Thermally assisted macroscopic quantum tunneling of a ferromagnetic particle in a magnetic field at an arbitrary angle

At finite temperature we study the quantum tunneling of magnetization for a small ferromagnetic particle with biaxial symmetry placed in a magnetic field dt an arbitrary angle. We present a numerical WKB exponent below the crossover temperature in which the quantum tunneling is affected by thermal activation and the approximate form of the WKB exponent around the crossover region. The effects of quantum fluctuations on the thermal activation rate beyond the crossover regime are discussed. [S0163-1829(98)08017-5].