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 (φ) 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(φ) is well fitted by considering cos3φ/sin3φ 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.