For novel harddisks (»perpendicular recording«) the strong magnetic axis has to be normal to the layer, i.e. only one variant of the L10 superstructure is allowed. This was achieved experimentally by ion irradiation of FePd layers. Computer simulations based on experimental data led us to a deeper understanding of the selection mechanism of the variant: During the deposition of the FePd layer a small disturbance of the isotropy of the chemical disorder forms (the directional short range order DSRO has been measured) which causes during the L10 ordering a selection of the axes of strong anisotropy normal to the surface (see the lower right figure).
The He+ irradiation is less efficient for FePt nanoparticles because the vacancies escape faster to the surface (see in the lower figure a sequence of snapshot of the formation of the L10 superstructure; arbitrary Monte-Carlo time unit). Mostly single-variant-particles form after long lasting irradiation due to symmetry breaking by fluctuations. It should be noted that Pd covers the particle surface.
|