55Mn NMR in Mn12 acetate:: Hyperfine interaction and magnetic relaxation of the cluster -: art. no. 224425

The Mn-55 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 Mn-55 NMR composed of fivefold quadrupole-split lines for I=5/2 nuclei have been interpreted to arise from a Mn4+ ion and two crystallographically inequivalent Mn3+ ions. The isotropic hyperfine field in the Mn4+ ion indicates a large amount of reduction (26%) as compared with the theoretical evaluation. In the analysis of the hyperfine field in Mn3+ ions, the anisotropic dipolar contribution has been taken into account in addition to the Fermi-contact term. Adopting the reduction factor 8% of the dipolar term, which is estimated from the magnetic moment determined by a polarized-neutron-diffraction experiment, we obtained a reduction of 15% for the Fermi-contact term. We suppose that such an appreciable amount of the reduction factor is due to the covalence and strong exchange interaction among manganese ions via the oxygen ions. By using the hyperfine coupling constants of 12 manganese ions in the Mn-12 cluster, 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 0.32 kG seen by the cluster spin. The relaxation of the cluster magnetization after reversal of the external field was investigated by observing the recovery of the Mn-55 spin-echo intensity in the fields of 0.20-1.90 T along the c axis at 2.0 K. The magnetization of the cluster exhibited the square-root t recovery in the short-time regime. With increasing external field, the relaxation time decreased following significant dips at every 0.45 T, which is due to the effects of phonon-assisted quantum tunneling between the spin states at magnetic level crossings.