arXiv subjects

R J Choudhary

Publications and source records attributed to R J Choudhary.

3 recordsLinked to original sources

arXiv2025

Local symmetry breaking and orbital glass behaviour in CoFe2O4

The structural distortions, orbital correlations, and electronic states in cobalt ferrite (CoFe2O4) were investigated using complementary characterisation techniques, including SR-XRD, HAXPES, XANES, EXAFS, and Raman spectroscopy. SR-XRD confirms phase purity and reveals a temperature-dependent superlattice reflection between 200 K and 100 K, consistent with the emergence of short-range orbital ordering driven by cooperative Jahn-Teller distortion (JTD). The disappearance of this feature below 100 K signals orbital freezing and the onset of a glass-like orbital state. HAXPES measurements show multiplet splitting and charge-transfer satellite features in the Co and Fe 2p core levels, indicating mixed valence states and strong electron correlations. XANES analysis reveals hybridized p-d states and local coordination distortions. Temperature-dependent EXAFS measurements indicate increasing local disorder-particularly in Fe-O and Fe-Fe octahedral bonds as evidenced by enhanced Debye-Waller factors. These distortions, attributed to cation redistribution and oxygen vacancies, are static and asymmetric, primarily affecting the octahedral sublattice. Notably, signatures of cooperative Jahn-Teller distortions emerge in the intermediate temperature range (200-100 K) and disappear upon further cooling. Raman spectroscopy further supports these findings, revealing phonon anomalies and enhanced spin-phonon coupling in the same temperature range. Magnetic measurements indicate spin reorientation and exchange interaction anomalies that align with the orbital behaviour. Together, these results hint at a frustrated orbital state in CoFe2O4 possibly involving cooperative Jahn-Teller distortions, disrupted long-range coherence, and orbital glass behaviour offering new insights into the coupling of orbital, spin, and lattice degrees of freedom in spinel systems.

cond-mat.str-el↗

arXiv2022

Robust electronic and tunable magnetic states in Sm$ _{2} $NiMnO$ _{6} $ ferromagnetic insulator

Ferromagnetic insulators (FM-Is) are the materials of interest for new generation quantum electronic applications. Here, we have investigated the physical observables depicting FM-I ground states in epitaxial Sm$ _{2} $NiMnO$ _{6} $ (SNMO) double perovskite thin films fabricated under different conditions to realize different level of Ni/Mn anti-site disorders (ASDs). The presence of ASDs immensely influence the characteristic magnetic and anisotropy behaviors in SNMO system by introducing short scale antiferromagnetic interactions in predominant long range FM ordered host matrix. Charge disproportion between cation sites in form of $ Ni^{2+}+Mn^{4+} \longrightarrow Ni^{3+}+Mn^{3+} $, causes mixed valency in both Ni and Mn species, which is found insensitive to ASD concentrations. Temperature dependent photo emission, photo absorption measurements duly combined with cluster model configuration interaction simulations, suggest that the eigenstates of Ni and Mn cations can be satisfactorily described as a linear combination of the unscreened $ d^{n} $ and screened $ d^{n+1} \underline{L} $ ($ \underline{L} $: O 2\textit{p} hole) states. The electronic structure across the Fermi level (E$ _{F} $) exhibits closely spaced Ni $ 3d $, Mn $ 3d $ and O $ 2p $ states. From occupied and unoccupied bands, estimated values of the Coulomb repulsion energy ($ U $) and ligand to metal charge transfer energy ($ Δ$), indicate charge transfer insulating nature, where remarkable modification in Ni/Mn $ 3d $ - O $ 2p $ hybridization takes place across the FM transition temperature. Existence of ASD broadens the Ni, Mn $ 3d $ spectral features, whereas spectral positions are found to be unaltered. Hereby, present work demonstrates SNMO thin film as a FM-I system, where FM state can be tuned by manipulating ASD in the crystal structure, while I state remains intact.

cond-mat.mtrl-sci↗

arXiv2021

Evolution from sinusoidal to collinear A-type antiferromagnetic spin-ordered magnetic phase transition in Tb0.6Pr0.4MnO3

The present study reports on the structural and magnetic phase transitions in Pr-doped polycrystalline Tb0.6Pr0.4MnO3, using high-resolution neutron powder diffraction (NPD) collected at SINQ spallation source (PSI), to emphasize the suppression of the sinusoidal magnetic structure of pure TbMnO3 and the evolution to a collinear A-type antiferromagnetic ordering. The phase purity, Jahn-Teller distortion, and one-electron bandwidth for eg orbital of Mn3+ cation have been calculated for polycrystalline Tb0.6Pr0.4MnO3, in comparison to the parent materials TbMnO3 and PrMnO3, through the Rietveld refinement study from X-ray diffraction data at room temperature. The temperature-dependent zero field-cooled and field-cooled dc magnetization study at low temperature down to 5 K reveals a variation in the magnetic phase transition due to the effect of Pr3+ substitution at the Tb3+ site, which gives the signature of the antiferromagnetic nature of the sample, with a weak ferromagnetic component at low temperature induced by an external magnetic field. The field-dependent magnetization study at low temperatures gives the weak coercivity having the order of 2 kOe, which is expected due to canted-spin arrangement or ferromagnetic nature of Terbium ordering. The NPD data for Tb0.6Pr0.4MnO3 confirms that the nuclear structure of the synthesized sample maintains its orthorhombic symmetry down to 1.5 K. Also, the magnetic structures have been solved at 50 K, 25 K, and 1.5 K through the NPD study, which shows A-type antiferromagnetic spin arrangement.

cond-mat.mtrl-sci↗