  We study the model of a biaxial single ferromagnetic spin Hamiltonian with an
external magnetic field applied along the medium axis. The phase transition of
the escape rate is investigated. Two different but equivalent methods are
implemented. Firstly, we derive the semi-classical description of the model
which yields a potential and a coordinate dependent mass. Secondly, we employ
the method of spin-particle mapping which yields a similar potential to that of
semi-classical description but with a constant mass. The exact instanton
trajectory and its corresponding action, which have not been reported in any
literature is being derived. Also, the analytical expressions for the first-
and second-order crossover temperatures at the phase boundary are derived. We
show that the boundary between the first-and the second-order phase transitions
is greatly influenced by the magnetic field.
