Magnetic Hysteresis of CoPt Films

Abstract

Co-Pt bulk alloys have been of interest due to their high coercivity and anisot-ropy.^(1,2) Several uses of these alloys have been suggested in magnetic recording, such as longitudinal recording⁽³⁾ and magnetooptical recording.⁽⁴⁾ Of particular interest are the alloys between 25 and 60 at.%. Within this composition range the alloys undergo an ordering phase transition.^(5,6) Direct evidence of this ordering was first reported by Gebhardt and Koster⁽⁷⁾ who showed that the face centered cubic (fcc) structure of equiatomic alloys transforms into a face centered tetragonal (fct) at temperatures below 835°C. The fct phase is highly anisotropic with K ≃ 4 × 10⁷erg/cc.⁽⁸⁾ The coercivity of these films is small for the disordered fcc Co₅₀-Pt₅₀ alloys, but increases with annealing because of the transformation to fct and then decreases in overaged state because of coarsening of the microstructure. Higher Pt content alloys ~ 75%, transform from disordered fcc to ordered fcc. Although there are several reports on the magnetic properties of Co-Pt thin films, most of the work has been on films with high Co concentrations. Yanagisawa et al⁽³⁾ reported a peak in coercivity as high as 1700 Oe for a 350Å thick film with 20% Pt. The coercivity decreased on either side of this composition. Kitada and Shimizu⁽⁹⁾ related this anomaly in coercivity to the microstructure of the films. It was assumed that the increase in coercivity was due to both grain growth and the tendency for the c-axis of α-Co (hep structure) to collapse into the film surface. Aboaf and co-workers^(10,11) reported the results of annealing in Co₅₀-Pt₅₀ films. The coercivity was reported to increase from 90 Oe for the as-grown films to “very high” values after annealing at 700°C for one hour. They also related this increase in coercivity to the solid state transformation from fcc to ordered fct.