Investigation of the Microstructure of Co/Cu/Co/NiMn Spin Valve Systems
NiMn is an interesting material for achieving a high exchange bias in spin valve systems. However, the antiferromagnetic ordering necessary for the exchange bias effect is found only after an annealing treatment. Since this treatment may lead to device degradation by interdiffusion, it is crucial to understand the details of the microstructural changes during the annealing in order to optimize the procedure. In this study, we investigated the microstructure of Co/Cu/Co/NiMn spin valve structures during the annealing treatment. The transformation to the ordered state is directly observed by the splitting of (022) and (220) rings in the electron diffraction pattern of NiMn, which accompanies the transformation of NiMn from disordered face centered cubic (FCC) to ordered face centered tetragonal (FCT). The antiferromagnetism of the FCT phase of NiMn is confirmed by the change in magnetic and magnetoresistance characteristics of the multilayer structure after annealing even without external magnetic field. The size and distribution of the ordered domains is investigated by dark field imaging. We find that the ordered FCT domains are smaller than the NiMn grains. High resolution TEM reveals a number of stacking faults in the structure of Co/Cu and NiMn layers. The influence of annealing on the roughness of different layers is analyzed by small angle X-ray reflectivity (SAXR). We find that both the surface roughness and the roughness of the internal interfaces increase during the annealing treatment, which may be explained by partial interdiffusion of the individual layers.
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Document Type: Research Article
Publication date: August 1, 2009
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