Magnetic field-induced metamagnetism and magnetocaloric effect in double perovskites Re2CoMnO6 (Re=Sm, Dy)

Double perovskites Re2CoMnO6 (Re=Sm, Dy) have been synthesized by a solid-state reaction method. Magnetic field-induced metamagnetic behavior is observed in the samples below the ferromagnetic Curie temperature (TC), which are respectively 123 K and 88 K in Sm2CoMnO6 and Dy2CoMnO6. The origin of this effect is attributed to the competition between the nearest-neighbor ferromagnetic super-exchange interaction (Co2+-Mn4+) and the next nearest-neighbor antiferromagnetic exchange interaction (Co2+-Co2+, Mn4+-Mn4+). Arrott plots combined with the thermal hysteresis confirm that the transition from ferromagnetic to antiferromagnetic state with negative slope is the first-order magnetic phase transition. Moreover, the positive slope closely to TC indicates that the transition from paramagnetic to ferromagnetic state is the second-order transition. Magnetocaloric effect associated with Maxwell's relation is studied and the maximal value of |ΔSM| is 1.4 and 3.5 J kg−1 K−1 at 6 T near TC for Sm2CoMnO6 and Dy2CoMnO6. The large value of |ΔSM| for Dy2CoMnO6 is obtained which is 9.1 J kg−1 K−1 at 20 K. The relatively smaller magneto entropy change for Sm2CoMnO6 may be attributed to the higher coercive field and the weaker magnetism.