Determination of anisotropy field in polycrystalline lithium ferrites from FMR linewidths

Abstract

A technique for separating anisotropy and porosity induced FMR line-broadening contributions in narrow linewidth polycrystals has been extended to large linewidth lithium ferrite materials. The FMR linewidth is measured versus frequency and temperature T. Since the porosity contribution is proportional to the magnetization 4πMs, and the anisotropy field Ha falls off much faster at high T than does 4πMs, the two contributions to the linebroadening can be separated. Three samples were studied: Li-ferrite with and without cobalt addition (0.01 Co atoms/formula unit), and a Li-ferrite with 0.01 Co and 0.26 Ti at./f.u. The anisotropy field for unsubstituted Li-ferrite was found to decrease monotonically with T with Ha≊ 350 Oe at 25 °C, in agreement with single crystal data. Cobalt addition was found to decrease the room temperature Ha in the Li-ferrite without titanium but not in the Li-Ti-ferrite. This agrees with previous observations of Ti decreasing the effectiveness of Co in reducing the anisotropy.