Tunneling Rate for Superparamagnetic Particles by the Instanton Method
  We derive the tunneling rate for paramagnetic molecules in the context of a collective spin model. By means of path integral methods an analytical expression is derived. Given the very large spins in question (s ~ 3000 hbar), the observation of magnetization changes due to pure unitary tunnel effects is unlikely. 
E.H. Martins Ferreira and M.C. Nemes