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The nuclear magnetic relaxation times T2 and T1 of 55 Mn in the molecular cluster magnet Mn12 Ac have been measured, using the spin-echo method for oriented powder sample, at low temperatures below 2.5 K down to 200 mK in the fields up to 9 T applied along the c-axis. Above about 1.5 K both of relaxation rates T −1 were also interpreted reasonably in terms of the above theoretical treatment. The quantitative comparison between the experimental results and the theoretical equations was made using hyperfine interaction tensors for each of three manganese ions determined from the analysis for the NMR spectra in zero field.

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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 at low temperatures. Both of 1/T 2 and 1/T 1 exhibit remarkable decreases with decreasing temperature and with increasing field, with the relative relation like T 1 /T 2 ≈200. We adopt a simple model that the thermal fluctuation of the cluster spin S = 10, which is associated with the spin-phonon interactions, is only due to the excitation to the first excited state from the ground state with the average lifetimes τ 1 and τ 0 (τ 0 τ 1 ). We show that 1/T 2 is interpreted in terms of the strong collision regime as given by 1/τ 0 , and that 1/T 1 is understood by the high-frequency regime based on standard perturbation treatment for the step-wise fluctuating field, thus being proportional to 1/τ 0 ω 2 N . §1. IntroductionRecently there has been a great interest in nano-scale molecular cluster magnets in view of mesoscopic behavior such as quantum tunneling of magnetization. 1) As a typical compound, Mn 12 Ac has been studied extensively so far. In each cluster of Mn 12 Ac, a high spin state of S=10 is established as the ground state because of antiferromagnetic couplings between four Mn +4 (s = 3/2) ions in a central tetrahedron and surrounding eight Mn +3 (s = 2) ions with two in-equivalent sites, both groupes being arranged ferromagnetically. The magnetic moments of each cluster are parallel (+) or anti-parallel (−) to the c-axis due to an effective Ising-like anisotropy of D/k B = 0.55 K caused by Jahn-Teller effect in Mn +3 ions. Most of magnetic properties have been satisfactorily understood by treating the cluster as a single giant spin of S=10 with the discrete energy levels |S, m with the quantization axis of the z-axis along the c-axis.In order to understand the magnetic properties of Mn 12 Ac from dynamical as well as statistical natures of each of manganese ions constituting the cluster spin, we are studying 55 Mn NMR and the nuclear magnetic relaxations. Recently we have reported on nuclear hyperfine interactions of 55 Mn in Mn 12 Ac obtained from the analysis of 55 Mn NMR results. 2) In the present work, we have measured, using oriented powder sample, the temperature and field dependence of the transverse relaxation time T 2 and the spin-lattice relaxation time T