Effect of the hydrostatic pressure on the susceptibility frequency spectrum of polycrystalline Mn-Zn and Ni-Zn ferrites

This paper is concerned with the study of the effect of hydrostatic pressure up to 1500 bar on the frequency spectra of polycristalline spinel ferrites. In the audio-frequencies range, this study, at different temperatures, had led to find a general hyperbolic law for the variation of the initial susceptibility versus the hydrostatic pressure where α is a coefficient independent of the temperature. According to this law the curves , for different temperatures, can be reduced to a single one . The relaxation frequency fcr, at constant temperature, increases linearly versus hydrostatic pressure, the product being constant. The two phenomena, the decrease of the initial susceptibility and the increase of the relaxation frequency have the same origin and are interpretated as a change in the topography of the Bloch walls in the grains, due to random stresses induced by pressure, the stresses having for origin the compressibility anisotropies of the material, particularly at the grain boundaries where the defects are the most important.