Two series of epitaxial CoPt and FePt films, with nominal thicknesses of 42 or 50 nm, were prepared by sputtering onto single-crystal MgO͑001͒ substrates in order to investigate the chemical ordering and the resultant magnetic properties as a function of alloy composition. In the first series, the film composition was kept constant, while the substrate temperature was increased from 144 to 704°C. In the second series the substrate temperature was kept constant at 704°C for CoPt and 620°C for FePt, while the alloy stoichiometry was varied in the nominal range of 40-60-at. % Co͑Fe͒. Film compositions and thicknesses were measured via Rutherford backscattering spectrometry. The lattice and long-range order parameter for the L1 0 phase were obtained for both sets of films using x-ray diffraction. The room-temperature magnetocrystalline anisotropy constants were determined for a subset of the films using torque magnetometry. The order parameter was found to increase with increasing temperature, with ordering occurring more readily in FePt when compared with CoPt. A perpendicular anisotropy developed in CoPt for substrate temperatures above 534°C and in FePt above 321°C. The structure and width of the magnetic domains in CoPt and FePt, as seen by magnetic force microscopy, also demonstrated an increase in magnetic anisotropy with increasing temperature. For the films deposited at the highest temperatures ͑704°C for CoPt and 620°C for FePt͒, the order parameter reached a maximum near the equiatomic composition, whereas the magnetocrystalline anisotropy increased as the concentration of Co or Fe was increased from below to slightly above the equiatomic composition. It is concluded that nonstoichiometric L1 0 CoPt and FePt, with a slight excess of Co or Fe, are preferable for applications requiring the highest anisotropies.

Using differential scanning calorimetry, the enthalpy (activation energy) for the A1 to L10 transformation in FePt and CoPt thin films have been measured as −10.2±2.1 kJ/g -atom (1.7±0.1 eV) and −3.1±0.2 kJ/g-atom (2.8±0.2 eV), respectively. FePt is further seen to transform at temperatures that are approximately 120 °C lower than those for CoPt. The enthalpy of grain growth that accompanies the transformation is shown to be negligibly small by comparison to the transformation enthalpy in both films. The impact of our findings on alloy development for ultrahigh density magnetic storage media is discussed.

Differential scanning calorimetry, in conjunction with x-ray and electron diffraction, is used to investigate the A1 to L10 ordering transformation in binary FePt films with compositions in the range of 47.5–54.4at.% Fe. The kinetic ordering temperature, taken as the calorimetric peak temperature at a heating rate of 40°C∕min, decreases from 447to357°C in this composition range. In contrast with the kinetic ordering temperature, the Curie temperature of the L10 ordered phase increases from 384to455°C as the Fe content is increased. The activation energies of ordering lie between 1.4 and 2.0eV, and the transformation enthalpies are in the range of −8.2to−13.6kJ∕g-at. The Avrami exponent for the transformation is lower than expected and lies in the range of 1.1–1.8. The lattice parameter of the A1 phase and the c∕a ratio of the L10 phase decrease with increasing Fe content.

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