Relaxation Effects in Para- and Ferromagnetic Resonance

Magnetic resonance experiments have been carried out at 3-cm wavelength in para-magnetic and ferromagnetic samples at very high microwave power levels, in a temperature range between 77°K and 700°K. Changes in the microwave susceptibility and the dc magnetization have been observed for microwave amplitudes between 1 and 50 oersted.

For a para-magnetic salt, MnS${\mathrm{O}}_4$·4${\mathrm{H}}_2$O, these changes are readily interpreted in terms of a spin-lattice relaxation mechanism. The value for the spin-lattice relaxation time is derived in three different ways and agrees well with that obtained by Gorter's nonresonant method.

When a large exchange interaction occurs between the spins, the situation above the Curie point can be described in terms of a conversion of magnetic into exchange energy. The magnetic and the spin-exchange systems are not always in thermal equilibrium. The characteristic time for the transfer of energy between these systems is equal to the inverse of the line width, which is given by the Van Vleck-Anderson formula for exchange narrowing. Experimental results for an organic free radical and some ferrites confirm this point of view.

Below the Curie temperature the situation is more complicated. The experimental data for several ferrites and supermalloy show qualitatively the same behavior.

The absorbed magnetic energy is again converted into exchange energy with a characteristic time which is always shorter than 3×${10}^{-8\mathrm{}}$ sec. At high temperatures this time is equal to the inverse line width and the transition to the para-magnetic region is continuous. At low temperatures the relaxation time increases roughly inversely proportional to the temperature although the width remains constant. The microwave susceptibility has an anomalous decrease at high power levels. No satisfactory explanation has been found for these effects in existing theories.