Magnetic entropy change in the Ge-rich alloys Gd-Si-Ge

We have studied the temperature and field dependence of the magnetocaloric effect in the Ge-rich alloys Gd₅(Si_xGe_1-x)₄, where x=0.0825, 0.15, and 0.1875. The low field susceptibility shows a two-step magnetic ordering at the Curie temperature T_C and the Neel temperature T_N. The T_C of the different compositions covers the temperature range 67 K-108 K while T_N remains constant at around 133 K. The magnetic entropy change ΔS_M(T) as a function of applied field up to 7 Tesla is characterized by a second-order magnetic phase transition around T_N and a first-order transition around T_C. The maximum magnetic entropy change around T_N is only about 3 J·kg^-1·K^-1, but it is much larger around T_C. It rapidly reaches a saturation plateau of about 40-50 J·kg^-1·K^-1, which widens with increasing field. The width of the plateau at an applied field of 7 T is about 20 K. These results indicate that the Ge-rich alloys are potentially good candidates for MR applications at low temperature.