Exchange Anisotropy of Polycrystalline Ferromagnetic/Antiferromagnetic Bilayers

Abstract

The role of magnetic anisotropy of the antiferromagnetic layer on the magnetization process of exchange coupled polycrystalline ferromagnetic/antiferromagnetic bilayers is discussed. In order to elucidate the magnetic torque response of Ni-Fe/Mn-Ir bilayers, the single spin ensemble model is newly introduced, taking into account the two-dimensionally random distribution of the magnetic anisotropy axes of the antiferromagnetic grains. The mechanism of the reversible inducement of the exchange anisotropy along desirable directions by field cooling procedure is successfully explained with the new model. Unidirectional anisotropy constant, J_K, of polycrystalline Ni-Fe/Mn-Ir and Co-Fe/Mn-Ir bilayers is investigated as functions of the chemical composition of both the ferromagnetic layer and the antiferromagnetic layer. The effects of microstructure and surface modification of the antiferromagnetic layer on J_K are also discussed. As a notable result, an extra large value of J_K, which exceeds 0.5 erg/㎠, is obtained for Co_(70)Fe_(30)/Mn_(75)Ir_(25) bilayer with the ultra-thin (50 Å~100 Å) Mn-Ir layer. The exchange anisotropy of Co_(70)Fe_(30) 40Å/Mn_(75)Ir_(25) 100 Å bilayer is stable for thermal annealing up to 400℃, which is sufficiently high for the application of spin valve magnetoresistive devices.