Magnon-induced pseudoquadrupole interaction in ordered systems

Temperature-dependent aspects of the Suhl-Nakamura process, contributing to the measured quadrupole interaction in ordered systems, are described. The mechanism involves a pseudoquadrupolar coupling of magnons with nuclei via the magnetic hyperfine or transfered hyperfine $I_z{S}_z$ interaction. Within the framework of the long-wave magnon approximation, the process is linear in the magnetization and explains recent experimental results for ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ in Gd${\mathrm{Al}}_2$. A simple method of utilizing the new mechanism for the sign determination of the electric field gradient $q$ is described, and the method is applied to determine $q<\mathrm{}0\mathrm{}$ at the Al sites in Gd${\mathrm{Al}}_2$. Various implications of the new process are briefly discussed.