Quantum Tunneling and Phase Transitions in Spin Systems with an Applied   Magnetic Field
  Transitions from classical to quantum behaviour in a spin system with two degenerate ground states separated by twin energy barriers which are asymmetric due to an applied magnetic field are investigated. It is shown that these transitions can be interpreted as first- or second-order phase transitions depending on the anisotropy and magnetic parameters defining the system in an effective Lagrangian description. 
S.-Y. Lee, H.J.W. Mueller-Kirsten, D.K. Park, F. Zimmerschied