Magnetic transitions in disordered GdAl2

The role of disorder in magnetic ordering transitions is investigated using mechanically milled GdAl2. Crystalline GdAl2 is a ferromagnet while amorphous GdAl2 is a spin glass. Nanostructured GdAl2 shows a paramagnetic-to-ferromagnetic transition and glassy behavior, with the temperature and magnitude of each transition dependent on the degree and type of disorder. Disorder is parametrized by a Gaussian distribution of Curie temperatures T-C with mean (T) over bar (C) and breadth DeltaT(C). A nonzero coercivity is observed at temperatures more than 20 K above the highest T-C of any known Gd-Al phase; however, the coercivity decreases with decreasing temperature over the same temperature range where the GdAl2 grains ferromagnetically order. Models for the anomalous coercivity behavior are proposed and evaluated for their ability to explain the origin of the low-temperature glassy magnetization peak.