| 1. |
- Cedervall, Johan, et al.
(författare)
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Phase stability and structural transitions in compositionally complex LnMO 3 perovskites
- 2021
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 300
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Tidskriftsartikel (refereegranskat)abstract
- Entropy stabilised materials have possibilities for tailoring functionalities to overcome challenges in materials science. The concept of configurational entropy can also be applied to metal oxides, but it is unclear whether these could be considered as solid solutions in the case of perovskite-structured oxides and if the configurational entropy plays a stabilising role. In this study, compositionally complex perovskite oxides, LnMO3 (Ln = La, Nd, Sm, Ca and Sr, M = Ti, Cr, Mn, Fe, Co, Ni, and Cu), are investigated for their phase stability and magnetic behaviour. Phase-pure samples were synthesised, and the room temperature structures were found to crystallise in either Pnma or R3¯c space groups, depending on the composition and the resulting tolerance factor, while the structural transition temperatures correlate with the pseudo cubic unit cell volume. The techniques used included diffraction with X-rays and neutrons, both ex- and in-situ, X-ray photoelectron spectroscopy, magnetometry as well as electron microscopy. Neutron diffraction studies on one sample reveal that no oxygen vacancies are found in the structure and that the magnetic properties are ferrimagnetic-like with magnetic moments mainly coupled antiferromagnetically along the crystallographic c-direction. X-ray photoelectron spectroscopy gave indications of the oxidation states of the constituting ions where several mixed oxidation states are observed in these valence-compensated perovskites.
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| 2. |
- Datt, Gopal, et al.
(författare)
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Combined Bottom-Up and Top-Down Approach for Highly Ordered One-Dimensional Composite Nanostructures for Spin Insulatronics
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:31, s. 37490-37499
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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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| 3. |
- Dokala, Ravi Kiran, et al.
(författare)
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Elastic strain control of electronic structure, and magnetic properties of [Pr1-xCaxMnO3/SrTiO3](15) superlattices
- 2020
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Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 127:17
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Tidskriftsartikel (refereegranskat)abstract
- We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr 0.7Ca 0.3MnO 3/SrTiO 3] 15 and [Pr 0.5Ca 0.5MnO 3/SrTiO 3] 15 on (001) oriented SrTiO 3 and LaAlO 3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO 3 as compared to those grown on SrTiO 3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (H K) and cubic anisotropy constant ( K 1) for all the investigated superlattices. This analysis yields the highest H K similar to 9kOe and K 1 similar to 8 x 10 5erg/cc for the [Pr 0.7Ca 0.3MnO 3/SrTiO 3] 15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in T C ( < 100K) was observed in the case of LaAlO 3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO 6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of e g levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.
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| 4. |
- Ivanov, Sergey, et al.
(författare)
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Partial cation ordering, relaxor ferroelectricity, and ferrimagnetism in Pb(Fe1-xYbx)(2/3)W1/3O3 solid solutions
- 2020
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Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 128:13
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Tidskriftsartikel (refereegranskat)abstract
- The structural, magnetic, and dielectric properties of ceramic samples of Yb-doped PbFe2/3W1/3O3 have been investigated by a variety of methods including x-ray powder diffraction, magnetometry, and dielectric spectroscopy. In addition, theoretical investigations were made using first-principles density functional calculations. All the doped samples Pb(Fe1-xYbx)(2/3)W1/3O3 (PFYWO) (0.1 <= x <= 0.5) were found to crystallize in an ordered cubic ( F m 3 m ) structure with partial ordering in the B-perovskite sites. Observed changes in the cationic order were accompanied by differences in the dielectric and magnetic responses of the system. While pure PbFe2/3W1/3O3 is antiferromagnetic, the doped Pb(Fe1-xYbx)(2/3)W1/3O3 PFYWO samples display excess moments and ferrimagnetic-like behavior, associated with differences in B ' and B '' site occupancies of the magnetic Fe3+ cations. The magnetic transition temperature of the ferrimagnetic phase is found to decrease with increasing Yb content, from T-N similar to 350K of the undoped sample down to 137K for x=0.5. All PFYWO compounds display a ferroelectric relaxor behavior akin to that of PbFe2/3W1/3O3, albeit our results show significant changes of the frequency and temperature dependence of the dielectric properties. The changes of the properties of PFYWO with increasing Yb substitution can be explained by the changes in the cation size/charge mismatch and the size difference of the two ordered positions.
