Losses in ferrites: single-crystal studies

A simple experiment on ferromagnetic resonance absorption in single crystals of ferrites is described, and the method of deducing physically significant parameters of the ferrite from the results of such experiments is derived. Results at 24,000 Mc/s, and the parameters deduced from them in this way, are given for (NiO)0.75(FeO)0.25Fe2O3 and for (NiO)0.95(FeO)0.05Fe2O3. The results indicate that the presence of divalent iron gives rise to an important loss mechanism. Another experiment is described in which the motion of an individual ferromagnetic domain boundary is observed in a single crystal, and results of such an experiment on (NiO)0.75(FeO)0.25 Fe2O3 at various temperatures are given. They indicate that the damping of domain-wall motion in this material increases by a large factor (100 or more) as the temperature decreases from 300 to 77° K. An elementary thermodynamical calculation is given which describes both types of experimental results in terms of a torque relaxation. Elastic-constant and conductivity measurements which yield relevant results are also discussed briefly.