Electron paramagnetic resonance in exchange-coupled systems with unlike spins

Abstract

Electron paramagnetic resonance in exchange-coupled systems with unlike spins (i.e, different $g$ factors) is studied in the strong-isotropic-exchange limit. Arguments are advanced which show that in the long-wavelength low-frequency regime the dynamic magnetic susceptibility has a hydrodynamic component which can be identified with the dynamic susceptibility associated with the total spin. In an applied field the dynamic transverse susceptibility for the total spin develops an exchange-narrowed resonant peak. An expression for the effective $g$ factor characterizing the resonance is obtained and evaluated. An equation for the linewidth is derived and estimates are given for the width in the infinite-temperature limit. Comparisons are made with previously published work on this problem, and possible experimental tests of the theory are suggested. The implications of this work for the analysis of the paramagnetic resonance of localized moments in nonmagnetic metals are discussed.