Low-temperature magnetic hysteresis in Mn12 acetate single crystals

– Precise magnetic-hysteresis measurements of small single crystals of Mn12 acetate of spin 10 have been conducted down to 0.4 K using a high-sensitivity Hall magnetometer. At higher temperature (> 1.6 K) step-like changes in magnetization are observed at regularly spaced magnetic-field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a higher-order uniaxial magnetic anisotropy (fourth order in Sz), first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory. Introduction. – The high-spin (S = 10) molecular magnets Mn12 acetate and Fe8 have become prototypes for the study of the transition from classical superparamagnetism to quantum tunnelling of mesoscopic spins. Much of the recent interest in these materials has been stimulated by the observation of a remarkably regular series of steps and plateaus in the magnetic-hysteresis loops of Mn12 at low temperature (below a blocking temperature of 3 K), first in oriented powders [1] and shortly thereafter in single crystals [2]. These results indicate that the relaxation rate of the magnetization toward equilibrium is greatly enhanced at welldefined intervals of magnetic field. These observations have been interpreted within a simple effective spin Hamiltonian for these molecules and a model of thermally assisted tunnelling of the magnetization, suggested in ref. [3]. This model describes a regime intermediate between thermal activation over the anisotropy barrier (superparamagnetism) and pure quantum tunnelling (T = 0) in which both thermal activation and quantum tunnelling are important to the magnetization reversal. (∗) E-mail: [email protected]