Magnetic anisotropy in antiferromagnetic layers affecting exchange bias ofNi-Fe/Mn-Ir bilayers

The magnetic anisotropy of antiferromagnetic layers (K-AF) was estimated fo r Ni-Fe 50 Angstrom/Mn-Ir d(AF) bilayers using Mauri's method (d(AF)(cr)=J( K)(s)/K-AF), where J(K)(s) is the saturation value of the unidirectional an isotropy constant (J(K)). The critical thickness of the antiferromagnetic l ayers (d(AF)(cr)), at which J(K) took half the value of J(K)(s), was determ ined from the dependence of J(K) on d(AF). The d(AF)(cr) was found to be al most constant (35 +/- 2 Angstrom) independent of J(K)(s). Thus, the relatio n of J(K)(s)proportional to K-AF was derived, suggesting that the variation in J(K)(s) is due to a change in the value of K-AF. J(K)(s), however, was found to vary considerably for various Mn-Ir films possessing an almost ide ntical Ir content, and thus probably the same value of K-AF. In addition, s tudies by x-ray diffraction, transmission electron microscopy, and electron diffraction revealed that the change in J(K)(s) was independent of the mic rostructure and phase of the antiferromagnetic (AF) Mn-Ir films, both of wh ich control K-AF. Thus, J(K)(s) was found to be independent of K-AF contrad icting the relation, J(K)(s)proportional to K-AF. This contradiction result s from the assumption by Mauri that the coupling energy (J) is equal to J(K )(s) even in the polycrystalline exchange-coupled bilayers. A model that to ok account of the distribution of K-AF axes of AF grains in the plane of th e AF film successfully explained the behavior of J(K). J(K)(s) was found to change independent of both J and K-AF, and furthermore, it has been shown that the dependence of J(K)(s) on the sputtering conditions for Mn-Ir films is probably due to the effective temperature of the films during depositio n. (C) 2000 American Institute of Physics. [S0021-8979(00)35908-4].