Magnetotransport Properties of CrO2.

When conduction electrons are strongly coupled to core spins, as in double-exchange systems, they acquire a quantum (Berry) phase while moving through a crystal and are affected by a gauge field, with the result that they produce an anomalous Hall effect. We measured the transport properties of CrO2 as a pure double-exchange ferromagnet, and show that the experimental results for the anomalous Hall effect of CrO2 are consistent with this Berry phase mechanism caused by topological spin defects. We also discuss the possibility of magnetoresistance resulting from such defects.