RESUMO
Rare-earth double perovskite oxides have intriguing magnetocaloric properties at cryogenic temperatures. In this study, Ho2NiMnO6 and Ho2CoMnO6 were synthesized using the sol-gel method, which crystallized in a monoclinic structure in the P21/n space group. The magnetic phase transition was observed at 81.2 K for Ho2NiMnO6 and 73.5 K for Ho2CoMnO6. The presence of a paramagnetic matrix and short-range ferromagnetic clusters causes magnetic disorder in these double perovskites, resulting in Griffiths phase formation. The Arrott plot confirms that compounds undergo second-order phase transition. At an applied magnetic field of 5 T, the maximum magnetic entropy change (-ΔS) for the studied compounds is 1.7 and 2.2 J kg-1 K-1, respectively. The transition metals Ni and Co in a double perovskite cause lattice distortion in the structural parameters and oxidation states of manganese (Mn3+/Mn4+), which changes the magnetic and magnetocaloric properties. The quantitative approach provides a systematic study of magnetocaloric properties of the rare earth double perovskite compounds with ferromagnetic 3d transition elements.
RESUMO
The crystal structure, cryogenic magnetic properties, and magnetocaloric performance of double perovskite Eu2NiMnO6 (ENMO), Gd2NiMnO6 (GNMO), and Tb2NiMnO6 (TNMO) ceramic powder samples synthesized by solid-state method have been investigated. X-ray diffraction structural investigation reveal that all compounds crystallize in the monoclinic structure with a P21/n space group. A ferromagnetic to paramagnetic (FM-PM) second-order phase transition occurred in ENMO, GNMO, and TNMO at 143, 130, and 112 K, respectively. Maximum magnetic entropy changes and relative cooling power with a 5 T applied magnetic field are determined to be 3.2, 3.8, 3.5 J/kgK and 150, 182, 176 J/kg for the investigated samples, respectively. The change in structural, magnetic, and magnetocaloric effect attributed to the superexchange mechanism of Ni2+-O-Mn3+ and Ni2+-O-Mn4+. The various atomic sizes of Eu, Gd, and Tb affect the ratio of Mn4+/Mn3+, which is responsible for the considerable change in properties of double perovskite.
