Single-crystalline model spin valves using single-crystallineNiO(111)substrates

The preparation of fully epitaxial spin valves elaborated on NiO(111) single crystals, with permalloy (Ni 8 0 Fe 2 0 ), Co, Co 3 0 Fe 7 0 , and Co/Co 3 0 Fe 7 0 as pinned layers, is reported. The best results were obtained with permalloy being the pinned layer. A giant magneto-resistance (GMR) of 3.5% at room temperature was obtained easily. The detrimental effect of an inter-diffusion at the ferromagnet/NiO interface is shown. The role of the initial surface preparation in terms of roughness and the magnetic properties of the pinned layer are discussed. It appeared that a 0.8-nm-thick CoFe layer is an excellent and efficient wetting layer for Co. These model structures allowed a decoupled investigation of the different parameters although the nearly isotropic GMR response of these structures makes them also interesting devices. The microstructure and the exchange coupling were investigated together. At room temperature, the major effect of the exchange coupling on single crystalline NiO(111) substrates is an inversely proportional behavior of the coercive field of the pinned layer with respect to the ferromagnetic layer thickness. The effect is discussed with respect to recent models and understood as resulting from the energy losses in the antiferromagnet because of irreversible losses in the antiferromagnetic order.