Specific heat and magnetic relaxation of the quantum nanomagnetMn12Ac

Abstract

We present specific-heat and ac-susceptibility measurements providing more evidence that magnetic quantum tunneling occurs in ${\mathrm{Mn}}_{12}$Ac 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.