Random magnet with competing anisotropies in Fe$_{x}$Ni$_{1-x}$F$_{2}$ alloys

A series of epitaxial (110) Fe$_{x}$Ni$_{1-x}$F$_{2}$ films were deposited on (110) MgF$_{2}$ via molecular beam epitaxy. The Fe concentration $x$ was determined by measuring the lattice parameter along the [110] direction using x-ray diffraction. The film thicknesses and the roughness of each interface were found by fitting of x-ray reflectivity data. The magnetic ordering as a function of x was analyzed by SQUID magnetometry. Enhancement of the N\'{e}el temperature in alloys as well as evidence of spontaneous magnetization along the c-axis after field-cooling were observed for samples with x$>$0.1; for samples with x$<$0.1 the magnetization was perpendicular to the c-axis. Two phase transitions were observed for alloy samples with x$>$0.1. The phase diagram of the upper transition was consistent with mean field theory of a system with competing anisotropies. The transition at lower temperatures was unaffected by the application of a magnetic field, whereas the upper temperature transition was broadened by the application of fields as small as 50 Oe. This suggests the presence of a spin-glass phase at lower temperatures, followed by melting of the spin glass prior to the main transition to paramagnetic behavior as the temperature is raised.