Electron microscopy studies of spin-valve materials

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Abstract
Since the discovery of the giant magnetoresistance effect and more recently its application in magnetic recording technology, the interest in spin-valve (SV) structures used in devices such as magnetoresistive sensors and random access memories, has increased greatly. As the size of these devices becomes smaller and smaller, the need to investigate the local microstructure and the micromagnetic behaviour of SV materials becomes obvious. High-resolution electron microscopy (HREM) analyses have been carried out to investigate the microstructure of these metallic layered films. The choice of the materials used is crucial for the resulting magnetic properties. After a summary of the most common configurations found in the literature and some comments on their advantages and disadvantages, we will present some HREM studies which have clarified the differences in magnetic properties between top and bottom SVs. The growth conditions, the use of a seed layer and the thermal behaviour of SVs annealed at different temperatures will be discussed. In addition, some magnetostatic effects have been explained by microstructural considerations. In addition to the HREM experiments, one of the techniques enabling micromagnetic studies at the micron scale to be carried out is Lorentz transmission electron microscopy (LTEM). This technique, which allows the magnetic domain structure of a magnetic material to be observed in situ, has been improved over the past few years. Very recently, the development of in situ magnetizing experiments in LTEM has enabled us to apply simultaneously an external field as well as a current through an SV element during the observation of the magnetization reversal. As a result, both electronic properties, via the giant magnetoresistance (GMR) curve, and local magnetic properties, via observation of the domain structure, can be analysed and correlated. Furthermore, the use of a mapping technique which allows quantitative analysis of the in-plane magnetization of the SV element, based on the analysis of Foucault images, has shown a clear correlation between the resistance values and the domain structure of the element. Such facilities have also resulted in a better understanding of the behaviour of various SV elements under real operating conditions. In particular, the effect on the reversal mechanism of the current density, the stray-field coupling at the edges of the element for different shapes of elements and the current direction through the element have been carefully studied.