Specific heat and magnetic relaxation of the quantum nanomagnet Mn12Ac

We present specific-heat and ac-susceptibility measurements providing more evidence that magnetic quantum tunneling occurs in Mn12Ac spin clusters. The total anisotropy energy barrier determined by fitting an Arrhenius law to the magnetic relaxation time is underestimated when compared to the specific-heat data. The expected magnetic contribution using a quadratic spin Hamiltonian needs higher-order terms to improve the agreement with the experiments, as recently done for electron-paramagnetic-resonance spectra. In addition, another interesting effect was observed in the specific-heat measurements when a dc magnetic field is applied. The extra magnetic contribution due to Zeeman splitting is suppressed below the;expected superparamagnetic blocking temperature in the time scale of the measurements.