxB2+Co3+](B)O4-delta. The strength of exchange interaction J(ij) between the magnetic ions in a bulk spinel lattice decreases by similar to 15% for A-site dilution relative to the undiluted compound; however, B-site dilution results in an enhancement in J(ij) by 17%. The frequency and temperature dependence of dynamic-susceptibility [chi(ac)(f, T)] studies of nanostructured compounds reveals the existence of spin-glass like behavior below the freezing temperature T-F similar to 125.7 K (for x(B) = 0.2) and 154.3 K (y(A) = 0.1). Relaxation time tau follows the Power-Law variation with a dynamical critical exponent zv = 6.17 and microscopic spin relaxation time tau(o) = 4.4 x 10(-15) s for x(B) = 0.2 (for y(A) = 0.1, zv = 5.2 and tau(o) = 5.4 x 10(-13) s). The amplitude and peak position in chi(ac)(T) decreases with an increase in the DC bias field, which indicates that the spin-glass phase can survive in the presence of low fields forming a critical line with an exponent 2/3. This behavior is similar to the de Almeida-Thouless (AT-line) analysis in the T-H phase diagram which supports the existence of spin-glass like behavior below T-F in these Zn diluted spinels. 
  •  
39.
  • Pramanik, Prativa, et al. (författare)
  • Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in noncollinear spinel Ti x Mn1-x (Fe y Co1-y )2O4 : a neutron diffraction study
  • 2024
  • Ingår i: Journal of Physics. - : Institute of Physics (IOP). - 0953-8984 .- 1361-648X. ; 36:35
  • Tidskriftsartikel (refereegranskat)abstract
    • Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel TixMn1-x(FeyCo1-y)(2)O-4 has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i) x(Ti) = 0.20 and y(Fe) = 0.18; (ii) x(Ti) = 0.40 and y(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti0.20Mn0.80Fe0.36Co1.64O4 exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperature T-C = 258 K in comparison to the end compound MnCo2O4 (T-C similar to 180 K) with a characteristic ratio c(t)/root 2a(t) of 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti0.40Mn0.60Fe0.87Co1.13O4 with higher B-site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility, chi(ac)(T) reveals a cluster spin-glass state with Gabay-Toulouse (GT) like mixed phases behaviour below T-C. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence of chi(ac)(T) follows the weak anisotropic GT-line behaviour with crossover exponent Phi lies in the range 1.38-1.52 on the H-T plane which is in contrast to the B-site Ti substituted MnCo2O4 spinel that appears to follow irreversible non-mean-field AT-line behaviour (Phi similar to 3 + delta). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (T < 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.
  •  
40.
  • Pramanik, P., et al. (författare)
  • Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti1-xMnxCo2O4 spinel
  • 2020
  • Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 32:24
  • Tidskriftsartikel (refereegranskat)abstract
    • A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1-xMnxCo2O4 in the temperature interval 1.6 K T 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1-xMnxCo2O4 system for all the Jahn-Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d(1), 3d(4) and 3d(6), respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length d(c)(T-B-O) is longer than the equatorial bond length d(ab)(T-B-O) due to the splitting of the e(g) level of Mn3+ ions into and . For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T-COMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of 110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T-COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.
  •  
41.
  • R., Rajesh Kumar, et al. (författare)
  • Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits
  • 2024
  • Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 16:15, s. 19225-19234
  • Tidskriftsartikel (refereegranskat)abstract
    • Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.
  •  
42.
  • Retuerto, Maria, et al. (författare)
  • Pb2MnTeO6 Double Perovskite : An Antipolar Anti-ferromagnet
  • 2016
  • Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 55:9, s. 4320-4329
  • Tidskriftsartikel (refereegranskat)abstract
    • Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to similar to 120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s2 lone-pair electrons, together with a surprising off-centering of Mn2+ (d5) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN approximate to 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near similar to 150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.
  •  
43.
  • Sarkar, Tapati, et al. (författare)
  • Compositional dependence of the magnetic state of Co3-xZnxTeO6 solid solutions
  • 2021
  • Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 884
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic properties of phase-pure solid solutions of Co3-xZnxTeO6 are investigated using magnetometry (single crystals) and neutron diffraction (polycrystalline powders), and compared to results of detailed crystallographic studies. There are five unique Co sites in Co3TeO6, including a tetrahedrally coordinated Co-2 site which Zn preferentially occupies. The magnetic interaction is found to monotonically decrease as the Co-2-O bond length decreases with increasing Zn content. The results suggest a modification of the magnetic structure in the samples containing Zn.
  •  
44.
