Positive exchange-bias induced by interface domain wall quenching in GdFe/TbFe films

Exchange-bias in a soft/hard ferrimagnetic (sFi/hFi) GdFe/TbFe bilayer with antiferromagnetic interface coupling has been studied as a function of the magnitude H-cf and angle psi(cf) of the cooling field. A continuous transition from negative exchange bias to positive exchange bias is observed with increasing H-cf. The transition is shown to arise from a progressive rotation of the direction of the interfacial pinning acting on the sFi magnetization in the plane of the film. This is explained by the presence of a partial interface domain wall (iDW) quenched in the hFi layer, the thickness and angular span of which depend on H-cf. The presence of the frozen iDW at the end of cooling is clearly evidenced thanks in particular to the strong effect a change of its handedness produces on the psi(cf) dependence of the exchange-bias field H-E. Overall, H-E is shown to be uniquely determined by the orientation of the hFi magnetization at the interface, whereas the coercivity of the sFi layer proves to be dependent also on the detailed micromagnetic structure deeper in the hFi layer.