Thickness of the pinned layer as a controlling factor in domain wall formation during training in IrMn-based spin valves

Studies of CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 1.6 nm have demonstrated that a domain wall parallel to the surface develops in the pinned layer after training at the magnetoresistance (MR) maximum. To investigate the effects of domain wall formation on the MR, we have studied the depth profile of the vector magnetization in comparable spin valves, with pinned ferromagnetic (FM) layer thicknesses, from 1 to 15 nm , using polarized neutron reflectivity. At the maximum MR achieved after training, the antiparallel magnetization of the pinned layer, in a 2 nm sample, is reduced to 5% of its saturation value, suggesting the formation of domain walls perpendicular to the surface. In a 9 nm sample, the pinned layer magnetization is instead canted away from the field at the MR maximum. A transition from perpendicular to parallel domain wall formation occurs for pinned layer thicknesses greater than 4 nm , and the magnitude of the maximum MR subsequently depends on the type of domain wall that develops.