  • Sarkar, Tapati, et al. (författare)
  • Successive phase transitions in the orthovanadate TmVO3
  • 2015
  • Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 48:34
  • Tidskriftsartikel (refereegranskat)abstract
    • Synthesis and crystal structure, magnetization and heat capacity measurements of phase pure polycrystalline TmVO3 are reported. TmVO3 was stabilized in the orthorhombic structure by thermal treatment of the precursor TmVO4 in a reducing atmosphere. Magnetization and heat capacity measurements reveal the presence of several successive structural and magnetic phase transitions in this compound. At T = 108 K, the sample undergoes a transition from a paramagnetic state to an antiferromagnetic state, followed by a second transition at 78 K which is related to spin and orbital reorientation. The heat capacity measurements reveal the presence of a third transition in the paramagnetic state (at T = 175 K), which corresponds to a structural phase transition and orbital ordering. At low temperatures (similar to 15 K) and weak fields, there is an anomaly in the magnetization, which may be associated with antiferromagnetic short range ordering of the Tm3+ ions.
  •  
45.
  • Sarkar, Tapati, et al. (författare)
  • The role of Tb-doping on the structural and functional properties of Bi4-xTbxTi3O12 ferroelectric phases with the Aurivillius type structure
  • 2017
  • Ingår i: Journal of materials science. Materials in electronics. - : SPRINGER. - 0957-4522 .- 1573-482X. ; 28:6, s. 4914-4924
  • Tidskriftsartikel (refereegranskat)abstract
    • Synthesis, crystal structure, dielectric, and magnetic properties of the Aurivillius phase Bi4-xTbxTi3O12 (x = 0.0, 0.4, 0.6, 0.8) are reported. The samples were synthesized using standard solid state reaction technique. The thermal stability of the obtained solid solutions was investigated. For x ae 0.8, the samples crystallized in an orthorhombic symmetry. All the samples showed finite second harmonic generation response indicating a non-centrosymmetric structure. The structural data could be refined using the polar orthorhombic space group B2cb. The orthorhombicity decreases with an increase in the Tb3+ concentration. The orthorhombic distortions in these compositions are related to the Bi3+-based perovskite sublattice. Our results indicate that the non-lone pair Tb3+ cations preferentially occupy the perovskite sublattice initially, but with an increase in the doping concentration they can partially substitute the Bi3+ ions in the fluorite block. Temperature dependent dielectric measurements revealed a decrease in the ferroelectric Curie temperature T-C with an increase in x from T-C = 904 K (for x = 0) to 877 K (for x = 0.4). Further increase in x led to a cross-over to a relaxor-type behavior. Magnetic measurements showed that the samples are paramagnetic down to 5 K.
  •  
46.
  • Sarkar, Tapati, et al. (författare)
  • Thermal evolution of the spin ordering at the concomitant spin-orbital rearrangement temperature in RVO3 (R=Lu, Yb and Tm)
  • 2016
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 409, s. 87-91
  • Tidskriftsartikel (refereegranskat)abstract
    • Synthesis, crystal structure and magnetization measurements of phase pure polycrystalline RVO3 (R=Lu, Yb and Tm) are reported. The compounds were stabilized in the orthorhombic structure by thermal treatment of the respective precursors (RVO4) in a reducing atmosphere. Special pressure treatment was carried out during the synthesis to ensure phase pure samples without secondary phases. Magnetization measurements reveal the presence of two spin ordering temperatures in the samples. Interestingly, at the lower spin ordering temperature, TsO2, the uncompensated excess moment of the antiferromagnetic spin structure has different field dependences above and below TsO2, causing a jump in the thermal evolution of the magnetization that changes sign with increasing field strength. This jump is associated with the reported magnetic and orbital rearrangement in the samples, and the different spin configurations in the C- and G-type antiferromagnetic structures.
  •  
47.
  • Sarkar, Tapati, et al. (författare)
  • Tunable single-phase magnetic behavior in chemically synthesized AFeO(3)-MFe2O4 (A = Bi or La, M = Co or Ni) nanocomposites
  • 2018
  • Ingår i: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 10:48, s. 22990-23000
  • Tidskriftsartikel (refereegranskat)abstract
    • The properties of magnetic nanocomposites rely strongly on the interplay between those of the constituent components. When the individual components themselves are complex systems belonging to the family of correlated electron oxide systems which typically exhibit exotic physical properties, it becomes nontrivial to customize the properties of the nanocomposite. In this paper, we demonstrate an easy, but effective method to synthesize and tune the magnetic properties of nanocomposites consisting of correlated electron oxide systems as the individual components. Our method is based on a novel synthesis technique by which the two components of the nanocomposite can be directly integrated with each other, yielding homogeneous samples on the nanoscale with magnetic behavior reminiscent of a single phase. We illustrate our method using multiferroic BiFeO3 (BFO) and LaFeO3 (LFO) as the major phase (i.e., matrix), and MFe2O4 (M = Co2+ or Ni2+) as the embedded magnetic phase. Furthermore, we show that by a proper selection of the second phase in the nanocomposite, it is possible to customize the magnetic properties of the matrix. We illustrate this by choosing CoFe2O4 and NiFe2O4, two oxides with widely differing magnetic anisotropies, as the embedded phase, and demonstrate that the coercivity of BFO and LFO can be increased or decreased depending on the choice of the embedded phase in the nanocomposite.
