Studies of transverse and longitudinal relaxations of ${}^{55}$Mn in molecular cluster magnet Mn${}_{12}$Ac

Abstract

The transverse and longitudinal relaxation rates 1/$T_2$ and 1/$T_1$ of ${}^{55}$Mn in molecular cluster magnet Mn${}_{12}$Ac have been measured al low temperatures down to 200mK and in the fields upto 9T. Both of 1/$T_2$ and 1/$T_1$ exhibit remarkable decreases with decreasing temperature and with increasing field, with the relative relation $T_1/T_2\approx 200$. In the analysis, we adopt a simple model that the thermal fluctuation of the cluster spin $S$=10 associated with the spin-phonon interactionis, is only due to the excitation to the first excited state from the ground state with the average life-times ${\tau }_1$ and ${\tau }_0$ (${\tau }_0$$\gg$${\tau }_1$). We show that 1/$T_2$ is interpreted in terms of the strong collision regime as given by 1/${\tau }_0$, and that 1/$T_1$ is understood by the high-frequency limit based on standard perturbation treatment for the step-wise fluctuating field, thus being proportional to 1/${\tau }_0{\omega }_N^2$.

12 pages, 11 fugures, revtex4