arXiv subjects

Subhash Thota

Publications and source records attributed to Subhash Thota.

5 recordsLinked to original sources

arXiv2025

Giant-Magnetocaloric effect and phonon dynamics in (GdCe)CrO$_3$

We investigate the effect of Ce$^{3+}$ substitution on the magnetic ordering and phonon dynamics of the GdCrO$_3$ orthorhombic perovskite. The Ce doped compound exhibits long-range canted antiferromagnetism with Neel transitions, T$_N$ at $\sim$ 173 K, accompanied by spin-flip, T$_{SF}$ at $\sim$ 10 K. Ce$^{3+}$ incorporation drives a modification of the spin-flip transition from the $\Gamma{(G_x,A_y,F_z)}$ configuration to $\Gamma$ inducing a reorientation of the spin axis between the (001) and (001) crystallographic planes. This spin reorientation is governed by Zeeman energy and produces pronounced field-induced irreversibility between FCC and FCW magnetization processes. The substituted compound Gd$_{0.9}$Ce$_{0.1}$CrO$_3$ (GCCO) exhibits a remarkably large magnetic entropy change, $\Delta$ S $\sim$ 45-40 J/kg-K for $\Delta$ H = 90-70 kOe at 3 K among the highest reported for rare-earth orthochromites. The interplay of spin-only magnetocrystalline anisotropy from Cr$^{3+}$ and spin-orbit-driven magnetic moments of Gd$^{3+}$ and Ce$^{3+}$ results in pronounced spin-phonon coupling, manifested through the A$_{1g}$(6) vibrational mode. The observed temperature-dependent spectral evolution is consistent with behaviour reported in isostructural magnetic perovskites.

cond-mat.mtrl-sci↗

arXiv2025

Spin-Reorientation Dynamics and Strong-Spin Phonon Coupling in Ce-substituted SmCrO$_3$

We report the influence of Ce$^{3+}$ substitution on the magnetic structures and phonon dynamics in SmCrO$_3$ perovskites. Magnetic landscapes are spanned by long-range canted anti-ferromagnetism, AFM with Neel temperatures $\sim$196 K accompanied by spin-reorientation transitions, T$_{SRPT}$ at 42 K. In Sm$_{0.9}$Ce$_{0.1}$CrO$_3$ (SCCO), Ce$^{3+}$ substitution at Sm$^{3+}$ sites transform the weak ferromagnetic (FM) $\Gamma_4$ state into robust AFM $\Gamma_1$ configuration through a gradual crossover. Such coexistence of magnetic spin configurations $\Gamma_1$(AFM) to $\Gamma_4$ (WFM) results in the enhanced high coercive field and a pronounced exchange bias-field, $H_{\mathrm{EB}}$ $\sim$ 2 kOe. Spin-only driven magneto-crystalline anisotropy of Cr$^{3+}$ and spin-orbit driven magnetic moment in Sm$^{3+}$, and Ce$^{3+}$ exhibits spin-phonon coupling through $A_{1g}(6)$ mode in SCCO are consistent with the temperature dependent spectral features of the isostructural magnetic systems and quite significant in SCCO which is in accordance with the higher structural distortion in SCCO. These results demonstrate that site-specific R$^{3+}$ substitution modulates lattice distortions, spin-phonon coupling, and spin-orbit interactions, offering pathways to optimize perovskites for diverse spintronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Optimized Analysis of the AC Magnetic Susceptibility Data in Several Spin-Glass Systems using the Vogel-Fulcher and Power Laws

