Magnetic structure and spin dynamics of the ground state of the molecular cluster Mn12O12 acetate studied by 55Mn NMR -: art. no. 104401

Mn-55 nuclear magnetic resonance (NMR) measurements have been carried out in an oriented powder sample of Mn12 acetate at low temperature (1.4-3 K) in order to investigate locally the static and dynamic magnetic properties of the molecule in its high-spin S=10 ground state. We report the observation of three 55Mn NMR lines under zero external magnetic field. From the resonance frequency and the width of the lines we derive the internal hyperfine field and the quadrupole coupling constant at each of the three nonequivalent Mn ion sites. From the field dependence of the spectrum we obtain a direct confirmation of the standard picture, in which spin moments of Mn4+ ions (S=3/2) of the inner tetrahedron are polarized antiparallel to that of Mn3+ ions (S=2) of the outer ring with no measurable canting from the easy axis up to an applied field of 6 T. It is found that the splitting of the Mn-55-NMR lines when a magnetic field is applied at low temperature allows one to monitor the off-equilibrium population of the molecules in the different low lying magnetic states. The measured nuclear spin-lattice relaxation time T, strongly depends on temperature and magnetic field. The behavior could be fitted well by considering the local-field fluctuations at the nuclear Mn-55 site due to the thermal reorientation of the total S=10 spin of the molecule. From the fit of the data one can derive the product of the spin-phonon coupling constant times the mean-square value of the fluctuating hyperfine field. The two constants could be estimated separately by making some assumptions. The comparison of the mean-square fluctuation from,relaxation with the static hyperfine field from the spectrum suggests that nonuniform terms (q not equal0) are important in describing the spin dynamics of the local Mn moments in the ground state.