Composition Dependence of Irradiation-Induced Uniaxial Anisotropy Energy of Permalloy Films

Abstract

The composition dependence of irradiation-induced uniaxial magnetic anisotropy energy has been examined in Permalloy thin films over the range from 40% to 90% Ni. The uniaxial anisotropy was developed at 60°C by irradiating the films with 2-MeV 3He particles in the presence of an orienting magnetic field. The values of the uniaxial anisotropy constant Ku were found to range from a maximum of 1.9×104 erg/cm3 for 52 Ni-48 Fe to a minimum of 1.3×103 erg/cm3 at 90 Ni-10 Fe. In the range from 40% to 70% Ni the values obtained are significantly greater than those found for films evaporated or annealed in the presence of a magnetic field. Because of radiation-enhanced diffusion, directional short-range order can occur at temperatures considerably lower than those necessary for magnetic annealing, with a concommitant increase in the uniaxial anisotropy energy. These data were compared with the theoretical relationship, as derived by Néel and Taniguchi, and with a related semiempirical expression of Ferguson: K0 = A′ (Tc−Ta) n2 (1−n)2, where n is the fraction of Fe atoms, Ta is the annealing or deposition temperature, and Tc is the Curie temperature. Both expressions agree with the data fairly well for nickel compositions above 70%, but for compositions of nickel between 40% and 70% the expression developed by Ferguson does not account for all of the observed anisotropy.