Saturation Magnetization and Perpendicular Anisotropy of Nickel Films

Publisher Website
Google Scholar
Abstract
Polycrystalline nickel films between 26 and 296 Å in thickness were obtained by evaporation at normal incidence from tungsten filaments onto microscope‐slide glass substrates maintained at 75°C, at pressures between 6×10−10 and 12×10−10 Torr. In the same vacuum the substrates with their films were arranged to be parts of an automated torsion magnetometer in which the sample rotated about an axis in its own plane perpendicular to the magnetic field. Measurements were made of the torque as a function of the magnetic field required to maintain the plane of the sample at various fixed angles θ0 ranging from 3° to 84° with respect to the magnetic field, at sample temperatures of about 300°, 77°, and 4.2°K. The torque data were analyzed by a new method which permits detailed investigation of models for the magnetization. It is shown that for a single‐domain model with an out‐of‐plane uniaxial anisotropy, the saturation magnetization MS and the total anisotropy field S can be calculated from the magnitude and slope of the torque/field curve at any field strength H′ and any angle θ0. The single‐domain model was verified for four samples with thicknesses of 87, 109, 182, and 296 Å. For these samples, both MS and S were independent of H′ and θ0. This shows that for films thicker than 87 Å, MS is independent of thickness within the experimental error of ±2%, and has the temperature dependence of bulk material, decreasing by 6.2±1.0% between 4.2° and 300°K. The anisotropy field was completely accounted for by the shape demagnetizing field, and by magnetostriction and strain in nickel film. The torque data from the thinnest film, which was 26‐ Å thick and electrically discontinuous, did not fit the single‐domain model. The characteristic torque behavior of such very thin films probably indicates that they contain islands small enough to be superparamagnetic, This conclusion was supported by a theoretical analysis which showed that a film model consisting of identical, noninteracting, superparamagnetic particles does indeed give rise to torque curves like those of the 26‐ Å film. If such a superparamagnetic film is erroneously assumed to be a single domain, its MS values appear to be less than the bulk values. Therefore, a detailed analysis of torque curve shapes is required to obtain reliable values for MS.