Recent progress in the magnetic and cryogenic magnetocaloric properties of RE2TMTM’O6 double perovskite oxides

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

Reported here is a comparison of the magnetic, magnetocaloric, and dielectric properties of 50% iron substituted GdCrO3 (GdFe0.5Cr0.5O3) bulk pellet and 960 nm thick film of GdFe0.5Cr0.5O3 (GFCO). The 960 nm film was synthesized on a platinized-silicon substrate by chemical solution deposition and spin-coating methods. The X-ray diffraction scans of the bulk sample and the film as well as the morphology of the film as examined by the field-emission scanning electron microscope indicate phase-pure and polycrystalline nature of these samples. X-ray photoelectron spectroscopy was used to determine the valence states of Gd, Fe, and Cr. The temperature dependence of the dielectric constant from 225 to 700 K shows peaks at TC = 525 K for the bulk and ∼450 K for the film due to ferroelectric to paraelectric transitions, since electric polarization vs electric field hysteresis loops are observed at room temperature. The dielectric studies in the bulk GFCO for T > TC indicate a relaxor-like behavior. The measurements of the magnetization (M) of the samples as a function of temperature (5–350 K) and magnetic field (H) up to 7 T (=70 kOe) depict hysteresis behavior at low temperatures due to the canted antiferromagnetic order of Fe3+/Cr3+ below the Neel temperature of ∼275 K. The M vs H isotherms at various temperatures are used to determine and compare the magnetic entropy change (−ΔS) and relative cooling power (RCP) of the two samples, yielding (−ΔS) = 30.7 J/kg K (18.8 J/kg K) and RCP = 566.5 J/kg (375 J/kg) for the bulk (960 nm film) samples of GFCO at 7 K and 7 T, respectively. The plot of RCP vs T shows that magnetic cooling for this system is most effective for T < 30 K. Comparatively smaller magnitudes of (−ΔS) and RCP for the film vis-a-vis the bulk sample of GFCO scale with its reduced magnetization. This suggests that further improvements in the quality of the films are needed to improve their magnetization and hence their magnetocaloric properties, possibly making them useful for on-chip cooling in miniaturized devices.

Comparison of the dielectric and magnetocaloric properties of bulk and film of GdFe0.5Cr0.5O3

Comparative study on the iso-structural compounds LaFe0.5Cr0.5O3 (LFCO) and YbFe0.5Cr0.5O3 (YFCO) are carried out to investigate the structural, optical and magnetic properties. Rietveld refinement and the derived structural parameters disclose the extent of distortion exhibited by the ions substituted in the orthorhombic lattice. Raman analysis describes the genuine differences in the phonon modes. Contradictory Raman shift observed in the oxygen breathing mode at ~679 cm−1 denotes the existence of the multi-valence state in LFCO than YFCO. EPR and XPS analysis validates the multi-valence state of Cr3+/4+/6 ions present in the LFCO compound. The optical transition between the Cr3+/Fe3+ ions is studied by diffuse reflectance spectroscopy, where the direct bandgap energy (Eg) is 2.05 and 2.08 eV, respectively. Magnetic measurement reveals complex magnetic Phenomena such as magnetisation reversal and spin reorientation transition, illustrating a contrast in the magnetic interaction present due to La3+(non-magnetic) and Yb3+ (magnetic) ion and with the interplay of Cr3+/Fe3+ ion between their sublattices.

Gurpreet Kaur

Papers

Magnetocaloric effect and spin-phonon correlations in RFe0.5Cr0.5O3 (R = Er and Yb) compounds