BERRYS PHASE FOR LARGE SPINS IN EXTERNAL FIELDS

It is shown that even for large spins J the fundamental difference bet ween integer and half-integer spins persists. In a quasiclassical desc ription this difference enters via Berry's connection. This general ph enomenon is derived and illustrated for large spins confined to a plan e by crystal electric fields. Physical realizations ate rare-earth nic kel borocarbides. Magnetic moments for half-integer spin (Dy3+,J = 15/ 2) and magnetic susceptibilities for integer spin (Ho3+,J = 8) are cal culated. Experiments are proposed to furnish evidence for the predicte d fundamental difference.