2002/10/16 by Takao Goto, T. Goto, Takeshi Koshiba +5 · 3 citations
Biochemistry, Genetics and Molecular Biology · Materials Science · Physics and Astronomy · #Electron Spin Resonance Studies #Lanthanide and Transition Metal Complexes #Magnetism in coordination complexes #cond-mat
paper · pdf · doi:10.1103/physrevb.67.104408
12 pages, 11 fugures, revtex4
arxiv created 2002/10/16 · openalex publication_date 2003/03/17 · arxiv updated 2009/11/30 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
The nuclear magnetic relaxation times T2 and T1 of 55Mn 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 T2^\ensuremath-1 and T1^\ensuremath-1 exhibit remarkable decreases with decreasing temperature in zero field with a relative relation similar to T2^\ensuremath-1/T1^\ensuremath-1\ensuremath≈200. At lower temperatures, T2^\ensuremath-1 tends to become constant with the value of about 102s^\ensuremath-1, while T1^\ensuremath-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 \ensuremathτ0 and \ensuremathτ1. By applying nonlinear theory for such a fluctuating local field, a general expression for T2 was obtained. It turns out that the experimental results for T2^\ensuremath-1 are explained in terms of the equation of T2^\ensuremath-1=\ensuremathτ0^\ensuremath-1, which corresponds to the strong collision regime under the condition \ensuremathτ0\ensuremath≫\ensuremathτ1. On the other hand, the results for T1 are well understood, on the basis of the standard perturbation method, by an equation for the high-frequency limit similar to T1^\ensuremath-1\ensuremath∼1/\ensuremathτ0\ensuremathωN2, where \ensuremathωN is the 55Mn Larmor frequency. The experimental results for the field dependence of T2^\ensuremath-1 and T1^\ensuremath-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.