Exchange bias flop in FexZn1-xF2/Co bilayers -: art. no. 094426

We have measured the angular dependence of the exchange bias field (H-E) in polycrystalline Co thin films deposited on antiferromagnetic (AF) FexZn1-xF2 (110) films. Single crystals and twinned AF samples were studied, the latter possessing small grains (similar to7.0 nm) with their in-plane [001] easy axes perpendicular to each other. The twinned samples were field cooled through the AF Neel temperature with the field at an angle alpha with respect to the twins' perpendicular bisector in the plane of the sample. The most negative H-E occurred at an angle phi=0 for 0less than or equal toalphaless than or equal to30degreessimilar to40degrees. An exchange bias flop occurred if alpha was increased further, where phi abruptly shifted by 90degrees. This means that two equivalent exchange bias field axes exist in this system. A 1.0-nm pure FeF2 layer deposited between the FexZn1-xF2 and Co layers resulted in a sharper exchange bias flop transition, indicating that the pure interface layer acts as a buffer for the interface interaction. In untwinned FeF2 samples, a large H-E was observed with the sample field cooled along the FeF2 easy axis, whereas two loops with the same exchange bias magnitude but of opposite sign were observed when the cooling field was applied 90degrees to the AF easy axis. Changing the cooling field direction to 91degrees caused the sample to acquire a significant positive H-E parallel to the AF easy axis. These experiments demonstrate two important notions: (1) that the interface exchange coupling responsible for H-E is extremely sensitive to the underlying magnetic anisotropy of the AF; and (2) that the direction of the cooling field does not necessarily determine the direction of H-E.