Nonlinear response of single-molecule nanomagnets: Equilibrium and dynamical.

We present an experimental study of the nonlinear susceptibility of Mn-12 single-molecule magnets. We investigate both their thermal-equilibrium and dynamical nonlinear responses. The equilibrium results show the sensitivity of the nonlinear susceptibility to the magnetic anisotropy, which is nearly absent in the linear response for axes distributed at random. The nonlinear dynamic response of Mn-12 was recently found to be very large, displaying peaks reversed with respect to classical superparamagnets [F. Luis , Phys. Rev. Lett. 92, 107201 (2004)]. Here we corroborate the proposed explanation-the strong field dependence of the relaxation rate due to the detuning of tunnel energy levels. This is done by studying the orientational dependence of the nonlinear susceptibility, which permits us to isolate the quantum detuning contribution. Besides, from the analysis of the longitudinal and transverse contributions we estimate a bound for the decoherence time due to the coupling to the phonon bath, which is much shorter than the energy-level lifetimes.