Dynamic Effects in Paramagnetic Resonance of Magnetic Ions in Metals

Abstract

The transverse susceptibility, which determines the paramagnetic resonance behavior, is studied for paramagnetic ions coupled by an effective scalar exchange interaction to the conduction electrons of a host metal. By means of a temperature Green's-function technique for ionic spins, it is found that the dynamical behavior of the conduction electrons influences the ionic resonance if the relaxation rate of the conduction electrons is not large compared to their resonance frequency. The predicted effects, namely, a broadening at low temperature, a shift around the Curie temperature, and a diminished broadening at high temperature, are compared with experimental data. The last effect can also be seen in nuclear resonance, whereas, the other effects appear only in ionic resonance experiments, since only the ionic Zeeman energy is comparable with the Zeeman energy of the conduction electrons. The theory suggests a possibility for distinguishing between direct and indirect exchange coupling of ions in metallic solutions, since in the first case the resonance signals are exchange-narrowed, whereas, in the second case they are broadened by a dissipative contribution of the conduction electrons.