Mechanically induced structural and magnetic changes in the ${\mathrm{GdAl}}_2$ Laves phase

Changes of the structure and magnetic behavior of the ${\mathrm{GdAl}}_2$ Laves phase by high-energy ball milling were followed by x-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements. The starting compound crystallizes in the cubic ${\mathrm{MgCu}}_2$ structure and is a ferromagnet at lower temperatures with a Curie temperature of about 170 K. Upon mechanical milling, the ordered ${\mathrm{GdAl}}_2$ compound is found to become atomically disordered nanocrystalline material. Atomic disorder in ${\mathrm{GdAl}}_2$ is evidenced by the disappearance of a number of x-ray diffraction peaks, the decrease of lattice parameter, the increase of lattice strain, and the drastic changes of high-field magnetization curve at 4.2 K and of the nature of magnetic ordering upon milling. On the basis of experimental observations, a special type of atomic disorder namely quadruple-defect disorder is suggested. A constant average-crystallite size of about 21 nm is found after relatively short periods of milling. The material remains in the same crystalline structure as the starting compound even after prolonged periods of milling. While the Curie temperature of ${\mathrm{GdAl}}_2$ is slightly decreased by milling, another magnetic phase is detected with a magnetic ordering temperature of about 45 K during the intermediate stage of milling. This magnetic phase has been proved to be a spin-glass phase [Zhou and Bakker, Phys. Rev. Lett. 73, 344 (1994)]. With increasing milling time, both the fraction of the spin-glass phase and its freezing temperature ${\mathit{T}}_{\mathit{f}}$ increase, while the amount of ferromagnetic phase decreases. After long-period milling the original ferromagnetic phase disappears and the material completely becomes the spin-glass phase with a final ${\mathit{T}}_{\mathit{f}}$ of 65 K. Therefore, by mechanical milling well-defined nanocrystalline spin glass is created. The appearance of this spin-glass phase is due to accumulation of quadruple-defect disordering in the lattice of the ${\mathrm{GdAl}}_2$ Laves phase. The importance of using magnetic measurements as a structural probe is emphasized.