Transverse and longitudinal relaxations of Mn-55 in the molecular cluster magnet Mn12O12(CH3COO)(16)(H2O)(4)

Article

关键词: HIGH-SPIN MOLECULES;    MN12O12 ACETATE;    MN-12 ACETATE;    GROUND-STATE;    PROTON NMR;    MU-SR;    MAGNETIZATION;    SUSCEPTIBILITY;    MN-12-ACETATE;    NANOMAGNET;   

DOI  :  10.1103/PhysRevB.67.104408

来源: SCIE

【 摘 要 】

The nuclear magnetic relaxation times T-2 and T-1 of Mn-55 in the molecular cluster magnet Mn12O12(CH3COO)(16)(H2O)(4) 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 relaxation rates T-2(-1) and T-1(-1) exhibit remarkable decreases with decreasing temperature in zero field with a relative relation similar to T-2(-1)/T-1(-1) approximate to 200. At lower temperatures, T-2(-1) tends to become constant with the value of about 10(2) s(-1), while T-1(-1) still exhibits an appreciable decrease down to around 0.5 K. The analysis for the experimental results was made on basis of the concept that the fluctuating local field responsible for the nuclear magnetic relaxation is caused by thermal fluctuations of the Zeeman levels of the cluster spin of S=10 due to the spin-phonon interactions. Then the problem was simplified by considering only the thermal excitation from the ground state to the first excited state, that is, a step-wise fluctuation with respective average life times tau(0) and tau(1). By applying nonlinear theory for such a fluctuating local field, a general expression for T-2 was obtained. It turns out that the experimental results for T-2(-1) are explained in terms of the equation of T-2(-1) = tau(0)(-1), which corresponds to the strong collision regime under the condition tau(0) >> tau(1). On the other hand, the results for T-1 are well understood, on the basis of the standard perturbation method, by an equation for the high-frequency limit similar to T(1)(-1)similar to1/tau(0)omega(N)(2), where omega(N) is the Mn-55 Larmor frequency. The experimental results for the field dependence of T-2(-1) and T-1(-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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