Noncollinear interlayer coupling and its effect on exchange bias in Co(32Å)/NiO(tNiOÅ)/[Co(4Å)/Pt(6Å)]4/Pt(50Å) multilayers

In the Co(32 Å)/NiO(tNiO Å)/[Co(4 Å)/Pt(6 Å)]4/Pt(50 Å) multilayers, the Co/Pt multilayer keeps its out-of-plane magnetic easy axis, while for the Co(32 Å) layer, its easy axis does no longer lie in-plane completely as observed in the Co(32 Å)/NiO(tNiO Å) bilayer, tilting a little out-of-plane due to the noncollinear interlayer coupling between the Co(32 Å) layer and the Co/Pt multilayer. After cooling down to 10 K from 300 K at the remanent state for tNiO=20Å, it has been observed that the in-plane exchange bias related to the 32 Å Co layer is changed from negative at low temperature to positive at high temperature, and the in-plane blocking temperature is enhanced to be 120 K much higher than 80 K observed in the Co(32 Å)/NiO(20 Å) bilayer. The transition of the in-plane exchange bias from negative to positive has been observed for the NiO spacer of at least 80 Å. This transition and the enhanced in-plane blocking temperature can be attributed to the noncollinear interlayer coupling between the 32 Å Co layer and the Co/Pt multilayer across the spin structure in the NiO spacer.