In the late 1990s Pecharksy and Gschneidner reported a magnetic entropy change in Gd 5 (Si 2 Ge 2) that was about 50% larger than that reported for Gd metal, which had the largest known magnetic entropy change at the time. This giant magnetocaloric effect (GMCE) occurred at 270 K, which is lower than that of Gd (294 K). Since the MCE occurs below room temperature these materials would not be suitable for refrigerators operating at room temperature. Since then other alloys have also demonstrated the giant magnetocaloric effect. These include Gd 5 (Si x Ge 1− x) 4, La(Fe x Si 1− x) 13 H x and MnFeP 1− x As x alloys. Gadolinium and its alloys undergo second-order phase transitions that have no magnetic or thermal hysteresis. However, the use of rare earth elements makes these materials very expensive.