The anisotropic magnetoresistance (AMR) model was improved in order to determinate the magnitude and direction of the rotational anisotropy in NiFe/FeMn bilayer structures having different thicknesses of the ferromagnetic and antiferromagnetic layers. There are in our model two important parameters, namely the ratios ? = Hrot/Hex and ß=?rot/?a, where Hrot, and are the effective magnetic field and the angle of the rotational anisotropy, and Hex and ?a are the exchange bias field and the angle of applied field. These parameters describe the magnitude and direction of the rotational anisotropy. As the thickness of FeMn layer decreased up to 3.5 nm, the value increased from about 0.15 to 0.55. This proves that the regions of a weak exchange coupling extended as the antiferromagnetic layer thickness decreases. The ? and ß almost did not change as the NiFe layer thickness increased from 7 nm up to 15 nm, even the Hex was then reduced from 170 Oe to 70 Oe.
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