The giant magnetocaloric effect of optimally prepared Gd_(5)Si_(2)Ge_(2)

摘要

The appropriate heat treatment of the Gd_(5)Si_(2)Ge_(2) alloy prepared from high-purity Gd results in a considerable enhancement of its magnetocaloric effect. The maximum magnetic entropy change increases by ～80 (from ～20 to 36 J/kg K) and the adiabatic temperature change increases by ～55 (from ～11 to 17 K) for a magnetic field change from 0 to 50 kOe when compared to the as arc-melted material. The magnetic ordering temperature is slightly reduced from ～277 K in the as arc-melted material to ～272 K in the fully homogenized and annealed Gd_(5)Si_(2)Ge_(2). The behavior of the isothermal magnetization as a function of magnetic field changes to some extent, while the heat capacity anomaly at the phase transition temperature is considerably sharper. The 1570 K heat treatment results in the partial ordering and redistribution of the Si and Ge atoms between their corresponding crystallographic sites in the monoclinic Gd_(5)Si_(2)Ge_(2)-type structure and to the removal of the polymorphic orthorhombic Gd_(5)Si_(2)Ge_(2) phase with the Gd_(5)Si_(4)-type structure, which coexists with the monoclinic phase in the as arc-melted alloy.