Ferromagnetic Resonance in Two Nickel-Iron Ferrites

Ferromagnetic resonance data are presented as a function of temperature from 4.2°K to 380°K in (Ni${\mathrm{O})}_{0.95}$(Fe${\mathrm{O})}_{0.05}$ ${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ and in (Ni${\mathrm{O})}_{0.75}$(Fe${\mathrm{O})}_{0.25}$ ${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$. When the effects of eddy currents are corrected for, most of the energy dissipation in (Ni${\mathrm{O})}_{0.75}$(Fe${\mathrm{O})}_{0.25}$ ${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ as measured by the resonance line width is shown to be due to a relaxation mechanism associated with the presence of divalent iron. The line width shows a maximum at 160°K at our frequency of measurement (24 000 Mc/sec). A thermodynamical analysis of the loss mechanism is presented which assumes that a torque associated with part of the free energy relaxes. This correlates the line width quite satisfactorily with the temperature dependence observed in the magneto-crystalline anisotropy and the spectroscopic splitting ($g$) factor.