Mn NMR in Mn12 acetate: Hyperfine interaction and magnetic relaxation of cluster

The 55 Mn NMR in oriented powder crystal of Mn12Ac has been investigated at 1.4−2.0 K in zero field and with external fields along the c-axis. Three kinds of 55 Mn NMR, with central frequencies at 230.2±0.1, 279.4±0.1 and 364.±0.1 MHz, composed of five-fold quadrupole-split lines for I=5/2 nuclei have been interpreted to arise from Mn 4+ ion, and two crystallographically-inequivalent Mn 3+ ions, respectively. It is found that the isotropic hyperfine field in the Mn 4+ ion with 3d 3 configuration indicates a large amount of reduction (26%) as compared with the theoretical evaluation. In the analysis for the hyperfine field of Mn 3+ ions with 3d 4 configuration, we have taken into account of the anisotropic dipolar contribution in addition to the Fermi-contact term in order to explain two kinds of 55 Mn NMR frequencies in Mn 3+ ions in inequivalent sites. Using the reduction factor for the magnetic moment determined by polarized neutron diffraction experiment, we obtained the reduction factor of 8% from the calculated values for the dipolar term so as to fit the observed frequency. We suppose that such an appreciable amount of the reduction factor is due to covalence and strong exchange interaction among manganese ions via the oxygen ions. By using the hyperfine coupling constants of twelve manganese ions in Mn12Ac, the total hyperfine interaction of the ferrimagnetic ground state of S=10 has been determined to amount to 0.3 cm -1 in magnitude at most, the magnitude of which corresponds to the nuclear hyperfine field he=0.32 kG seen by Mn12 cluster spin. The relaxation of the cluster magnetization was investigated after reversal of the external field by observing the recovery of the 55 Mn spin-echo intensity in the fields of 0.20—1.90 T along the c-axis at 2.0 K. It was found that the magnetization of the cluster exhibits the √t-recovery in the short time regime. The relaxation time decreases with increasing external field following significant dips at every 0.45 T. This is interpreted to be due to the effects of thermally-assisted quantum tunneling between the spin states at magnetic level crossings.