Phase separation and nanoparticle formation in Cr-dosed FePt thin films

Cr-dosed FePt films with total thicknesses of 1.8 and 3.6 nm were grown in an ultrahigh vacuum at room temperature and studied by means of the surface magneto-optic Kerr effect, scanning electron microscopy, and x-ray magnetic circular dichroism. After annealing at 550 °C, the L1₀ and A1 phases are spatially separated. In the L1₀ phase, round nanoparticles form that are 15±4.8 nm in diameter and possess out-of-plane magnetic easy axes. An addition of ≪0.01 nm Cr enhances the coercivity (∼0.9 T), which then decreases with additional Cr, and a spin reorientation transition occurs near 0.3 nm Cr dosage as the L1₀ islands disappear. Segregation of Cr during annealing is believed to be responsible for the formation of round FePt islands and the separation of the L1₀ islands and planar A1 phase.