Disorder-induced depression of the Curie temperature in mechanically milled GdAl₂

The effect of disorder on the ferromagnetic transition is investigated in mechanically milled GdAl₂. GdAl₂ is a ferromagnet when crystalline and a spin glass when amorphous. Mechanical milling progressively disorders the alloy, allowing observation of the change from ferromagnetic to a disordered magnetic state. X-ray diffraction and pair-distribution-function analysis are used to determine the grain size, lattice parameter, and mean-squared atomic displacements. The magnetization as a function of temperature is described by a Gaussian distribution of Curie temperatures. The mean Curie temperature decreases with decreasing lattice parameter, where lattice parameter serves as a measure of defect concentration. Two different rates of change with lattice parameter are observed: the first, slower rate occurs while grain size is changing, and the grain size is constant in the second regime. The breadth of the Curie temperature distribution is linear with lattice parameter. Milled and annealed GdAl₂ shows that the mean Curie temperature has the same general dependence on lattice parameter as milled, but unannealed GdAl₂, while the breadth of the Curie temperature distribution is different in annealed samples compared to the unannealed materials.