Suppression of tunneling by interference in half-integer--spin particles
  Within a wide class of ferromagnetic and antiferromagnetic systems, quantum tunneling of magnetization direction is spin-parity dependent: it vanishes for magnetic particles with half-integer spin, but is allowed for integer spin. A coherent-state path integral calculation shows that this topological effect results from interference between tunneling paths. 
Daniel Loss, David P. DiVincenzo, and G. Grinstein (IBM)