${}^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) measurements have been carried out on an oriented powder sample of Mn12 clusters in order to investigate microscopically the inner spin structure and spin dynamics of the molecule in its high-spin $S=10\mathrm{}$ ground state. The external magnetic-field dependence of ${}^{55}\mathrm{M}\mathrm{n}-\mathrm{N}\mathrm{M}\mathrm{R}$ spectrum up to 15 T gives a direct confirmation of the internal magnetic structure of the Mn12 cluster, in which spin moments of ${\mathrm{Mn}}^{4+}$ $\mathrm{}(S=3/2\mathrm{})\mathrm{}$ ions are polarized antiparallel to that of ${\mathrm{Mn}}^{3+}$ $\mathrm{}(S=2\mathrm{})\mathrm{}$ ions. It is proved that the microscopic spin configuration rotates rigidly when an external magnetic-field is applied perpendicular to the easy axis. When the magnetic-field is applied parallel to the easy axis of the Mn12 cluster, the nuclear-spin-lattice relaxation rate ${1/T}_1$ decreases as expected from the field dependence of the lifetime broadening of the low-lying sublevels due to spin-phonon interaction. On the other hand, for transverse fields ${1/T}_1$ increases with H displaying a broad maximum at $H=5\mathrm{}–6\mathrm{T}.$ This behavior can be accounted for only in part by spin-phonon interaction. The dominant contribution to ${1/T}_1$ for transverse field is shown to arise from quantum fluctuations of the component of the magnetization transverse to the nuclear Zeeman quantization axis.

• Received 2 October 2002

DOI:https://doi.org/10.1103/PhysRevB.67.064426