Periodic Instanton Calculations of Quantum-Classical Transitions in Spin Systems

The quantum-classical transitions of the escape rates in the molecular magnet Mn_l2Ac and a biaxial anisotropic ferromagnetic spin model are investigated by applying the periodic instanton method. The effective free energies are expanded around the top of the potential barrier in analogy to the Landau theory of phase transitions. We show that the first-order transitions occur below the critical external magnetic field h_x = 1/4 for Mn_l2Ac and beyond the critical anisotropy constant ratio λ = 1/2 for the biaxial ferromagnetic grains, which is in good agreement with earlier studies.