Antiferromagnetic fourfold anisotropy induced exchange bias and magnetization reversal behaviors in CoFeB/IrMn bilayers

Bilayers of an amorphous CoFeB ferromagnet coupled to an epitaxially grown IrMn antiferromagnet, whose anisotropy is fourfold symmetric, are modeled to study the angular (f) dependence of magnetization reversal mechanism and exchange bias behavior, based on a modified Monte Carlo method. It is found that positive and negative exchange-bias field (H-E) maximum values appear in the antiferromagnetic easy-axis directions, and the H-E zero-value transition/return points exist in the antiferromagnetic hard-axis directions. Furthermore, the value of H-E(f) is well fitted by considering cos3f/sin3f terms with larger or comparable coefficients as compared to the first-order terms in the Fourier expansion. Finally, the microscopic spin configuration results evidence these phenomena originating from the tailoring of antiferromagnetic spins by simply rotating the field-cooling direction between antiferromagnetic easy- and hard-axis directions. We open an exchange-bias mode governed by antiferromagnetic fourfold anisotropy and propose a method to precisely detect antiferromagnetic higher-order anisotropies on an exchange-bias support.

Automated summary

Bilayers of amorphous CoFeB ferromagnet and epitaxially grown IrMn antiferromagnet were simulated to study the angular dependence of magnetization reversal mechanism and exchange bias behavior. It was found that positive and negative exchange-bias field values appear in the antiferromagnetic easy-axis directions, while the transition/return points of the field with zero exchange-bias exist in the antiferromagnetic hard-axis directions. Additionally, the value of the exchange-bias field was well fitted by considering cos3f/sin3f terms with comparable or larger coefficients compared to the first-order terms in the Fourier expansion.