Hysteresis of ultrasmall Fe-Pt particles

The magnetization reversal in very small FePt particles is investigated by analytical and numerical calculations. The modeling focuses on particles with diameters from 3 to 15 nm, as produced by a salt-matrix annealing technique. Experiment shows that the particles exhibit a certain degree of structural inhomogenity, which has a far-reaching effect on the magnetic hysteresis. In particles larger than about 10 nm, the magnetization-reversal mode is strongly inhomogeneous, and there are several scenarios that depend on the symmetry of inhomogenity. Small particles reverse nearly coherently, and the coercivity is essentially equal to the volume-averaged anisotropy. In this case, nonrectangular hysteresis loops reflect factors such as grain misalignment, particle-size distribution, and different degrees of L 10 order in different particles.