Phase transformation and magnetic properties of bulk CoPt alloy

Abstract

The effect of nanoscale exchange coupling has been investigated in the CoPt alloy system. The experimental results of XRD and TEM indicate that in the early stage of the disorder–order transformation, the ordered fct regions with a mean size smaller than 8 nm are uniformly distributed and randomly oriented along the a-, b- and c-axes of the cubic structure of the original disordered fcc matrix phase. Both phases are completely coherent. The degree of atomic order in the ordered regions continuously increases, but this process is different for the various ordered regions. Also the degree of atomic order of the matrix phase increases, albeit at a low rate. There is a broad continuous distribution of the degree of order in the initial stage of the ordering transformation. An ordered region with higher degree of order has a higher magnetocrystalline anisotropy. The effect of nanocomposite exchange coupling between the regions with different degree of order is very clear. A remanence ratio of 0.78 is obtained after annealing the sample at 675 °C for 20 min. When the annealing time is longer than 60 min at 675 °C, the growth of ordered regions leads to a situation in which the disordered regions are swallowed up and the ordered regions meet each other to form antiphase boundaries. Cross-intersected polytwin bands of several hundreds nm length and a few nm width are formed along the {111} and {211} planes to relax the strain energy. It is suggested that the mechanism of magnetization reversal changes from coherent rotation to magnetic-domain wall movement. Since the widths of the polytwin bands are still within the length of exchange coupling, the strong remanence enhancement is preserved.