Approach to magnetic saturation in sputtered amorphous films: Effects of structural defects, microscopic anisotropy, and surface roughness

We have studied the approach to magnetic saturation in sputtered amorphous films by superconducting quantum interference device (SQUID) magnetometry. We observe a 1/H law for the deviation ΔM from complete saturation. The effect is an order of magnitude smaller than earlier results on liquid-quenched ribbons. We show that the effect cannot be due to surface roughness or macroscopic anisotropy. We attribute the behavior to magnetoelastic anisotropy arising from linear defects of a quasidislocation or cylindrical-volume-defect type, running perpendicular to the film plane. We deduce a defect density order 10¹⁵ m^-2 and effective Burgers vector of order 3 {} in an as-grown Fe-Ni-B film. We also show how this method can be used to determine the microscopic spatially uncorrelated anisotropy, and we set an upper limit of order 10⁶ J/m² in the Fe-Ni-B film.