  We have investigated the effects of rotating an applied field on the exchange
anisotropy in Co/FeMn thin films. When the applied field is initially along the
cooling field direction, the longitudinal hysteresis loop has a maximum
coercivity and the transverse hysteresis loop is flat, indicating that the
exchange field is along the cooling field direction. When the applied field
angle is rotated away and then restored to the original field cooling
direction, the exchange anisotropy direction has changed. The rotation of the
exchange field direction trails the applied field and is hysteretic. The
rotational hysteresis of the exchange field direction is due to the weak
anisotropy in thin FeMn layers, and decreases with increasing FeMn thickness.
