Field annealing of the uniaxial anisotropy of thin permalloy films

Field annealing of the uniaxial anisotropy was investigated for 76-percent Ni-24-percent Fe films in the temperature range between 300 and 500°C. The films were evaporated and annealed in ultrahigh vacuum. The discussion and evaluation of the results are based on a reordering model with constant numbers of anisotropy sources for a given constant annealing temperature. The model was confirmed by the measured dependence of the anisotropy on the direction of the annealing field. During annealing treatment below 400°C only parts of the anisotropy sources could be reoriented. Above 450°C the easy axis could be completely turned over to the direction of the applied field. Analysis of the anisotropy isotherms revealed three different contributions to the anisotropy. The first, with the smallest relaxation times, contributed only a few percent to the whole anisotropy. The second part gave a contribution of about 15 percent, while the main contribution comes from a process which is assumed to be iron pair reorientation. The relaxation times for this process were close to the data of Ferguson [8] for iron-pair ordering in bulk material. They were greater by a factor of ten than those reported by Kneer and Zinn [3].