Dynamic coupling between two paramagnetic spin species induced by resonant phonons

Abstract

Results of paramagnetic relaxation studies conducted at ≈11 GHz and liquid helium temperature, involving lanthanum magnesium nitrate doped with Ce 3+ and Pr 3+ in spatially separated regions, are presented. The Ce transition alone was excited by fast passage, and the Pr paramagnetic resonance signal was observed. The spectral separation of the center frequencies of the two transitions, and the time during which the separation was maintained, could be independently adjusted. In this way both the temporal and spectral properties of the phenomenon could be studied. The Pr transition was found to exhibit in time a delayed onset of rapid heating followed by a gradual return to thermal equilibrium. The Pr line shape exhibited heating in those portions of the line localized in frequency nearest to the center of the Ce line. The spectral and temporal behavior of the Pr signal were both sensitive to the degree of Ce excitation as well as the relative spectral separation between the two lines. The observed effects are attributed to heating of Pr spin packets induced by hot photons created in the course of the paramagnetic relaxation of the Ce transition. Two simple analytical descriptions of the Ce-Pr interaction are presented.