| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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).
|
|
| 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.
|
|
| 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.
|
|
| 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
|
|
| 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.
|
|
