Fundamental Aspects of Exchange Bias Effect in (Ni/Co)/NixMn 100-x /(Ni/Co) Bilayers, Trilayers and Multilayers on Cu3Au(001).

Abstract

This study is concerned with three broad topics facing the technological application of exchange bias (EB). The antiferromagnetism of single-crystalline NixMn100−x ultrathin films in contact with ferromagnetic (FM) Co (Ni/Co) film(s) in bilayers, trilayers and multilayers on Cu3Au(001) deposited under ultrahigh vacuum conditions, is investigated by means of magneto-optical Kerr effect (MOKE). In the first study, the aim is to identify the contribution of interface and bulk antiferromagnetic (AFM) spins to the EB. Structural or chemical defects are deliberately introduced at the surface of the AFM layer or at a certain depth inside the AFM layer. The creation of defects in the bulk of the AFM layer enhances the magnitude of EB and its blocking temperature. It is also observed that the deeper the insertion of defects, the higher the value of the EB field and coercivity. These findings are discussed as the effect of additional pinning centers in the bulk of the AFM layer. In the second problem, we compare artificially layered [Ni/Mn] films with the corresponding disordered NixMn100−x alloys with almost the same Ni/Mn ratio and the same film thickness. It is revealed that the perpendicular interatomic lattice distance is decreased in the artificially layered [Ni/Mn] samples. These changes in the structure are discussed as the buckling or reconstruction of Mn atoms (probably Ni atoms too) in the bulk and also at the surface, which causes higher coercivity, EB-field, and stronger interlayer exchange coupling. In the third project, we unveil that a cover of Mn films that exhibit contracted vertical-to-in-plane lattice constant ratio and expanded structures at different thickness levels, induces perpendicular magnetic anisotropy (PMA) in an FM layer, confirming that the interlayer distance in the AFM can be a crucial parameter for establishing perpendicular magnetization.