Magnetocaloric effect and critical behaviors of R2NiMnO6 (R=Eu and Dy) double perovskite oxides

We have investigated the magnetocaloric effects and critical behaviors of Eu2NiMnO6 and Dy2NiMnO6 double perovskite oxides. For a magnetic field varying from 0 to 7 T, the maximum values of the magnetic entropy change (vertical bar Delta S-M(max)vertical bar) reach 4.0 J/kg K and 5.2 J/kg K, and the relative refrigeration capacity (RCP) values are 241.5 J/kg and 385.1 J/kg for Eu2NiMnO6 and Dy2NiMnO6, respectively. The relatively high values of vertical bar Delta S-M vertical bar and RCP suggest that these double perovskite oxides can be used as candidates of magnetic refrigerants working in a wide temperature range. Various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis are used to estimate the critical exponents (beta, gamma and delta) as well as T-C for Eu2NiMnO6. These critical exponents not only obey the Widom scaling relation delta = 1 + gamma/beta, but also fulfill the scaling equation M(H, epsilon) = epsilon(beta)f(+/-) (H/epsilon(beta+gamma)) where epsilon = (T - TC)/T-C, f(+) (T > T-C) and f-(T < T-C) are the regular analytic functions. Therefore, the deduced critical exponents are reasonably accurate. Moreover, temperature variation in effective critical exponents resemble with those for disordered ferromagnet, which matches with the Griffiths-like phase behavior observed from inverse magnetic susceptibility just above T-C. (C) 2018 Published by Elsevier B.V.