Microwave absorption along minor hysteresis loops of single-domain particles with uniaxial magnetic anisotropy

The high frequency absorption of a random assembly of non-interacting single domain ferromagnetic particles with positive uniaxial magnetic anisotropy is calculated numerically for values of the signal frequency below that of the natural ferromagnetic resonance frequency. When the applied magnetic field is cycled along the primary hysteresis loop of the magnetization an additional absorption peak considerably larger than the usual ferromagnetic resonance absorption appears at values of the magnetic field strength corresponding to the second and the fourth quadrant of the loop. The results of detailed calculations of the absorption along some minor hysteresis loops are presented and compared to absorption measurements at microwave frequencies on polycrystalline barium ferrite with high perfection of the crystal grains prepared by chemical coprecipitation. Theory and experiment are in good agreement from which it follows that coherent rotation is the dominant magnetization process in this material.