In spin-glasses (SG), the relaxation time $\tau$ ($= 1/2{\pi}f$) vs. $T_f$ data at the peak position $T_f$ in the temperature variation of the ac magnetic susceptibilities at different frequencies f is often fit to the Vogel-Fulcher Law (VFL): $\tau=\tau_0\exp[E_a/k_b(T_f-T_0)]$ and to the Power Law (PL): $\tau = \tau_0^*[(T_f-T_{SG}/T_{SG}]^{-z\nu}$. Both these laws have three fitting parameters each, leaving a degree of uncertainty since the magnitudes of the evaluated parameters $\tau_0$, $E_a/k_B$, $\tau_{0^*}$ and $z\nu$ depend strongly on the choice of $T_0$ and $T_{SG}$. Here we report an optimized procedure for the analysis of $\tau$ vs. $T_f$ data on several SG systems for which we could extract such data from published sources. In this optimized method, the data of $\tau$ vs. $T_f$ are fit by varying $T_0$ in the linear plots of $\ln \tau$ vs $1/ (T_f - T_0)$ for the VFL and by varying $T_{SG}$ in the linear plot of $\ln \tau$ vs. $\ln (T_f - T_{SG})/ T_{SG}$ for the PL till optimum fits are obtained. The analysis of the associated magnitudes of $\tau_0$, $E_a/k_B$, $\tau_{0^*}$ and $z\nu$ for these optimum values of $T_0$ and $T_{SG}$ shows that magnitudes of $\tau_{0^*}$, $\tau_0$ and $z\nu$ fail to provide a clear distinction between canonical and cluster SG. However, new results emerge showing $E_a/(k_BT_0) < 1$ in canonical SG whereas $E_a/(k_BT_0) >1$ for cluster SG systems and the optimized $T_0 <$ optimized $T_{SG}$ in all cases. Although some interpretation of these new results is presented, a more rigorous theoretical justification of the boundary near $E_a/(k_BT_0) \sim 1$ is desired along with testing of these criteria in other SG systems.

cond-mat.dis-nn↗

arXiv2020

Lattice dynamics and magnetic exchange interactions in GeCo2O4, a spinel with S = 1/2 pyrochlore lattice

GeCo$_2$O$_4$ is a unique system in the family of cobalt spinels ACo$_2$O$_4$ (A= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of GeCo$_2$O$_4$ spinel has remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel GeCo$_2$O$_4$, which exhibits an AFM ordering below the N\'eel temperature $T_N$ ~21 K, followed by a cubic ($Fd{\bar 3}m$) to tetragonal ($I4_{1}/amd$) structural phase transition at $T_S$ ~16 K. Our density-functional theory (DFT+U) calculations reveal that one needs to consider magnetic-exchange interactions up to the third nearest neighbors to get an accurate description of the low-temperature AFM order in GeCo$_2$O$_4$. At room temperature three distinct IR-active modes ($T_{1u}$) are observed at frequencies 680, 413, and 325 cm$^{-1}$ along with four Raman-active modes $A_{1g}$, $T_{2g}(1)$, $T_{2g}(2)$, and $E_{g}$ at frequencies 760, 647, 550, and 308 cm$^{-1}$, respectively, which match reasonably well with our DFT+U calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes, is also discussed. Noticeable changes around $T_N$ and $T_S$ are observed in the Raman line parameters of the $E_{g}$ and $T_{2g}$ modes, which are associated with the modulation of the Co-O bonds in CoO$_6$ octahedra during the excitations of these modes.

cond-mat.mtrl-sci↗

arXiv2020

Experimental and Computational Studies of the Optical Properties of CuAl1-xFexO2

Delafossites are promising candidates for photocatalysis applications because of their chemical stability and absorption in the solar region of the electromagnetic spectrum. For example, CuAlO2 has good chemical stability but has a large indirect bandgap (~3 eV), so that efforts to improve its absorption in the solar region through alloying are investigated. The effect of dilute alloying on the optical absorption of powdered CuAl1-xFexO2 (x = 0.0-1.0) is measured and compared to electronic band structures calculations using a generalized gradient approximation with Hubbard exchange-correlation parameter and spin. A new absorption feature is observed at 1.8 eV for x = 0.01, which redshifts to 1.4 eV for x = 0.10. This feature is associated with transitions from the L-point valence band maximum to the Fe-3d state that appears below the conduction band of the spin-down band structure. The feature increases the optical absorption below the bandgap of pure CuAlO2, making dilute CuAl1-xFexO2 alloys better suited for solar photocatalysis.

cond-mat.mtrl-sci↗