Magnetocaloric effect and Griffiths phase analysis in a nanocrystalline Ho₂NiMnO₆ and Ho₂CoMnO₆ double perovskite

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.