Critical thickness effect in the exchange-coupled NiFe/CrMnPt_x bilayer system

We have investigated the thickness dependence of the exchange biasing field in the polycrystalline Ni₈₁Fe₁₉/CrMnPt_x (x=0, 3, 6, and 9) bilayer system. The critical thickness at which the onset of the exchange bias is observed increases with the increasing Pt content of the CrMnPt_x layer. A planar domain wall model in the antiferromagnet (AF) is used to explain this effect, and a relationship among the exchange field, critical thickness, and uniaxial anisotropy of the AF layer is obtained. The angular dependence of the exchange field has also been studied. For samples with a thick enough AF layer, an even symmetry and odd rotational symmetry are found. For samples with the AF layer thickness in the critical thickness region, the angular-dependent exchange field shows asymmetric behavior. It is proposed that the phenomenon is associated with a spin-flop state occurring at the bilayer interface.