The strong coupling between 3d and 4f based magnetic sublattices in double perovskite (DP) compounds results in various exotic complex magnetic interactions, and the ground state contains multiple fascinating and remarkable magnetic states. In this article, we have performed a detailed investigation of the crystal structure, magnetic, and magnetocaloric properties of the ordered monoclinic polycrystalline double perovskite Ho 2 CoMnO 6 (HCMO) compound. A study of the magnetization dynamics employing temperature and magnetic field shows a powerful correlation between Ho and Co/Mn sublattices. Due to the presence of the ferromagnetic superexchange interaction in between Co 2 + − O − Mn 4 + networks, the system undergoes an ordered state at the transition temperature, T C ≈ 77 K. Below T C, a clear compensation point continued by negative magnetization is noticed in the virgin state of the compound. The reduction of the saturation magnetization ( M S) in the hysteresis curves (M-H) can be explained by the existence of local anti-site defects or disorders and anti-phase boundaries in the system. Temperature dependence of magnetic entropy change ( − Δ S) curves shows a maximum value of 13.4 J/kg K for Δ H = 70 kOe at a low temperature along with a noticeable inverse magnetocaloric effect. Moreover, the material holds reasonable values of magnetocaloric parameters. The absence of thermal hysteresis along with a large value of | Δ S | makes the system a potential candidate for low temperature as well as liquid nitrogen temperature-based magnetic refrigeration. Additionally, our experimental findings should encourage further detailed studies on the complex 3d–4f exchange interaction in the double perovskite system.

Role of 3d–4f exchange interaction and local anti-site defects in the magnetic and magnetocaloric properties of double perovskite Ho2CoMnO6 compound

We investigate the structural, magnetic, and spin-state transitions, and magnetocaloric properties of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CoNbO}}_{6}$ ($x=0$--1) double perovskites. The structural transition from tetragonal to monoclinic phase at $x\ensuremath{\geqslant}0.6$, and an evolution of (101)/(103) superlattice reflections and Raman active modes indicate the enhancement in the B-site ordering with $x$. The magnetic susceptibility data reveal the transition from weak ferromagnetic (FM) to antiferromagnetic (AFM) ordering for $x\ensuremath{\geqslant}0.6$ with ${T}_{\mathrm{N}}\ensuremath{\approx}9$--15 K. Interestingly, the La substitution drives towards a more insulating state due to an increase in high-spin ${\mathrm{Co}}^{2+}$, whereas a spin-state crossover is observed in ${\mathrm{Co}}^{3+}$ from high-spin to intermediate-/low-spin states with $x$. We discuss the correlation between complex magnetic interactions and the presence of various Co spin states in the system. Moreover, the emergence of metamagnetic nature due to the competition between FM and AFM interactions as well as crossover from conventional to inverse magnetocaloric effect have been demonstrated by a detailed analysis of temperature and field dependent change in magnetic entropy.

Unraveling magnetic interactions and the spin state in insulating Sr 2 − x La x CoNbO 6

We have investigated the crystal structure and the nature of the magnetic ground state of the polycrystalline compound Pr2FeCrO6 (PFCO) through X-ray diffraction (XRD), magnetization, and magnetocaloric effect studies. Analysis of the XRD pattern reveals that the PFCO compound exhibits a B-site disordered orthorhombic crystal structure. The random distribution of Fe3+ and Cr3+ magnetic sublattices at the B-sites of the crystallographic unit cell helps to generate several fascinating magnetic properties. The compound exhibits three distinct anomalies in both the temperature dependence of the magnetization and the magnetic entropy change (-ΔS) curves, namely, (i) a G-type canted antiferromagnetic (AFM) ordering of the transition metal ions (TN1), (ii) a progressive spin reorientation (SR) transition (TSR), and (iii) an AFM ordering of Pr3+ sublattices at very low temperature (TN2). Surprisingly, a novel "diamagnetism-like" behavior appears in the low-temperature region for low applied field values. Moreover, we have also constructed the thermal evolution of the magnetic crystal structures in different transition regions with the help of irreducible representations of the crystal symmetry. Overall, our study of B-site disordered PFCO may help to encourage basic fundamental and applied research on disordered rare-earth and transition metal-based perovskite systems due to their interesting magnetic properties over a broad temperature range.

Structural, magnetic, and magnetocaloric properties of the multiferroic host double perovskite compound Pr2FeCrO6

Multiple Magnetic Phases in Van Der Waals Mn-Doped SnS2 Semiconductor

We report the detailed analysis of specific heat $[{C}_{P}(T)]$ and ac susceptibility for magnetically frustrated ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CoNbO}}_{6}$ $(x=0--1)$ double perovskites to understand low-temperature complex magnetic interactions and their evolution with $x$. Interestingly, the observed Schottky anomaly in the $x\ensuremath{\leqslant}0.4$ samples shifts gradually toward higher temperatures with magnetic field as well as $x$, and the analysis reveals the persistence of the discrete energy states in these samples resulting from the spin-orbit coupling and octahedral distortion. Moreover, the extracted values of the Land\'e $g$ factor indicate the existence of high-spin state ${\mathrm{Co}}^{3+}$ ions energetically close to a nonmagnetic low-spin state. The specific-heat data show the $\ensuremath{\lambda}$-type anomaly for the $x\ensuremath{\geqslant}0.6$ samples due to evolution of the long-range antiferromagnetic ordering. Our analysis of low-temperature ${C}_{P}(T)$ data for the $x\ensuremath{\geqslant}0.6$ samples demonstrates the 3D isotropic Heisenberg antiferromagnetic (AFM) interactions and the temperature-induced second-order AFM-paramagnetic phase transition. More interestingly, we demonstrate the presence of the free ${\mathrm{Co}}^{2+}$-like Kramers doublet ground state in the $x=1$ sample. Further, the ac susceptibility and time evolution of the magnetization data reveal the low-temperature cluster-glass-like behavior in the $x=0--0.4$ samples, where spin-spin correlation strength decreases with $x$.

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Evidence of discrete energy states and cluster-glass behavior in Sr 2 − x La x CoNbO 6

Antisite (B-site) disorder in double perovskite lattice is responsible for various magnetic phenomena such as exchange bias, spin glass, memory effect, colossal magnetoresistance, etc. By controlling the antisite disorder in the antiferromagnetic double perovskite ${\mathrm{LaSrCoFeO}}_{6}$, we achieve intrinsic exchange bias effect with a large exchange bias field ($\ensuremath{\sim}1.2$ kOe) and giant coercive field ($\ensuremath{\sim}12.8$ kOe). Further, we find that the effect of such antisite disorder induces a spin-glass state in ${\mathrm{LaSrCoFeO}}_{6}$. Multiple signatures of slow dynamics were confirmed by frequency-dependent peak shift, slow spin relaxation, and memory effect over a wide temperature regime ($5\ensuremath{-}80$ K). The AC susceptibility data near the spin-glass temperature ($\ensuremath{\sim}72.1\ifmmode\pm\else\textpm\fi{}0.6$ K) are best fit by a critical slowing down model described by a dynamical exponent $\mathrm{z}\ensuremath{\nu}=7.5\ifmmode\pm\else\textpm\fi{}0.5$ and ${\ensuremath{\tau}}_{0}=1.05\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ s. The origin of exchange bias and spin glass are briefly discussed.

R. C. Sahoo

Papers

Exchange bias and spin glass states driven by antisite disorder in the double perovskite compound LaSrCoFeO 6