First principles calculations of FePt, CoPt, Co3Pt, and Fe3Pt alloys

First principles calculations based upon density functional theory have been used to investigate the magnetic properties of various Fe-Pt and Co-Pt alloys. At the 50:50 composition, the technologically important L1₀ alloys CoPt and FePt show large magnetocrystalline anisotropies consistent with the natural layering of the crystal structure. Calculated values for the magnetocrystalline anisotropy and magnetizations are found to be in close agreement with measured values. Since the L1₀ phase forms over a range of compositions, the influence of composition on magnetic properties has also been examined. A simple expression, derived from the Ne´el model, relates the anisotropy to the composition, or degree of disorder in the structure, and is found to be of value for understanding anisotropy in imperfect structures. At greater Fe of Co compositions there are several interesting crystal structures including the metastable pmm₂ phase that is composed of alternating pure and mixed planes. Again, fairly large anisotropies are seen as a consequence of layering and symmetry. Growing Fe₃Pt pmm₂ films seems less promising than Co₃Pt pmm₂ films given the larger energy difference between the pmm₂ and cubic L1₂ phases.