Microstructural Changes and Hysteresis Losses in Fe-Doped Gd Si Ge

Magnetocaloric Gd 5 Si 2 Ge 2 -based alloys exhibit a well-documented giant magnetocaloric effect at around room temperature. However, as candidate materials for real-world refrigeration devices, they suffer from a number of problems, one of which is large hysteresis losses while cycling in and out of the magnetic field. It has, however, been reported that a small amount of iron reduces these hysteresis losses very significantly, so increasing the total potential cooling capacity of the material. We have investigated the magnetic properties as well as the macro- and microstructural changes for a wide range of iron substitutions for silicon, according to the formula Gd 5 Si 2-x Fe x Ge 2 , where x was varied between 0 and 1. The samples were prepared with an arc-melter and homogenized in an inert atmosphere, after which they were analyzed with a nano-resolution field-emission-gun scanning electron microscope (FEG SEM) and an X-ray diffractometer. The magnetic measurements were made with a cryogenic vibrating-sample magnetometer. The substitution of silicon for iron presented us with some unusual macrostructures and a consistent change in the microstructures of the samples. While most of the iron forms part of the grain-boundary phases, it also results in the formation of a new Gd 5 (SiGe) 3 main phase. We also observed that the addition of iron lowered the Curie temperature of the alloy, but only had a minimal effect on reducing the hysteresis losses, suggesting that this previously observed reduction in the hysteresis losses only occurs with very small substitutions of iron.