Antiferromagnetic cubic anisotropy governed exchange bias in CoFeB/IrMn bilayers

Abstract

Dependence of exchange bias (EB) on field-measuring direction is commonly studied to establish the relationship between unidirectional and magnetocrystalline anisotropies in ferromagnet/antiferromagnet (AFM) systems, and phenomenologically, a cosine series expansion with/without higher-order odd terms may be used to fit the experimental data. The role higher-order AFM anisotropy plays on EB and the reason for the existence of higher-order odd terms in the expansion are still ambiguous. Herein, the influence of AFM cubic anisotropy on EB and coercivity was studied in amorphous-CoFeB/epitaxially grown IrMn bilayers, based on experimental measurements and Monte Carlo simulations. The results show that EB is still unidirectionally symmetric with respect to the field-measuring direction, and the symmetric axis is parallel to the field-cooling (FC) direction. Monte Carlo simulation results evidence large enough AFM anisotropy capable of trapping the AFM spins in one of the easy-axis directions closest to the FC direction. These findings, on the one hand, reveal an EB mode with orthogonal major and minor axes by modifying the energy wells and thus patterning the spin alignments of the AFM IrMn layer by means of AFM in-plane fourfold anisotropy in layered thin films, external cooling, and measuring magnetic fields and, on the other hand, are an important step toward a complete microscopic understanding of EB.