Magnetic anisotropy in antiferromagnetic layers affecting exchange bias of Ni–Fe/Mn–Ir bilayers

Abstract

The magnetic anisotropy of antiferromagnetic layers (KAF) was estimated for Ni–Fe 50 Å/Mn–Ir dAF bilayers using Mauri’s method (dAFcr=JKs/KAF), where JKs is the saturation value of the unidirectional anisotropy constant (JK). The critical thickness of the antiferromagnetic layers (dAFcr), at which JK took half the value of JKs, was determined from the dependence of JK on dAF. The dAFcr was found to be almost constant (35±2 Å) independent of JKs. Thus, the relation of JKs∝KAF was derived, suggesting that the variation in JKs is due to a change in the value of KAF. JKs, however, was found to vary considerably for various Mn–Ir films possessing an almost identical Ir content, and thus probably the same value of KAF. In addition, studies by x-ray diffraction, transmission electron microscopy, and electron diffraction revealed that the change in JKs was independent of the microstructure and phase of the antiferromagnetic (AF) Mn–Ir films, both of which control KAF. Thus, JKs was found to be independent of KAF contradicting the relation, JKs∝KAF. This contradiction results from the assumption by Mauri that the coupling energy (J) is equal to JKs even in the polycrystalline exchange-coupled bilayers. A model that took account of the distribution of KAF axes of AF grains in the plane of the AF film successfully explained the behavior of JK. JKs was found to change independent of both J and KAF, and furthermore, it has been shown that the dependence of JKs on the sputtering conditions for Mn–Ir films is probably due to the effective temperature of the films during deposition.