Berry's phase for large spins in external fields

It is shown that even for large spins J the fundamental difference between integer and half-integer spins persists. In a quasiclassical description this difference enters via Berry's connection. This general phenomenon is derived and illustrated for large spins confined to a plane by crystal electric fields. Physical realizations ate rare-earth nickel borocarbides. Magnetic moments for half-integer spin (Dy3+,J = 15/2) and magnetic susceptibilities for integer spin (Ho3+,J = 8) are calculated. Experiments are proposed to furnish evidence for the predicted fundamental difference.