Quadratic transverse anisotropy term due to dislocations in Mn12-Ac directly obtained by EPR spectroscopy

High-Sensitivity Electron Paramagnetic Resonance experiments have been carried out in fresh and stressed Mn12-Acetate single crystals for frequencies ranging from 40 GHz up to 110 GHz. The high number of crystal dislocations formed in the stressing process introduces a E(S_x^2-S_y^2) transverse anisotropy term in the spin hamiltonian. From the behaviour of the resonant absorptions on the applied transverse magnetic field we have obtained an average value for E = 22 mK, corresponding to a concentration of dislocations per unit cell of c = 10^-3.