Magnetic transitions in disordered ${\mathrm{GdAl}}_2$

The role of disorder in magnetic ordering transitions is investigated using mechanically milled ${\mathrm{GdAl}}_2.$ Crystalline ${\mathrm{GdAl}}_2$ is a ferromagnet while amorphous ${\mathrm{GdAl}}_2$ is a spin glass. Nanostructured ${\mathrm{GdAl}}_2$ 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 $T_C$ and breadth $\Delta T_C.$ 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 ${\mathrm{GdAl}}_2$ 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.