Dependence of exchange coupling on NiO grain size in NiO/NiFe bilayers

Abstract

The texture and grain-size effects on the exchange bias in sputtered polycrystalline NiO/NiFe bilayers were studied. Two oriented antiferromagnetic NiO layers along (111) and (200) planes were fabricated on SiO2/Si(100) substrates by varying the Ar/O-2 ratio. An exchange anisotropy field H-ex was detected in both NiO/NiFe bilayers with a NiO(111) plane where Ni moments are in ferromagnetic (FM) order, and with a NiO(200) plane where Ni moments are in fully compensated antiferromagnetic (AF) order. In order to clarify the presence of the H-ex in a NiO(200)/NiFe bilayer, we prepared NiO(200) layers with different grain sizes by controlling a total pressure at a constant Ar/O-2 ratio in a sputter chamber. We observed that the H-ex of the bilayer films with small grains of NiO(200) is larger than the H-ex with large grains. This observation is consistent with a model that the exchange interaction is caused by the reorientation of the moments with AF layer spins rotating, rather than FM layer spins rotating at the interface of the bilayer. (C) 2003 American Institute of Physics.