The Berry phase of an anisotropic spin system that is adiabatically rotated
along a closed circuit C is investigated. It is shown that the Berry phase
consists of two contributions: (i) a geometric contribution which can be
interpreted as the flux through C of a non-quantized Dirac monopole, and (ii) a
topological contribution which can be interpreted as the flux through C of a
Dirac string carrying a non-quantized flux, i.e., a spin analogue of the
Aharonov-Bohm effect. Various experimental consequences of this novel effect
are discussed.Comment: 4 pages, 3 figures (RevTeX + eps); v2 (revised paper): 4 pages, 4
figure