Magnetization reversal in single molecule magnets.

摘要

I have studied the magnetization reversal in single molecule magnets (SMMs). SMMs are Van der Waals crystals, consisting of identical molecules containing transition metal ions, with high spin and large uniaxial magnetic anisotropy. They can be considered as ensembles of identical, iso-oriented nanomagnets. At high temperature, these materials behave as superparamagnets and their magnetization reversal occurs by thermal activation. At low temperature they become blocked, and their magnetic relaxation occurs via thermally assisted tunneling or pure quantum tunneling through the anisotropy barrier.; We have conducted detailed experimental studies of the magnetization reversal in SMM material Mn₁₂-acetate (Mn₁₂) with S = 10. Low temperature measurements were conducted using micro-Hall effect magnetometry. We performed hysteresis and relaxation studies as a function of temperature, transverse field, and magnetization state of the sample. We identified magnetic sublevels that dominate the tunneling at a given field, temperature and magnetization. We observed a crossover between thermally assisted and pure quantum tunneling. The form of this crossover depends on the magnitude and direction of the applied field. This crossover is abrupt (first-order) and occurs in a narrow temperature interval (0.1 K), when an external field is parallel to the easy axis of the sample. We have shown that in this case there are competing maxima in the relaxation rate versus energy, and the global maximum shifts abruptly from one energy to the other as a function of temperature. Strong longitudinal and transverse fields make the crossover more gradual (second-order), which occurs in an interval of 1 K.; We have demonstrated that a thermally independent quantum regime exists below the temperature of approximately 0.6 K. The existence of this regime was also supported by the results of the magnetization relaxation experiments. These results also show non-exponential form of relaxation, previously observed by other groups.; In order to understand the crossover, we have performed numerical simulations of a model Hamiltonian. The results of these calculations and their comparison of our experimental data suggest the presence of additional tunneling mechanisms in Mn₁₂.