Stress dependence and effect of plastic deformation on magnetic hysteresis and anhysteretic magnetization of FeNi32% films

The magnetic hysteresis and anhysteretic magnetization of FeNi32% films were investigated as a function of isotropic stress. The magnetostriction contribution to dc magnetization under elastic stress and the effect of the plastic strain on the hysteresis loops are discussed. Also, a role of the plastic deformation interrelated with the elastic stress in the magnetization process is established. An experimental system based on a conventional vibrating sample magnetometer equipped with a specially designed loading fixture and optical resonant spectroscopy tension monitoring technique are used to measure anhysteretic permeability and magnetization curve as a function of stress. Measurements of magnetostriction as a function of magnetic field were shown to be also possible using this fixture. Stresses are deduced from the characteristic resonant frequency of the sample in the fixture and verified via pulse propagation velocity measurement. Both indirect stress measurements are contactless, relying on remote vibration measurement using a laser Doppler vibrometer. Uniaxial stresses up to 1 GPa can be applied for samples down to 50 mu m specimens. Anhysteretic permeability was extracted from the anhysteretic B-H curves constructed by degaussing the sample at the given longitudinal (parallel to the stresses) dc field. The large positive magnetostriction constant leads to higher susceptibility and lower coercivity with tensile stress while the large volume magnetostriction results in reduced saturation magnetization. Large stresses imposed on the sample result in plastic strain of the sample which induces increase in dislocation density and domain wall pinning. This causes the gain in hysteresis loss and coercivity to increase at the highest stresses.