No clear frequency dependence of the magnetization was observed for 2 without a dc field because of the fast QTM (the quantum tunnelling of the magnetization) process. However, 2 shows the explicit frequency dependency of the magnetization with applied dc field was applied (Figure ). Furthermore, as the dc field was gradually increased, the maximum " shifted to a lower frequency, and an additional peak was observed above 4000 Oe dc fields. This second peak was not observed for 20.1 (Figure ). Yamashita et al. reported a similar phenomenon as a phononbottleneck. In-phase and out-of-phase data for 20.1 were fitted to extract the relaxation time  using the extended Debye model (details are shown in the ESI). The relaxation time was fitted by considering the direct, Raman, and Orbach processes using the equation