We thank Drs. B. Herr, M. Fuller, N. Petersen, and H. Soffel for constructive criticism and fruitful discussions. We are grateful to Dr. K.L. Verosub for making available his laboratory and cryogenic magnetometer for the u-channel measurements. Comments by Drs. G. Acton, C. Richter, and J.S. Stoner on the manuscript were appreciated. This research was supported by the Grants He 1814/8-2 and 8-3 from the Deutsche Forschungsgemeinschaft.