  •  
48.
  • Sayed, Fatima, et al. (författare)
  • Controlling magnetic coupling in bi-magnetic nanocomposites
  • 2019
  • Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 11:30, s. 14256-14265
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetic nanocomposites constitute a vital class of technologically relevant materials, in particular for next-generation applications ranging from biomedicine, catalysis, and energy devices. Key to designing such materials is determining and controlling the extent of magnetic coupling in them. In this work, we show how the magnetic coupling in bi-magnetic nanocomposites can be controlled by the growth technique. Using four different synthesis strategies to prepare prototypical LaFeO3-CoFe2O4 and LaFeO3-Co0.5Zn0.5Fe2O4 nanocomposite systems, and by performing comprehensive magnetic measurements, we demonstrate that the final material exhibits striking differences in their magnetic coupling that is distinct to the growth method. Through structural and morphological studies, we confirm the link between the magnetic coupling and growth methods due to distinct levels of particle agglomeration at the very microscopic scale. Our studies reveal an inverse relationship between the strength of magnetic coupling and the degree of particle agglomeration in the nanocomposites. Our work presents a basic concept of controlling the particle agglomeration to tune magnetic coupling, relevant for designing advanced bi-magnetic nanocomposites for novel applications.
  •  
49.
  • Sayed, F., et al. (författare)
  • LaFeO3-CoFe2O4 bi-magnetic composite thin films prepared using an all-in-one synthesis technique
  • 2020
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : ELSEVIER. - 0304-8853 .- 1873-4766. ; 503
  • Tidskriftsartikel (refereegranskat)abstract
    • Bi-phasic composite films are generally grown as multilayers that result in layer-by-layer morphology with each layer having a distinct chemical composition. In this work, we report an all-in-one chemical synthesis technique combined with spin-coating to prepare single-layer bi-magnetic LaFeO3 (LFO)-CoFe2O4 (CFO) composite thin films with both phases co-existing in the same layer. The films have been characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the magnetic properties have been probed using dc magnetometry at room and low temperature. The unique synthesis technique followed ensures homogeneity of the two phases on the nanoscale with grain sizes similar to 10 nm for CFO and few tens of nm for LFO, as observed from TEM images. XRD confirms the presence of only the desired LFO and CFO phases in the films without any undesired secondary phases. Magnetic hysteresis loops reveal a coercivity of similar to 0.2 T at room temperature that increases by nearly one order of magnitude at T = 5 K. The all-in-one synthesis technique reported here can be used to prepare different bi-phasic composites in the form of single-layer two-dimensional films as well as zero-dimensional nanoparticles by a suitable modification of the precursors, solvents, and chelating agents.
  •  
50.
  • Sayed, F., et al. (författare)
  • Symbiotic, low-temperature, and scalable synthesis of bi-magnetic complex oxide nanocomposites
  • 2020
  • Ingår i: Nanoscale Advances. - : ROYAL SOC CHEMISTRY. - 2516-0230. ; 2:2, s. 851-859
  • Tidskriftsartikel (refereegranskat)abstract
    • Functional oxide nanocomposites, where the individual components belong to the family of strongly correlated electron oxides, are an important class of materials, with potential applications in several areas such as spintronics and energy devices. For these materials to be technologically relevant, it is essential to design low-cost and scalable synthesis techniques. In this work, we report a low-temperature and scalable synthesis of prototypical bi-magnetic LaFeO3-CoFe2O4 nanocomposites using a unique sol-based synthesis route, where both the phases of the nanocomposite are formed during the same time. In this bottom-up approach, the heat of formation of one phase (CoFe2O4) allows the crystallization of the second phase (LaFeO3), and completely eliminates the need for conventional high-temperature annealing. A symbiotic effect is observed, as the second phase reduces grain growth of the first phase, thus yielding samples with lower particle sizes. Through thermogravimetric, structural, and morphological studies, we have confirmed the reaction mechanism. The magnetic properties of the bi-magnetic nanocomposites are studied, and reveal a distinct effect of the synthesis conditions on the coercivity of the particles. Our work presents a basic concept of significantly reducing the synthesis temperature of bi-phasic nanocomposites (and thus also the synthesis cost) by using one phase as nucleation sites for the second one, as well as using the heat of formation of one phase to crystallize the other.
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