Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

摘要

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy (Keb) as well as an accompanied uniaxial magnetic anisotropy (Ku). We observed an additional fourfold magnetic anisotropy (K-4) induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one-and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane K-4 along < 100 >. The ferromagnetic resonance measurements indicate that the specific strength of K-4 for EB along [100] is larger than that for EB along [110]. The induced K-4 can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane < 100 > axes. The different dependence of K-4 on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

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