We report the far-infrared vibrational spectra of the high-spin molecular magnet $\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}\right({\mathrm{CH}}_3\mathrm{COO}{)}_{16}\left({\mathrm{H}}_2\mathrm{O}{)}_4\right]$$\cdot 2{\mathrm{CH}}_3\mathrm{COOH}\cdot 4{\mathrm{H}}_2\mathrm{O}$ as a function of temperature (4–40 K) and applied magnetic field (0–30 T). The new data confirm previously reported field-dependent line-shape changes of features at 284, 306, and $409{\mathrm{cm}}^{-1},$ as well as the field dependence of several new, smaller peaks between 315 and $375{\mathrm{cm}}^{-1}.$ Initially, these features grow linearly with applied field, but they tend toward saturation at 30 T, indicating a nonlinear coupling of these structures to the magnetic field. There is a strong similarity between variable-temperature and variable-field effects; the field-induced features are a subset of the temperature-induced structures. Based on a line-shape analysis of the $284{\mathrm{cm}}^{-1}$ structure as well as a comparison between temperature and field effects over the full far-infrared frequency regime, we ascribe the field-dependent absorptions to a coupling between intramolecular vibrations and the spin system. Some plausible mechanisms, which can account for this coupling as well as the possible contribution of the coupled vibrational modes to the fourth-order anisotropy Hamiltonian, are briefly discussed.

• Received 22 April 2002

DOI:https://doi.org/10.1103/PhysRevB.66.144430