Ground state tunneling and staircase hysteresis loop in molecular magnet Mn12 acetate

Quantum tunneling and staircase hysteresis loop of molecular magnet Mn12 acetate are studied by means of the Landau–Zener method. It is focused on fitting the experimental hysteresis loop with five steps observed by Chiorescu et al. [Phys. Rev. Lett. 85 (2000) 4807] at a sweeping rate of applied field c=1.1×10-2T/s at temperature T<0.6K. Occurrence of five steps at ground-state tunneling region is an important criteria for determining the off-diagonal part of the Hamiltonian of Mn12. No more than four steps can be obtained if only fixed values of second-order and fourth-order transverse anisotropic terms are taken into account. We introduce the fluctuation of the parameter of second-order transverse anisotropic term. A Gaussian distribution of parameter is introduced and five steps in the hysteresis loop of Mn12 can be theoretically generated. The effects due to a rapid fluctuation field, which may be nuclear spins, is also analyzed under Landau–Zener model.