Tunneling of the Mn12-acetate spin cluster
  The term in spin Hamiltonian, which determines the quantum tunneling rate in Mn12, is measured with neutron scattering. It is much smaller than previously reported values. This may imply that other mechanisms such as hyperfine fields may be more important in driving the tunneling. 
Wei bao, R. A. Robinson, J. R. Friedman, H. Casalta, E. Rumberger and   D. N. Hendrickson