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| 5. |
- Joshi, Deep C., et al.
(författare)
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2D crystal structure and anisotropic magnetism of GdAu6.75−xAl0.5+x (x ≈ 0.54)
- 2022
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Exploration of the gold-rich part of the ternary Gd–Au–Al system afforded the intermetallic compound GdAu6.75−xAl0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu6.75−xAl0.5+x crystallizes in a previously unknown structure type featuring layers of Gd2(Au, Al)29 and Gd2(Au, Al)28 clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 36 nets (dGd–Gd = 5.30–5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu5.3Al in the same system. The magnetic properties of GdAu6.75−xAl0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.
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| 6. |
- Joshi, Deep C., et al.
(författare)
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Ferromagnetic excess moments and apparent exchange bias in FeF2 single crystals
- 2019
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The anisotropic antiferromagnet FeF2 has been extensively used as an antiferromagnetic layer to induce exchange bias effects in ferromagnetic/antiferromagnetic bilayers and heterostructures. In this work, an apparent exchange bias occurring in the low temperature hysteresis loops of FeF2 single crystals is investigated. A detailed investigation of the hysteresis and remnant magnetization indicates that the observation of an apparent exchange bias in FeF2 stems from an intrinsic excess moment associated with a distortion of the antiferromagnetic structure of piezomagnetic origin.
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| 7. |
- Joshi, Deep C., et al.
(författare)
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Memory and rejuvenation in a quasicrystal
- 2020
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 132:2
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Tidskriftsartikel (refereegranskat)abstract
- The glassy features of a single crystal of the icosahedral quasicrystal i-GdCd7.5 were investigated by means of squid magnetometry. The temperature-dependent zero-field-cooled magnetization was recorded on re-heating from low temperatures after halts in the cooling. The results evidence dynamical features akin to those of archetypal spin glasses, such as aging, memory, and rejuvenation. The results are compared to those of model spin glasses with different spin dimensionality, suggesting a qualitative similarity to the behaviour of metallic RKKY "Heisenberg" spin glasses. Copyright (C) 2020 The author(s)
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| 8. |
- Joshi, Deep C., et al.
(författare)
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Room temperature ferrimagnetism in Yb-doped relaxor ferroelectric PbFe2/3W1/3O3
- 2019
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 115:7
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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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| 9. |
- Kotnana, Ganesh, et al.
(författare)
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Novel mixed precursor approach to prepare multiferroic nanocomposites with enhanced interfacial coupling
- 2020
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 511
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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.
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| 10. |
- Kumar, Siddharth, et al.
(författare)
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Electronic and magnetic structures of a mixed triple perovskite : Ba3NiRuIrO9
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 110:2
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Tidskriftsartikel (refereegranskat)abstract
- In search of spin-orbit coupling driven nonmagnetic J = 0 ground state and excitonic magnetism, various pentavalent iridates have been studied in recent years. However, a finite moment was observed in most of the cases due to solid state effects. In this work, we investigate the electronic and magnetic structure of 6H hexagonal compound Ba3NiRuIrO9, where Ir5+ is present along with magnetic Ni2+ and Ru5+ ions. Magnetic susceptibility measurements and neutron powder diffraction (NPD) experiments demonstrate the appearance of short-range magnetic ordering below 170 K and a long-range antiferromagnetic ordering below 80 K. The refinement of the NPD pattern further shows that the Ru and Ir moments interact antiferromagnetically within the dimer and interact ferromagnetically with the Ni sublattice. These experimental findings have been complemented by first-principles density functional theory calculations incorporating spin-orbit coupling effects and electronic correlations for the transitional metal d states. The computed magnetocrystalline anisotropy is also found to be significant and the crystallographic c axis comes out to be the easy axis of magnetization, consistent with the spin alignment direction found from NPD. This study shows that the mixed ruthenate iridate triple perovskite series is a promising family to study the interplay among spin-orbit coupling, electron correlation, and electron filling as a variety of Ba3MRuIrO9 with M as a transition metal ion, rare-earth ion, and alkali metal ions can be synthesized.
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