Magnetic structures at the ferromagnetic NiFe and antiferromagnetic NiMn interface in exchange-biased films: Role of noncollinear magnetism and roughness

The magnetic structures at the compensated ferromagnetic/antiferromagnetic (FM/AFM) interface of exchange bias FM NiFe/AFM NiMn films were determined with first-principles full-potential linearized augmented plane-wave calculations including noncollinear magnetism. The results predict that the magnetic moments of the FM NiFe layer lie perpendicular to those of the AFM NiMn layer. The intra-atomic noncollinear magnetism that arises near the interface is found to play an important role in stabilizing the perpendicular coupling that leads to a large biquadratic exchange energy (BEE). The BEE is large enough to require formation of a magnetic domain wall (with an estimated thickness ∼ 370 Å) in the AFM NiMn layers, which may account for the observed large coercivity and exchange bias. We also discuss magnetic structures at a rough FM/AFM interface-as simulated in model calculations with the inclusion of line step defects-which may contribute to a unidirectional magnetic anisotropy.