Nuclear Magnetic Relaxation Time near Compensation Temperature in Ferrimagnetic Insulator

The nuclear magnetic relaxation time $T_1$ in ferrimagnetic insulators is calculated by a Raman process of hyperfine interaction with a meanfield approximation. It is found that the 1/$T_1$ on one site rapidly increases near the compensation temperature $T_0$, whereas that on another site does not increase up to Curie temperature $T_c$. This is due to that the band width of soft magnon becomes comparable to $T_0$. The increasing behavior of 1/$T_1$ below $T_c$ is found also in another type ferrimagnet, which shows hump structure in the temperature dependence of magnetization instead of compensation. Also in this case, we find the rapid increase of 1/$T_1$ below $T_c$, even though the magnetization does not show the compensation. Such a coexistence of soft and hard magnons will lead to remarkable properties of ferrimagnet.