  EPR spectra have been recorded in very high field, up to 25T, and at high
frequency, up to 525 GHz, on a polycristalline sample of Mn12ac (see paper for
detailed formula), the first example of molecular cluster behaving like a
nanomagnet. The simulation of the spectra has provided an accurate
determination of the parameters of the spin hamiltonian (see paper for formula
and values of the various parameters). The presence of the fourth order term in
the total spin justifies the irregularities in the spacing of the jumps,
recently observed in the hysteresis loop of Mn12ac and attributed to
acceleration of the relaxation of the magnetization due to Quantum Tunneling
between degenerate M states of the ground S=10 multiplet of the cluster. The
term in (S_+^4 + S_-^4) is responsible of the transverse magnetic anisotropy
and plays a crucial role in the mechanism of Quantum Tunneling. The HF-EPR
spectra have for the first time evidenced its presence and quantified it.
