Effect of atomic ordering on magnetization decay and activation volume of FePt nanoparticles

The magnetic viscosity of two kinds of FePt nanoparticles with different order parameters (0.66 and 0.99) is investigated. Although the sample with the lower order parameter showed greater magnetization decay than that with higher order parameter, the decay was much smaller than that of a Co-Cr longitudinal thin-film medium with areal density capability of 40 Gb/si. The two FePt samples had almost the same value of activation volume (~5 * 10/sup -19/ cm/sup 3/) at around their coercivities in spite of the differences in the order parameter and the physical volume of the nanoparticles; their activation volume was equal to or smaller than their physical volume.