Electron microscopy studies of spin-valve materials

Since the discovery of the giant magnetoresistance effect and more recently its application in magnetic recording technology, the interest in spin-val ve (SV) structures used in devices such as magnetoresistive sensors and ran dom access memories, has increased greatly. As the size of these devices be comes 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 t o investigate the microstructure of these metallic layered films. The choic e of the materials used is crucial for the resulting magnetic properties. A fter a summary of the most common configurations found in the literature an d some comments on their advantages and disadvantages, we will present some HREM studies which have clarified the differences in magnetic properties b etween top and bottom SVs. The growth conditions, the use of a seed layer a nd the thermal behaviour of SVs annealed at different temperatures will be discussed. In addition, some magnetostatic effects have been explained by m icrostructural considerations. In addition to the HREM experiments, one of the techniques enabling microma gnetic studies at the micron scale to be carried out is Lorentz transmissio n electron microscopy (LTEM). This technique, which allows the magnetic dom ain structure of a magnetic material to be observed in situ, has been impro ved over the past few years. Very recently, the development of ill situ mag netizing experiments in LTEM has enabled us to apply simultaneously an exte rnal field as well as a current through an SV element during the observatio n of the magnetization reversal. As a result, both electronic properties, v ia the giant magnetoresistance (GMR) curve, and local magnetic properties, via observation of the domain structure, can be analysed and correlated. Fu rthermore, the use of a mapping technique which allows quantitative analysi s of the in-plane magnetization of the SV element, based on the analysis of Foucault images, has shown a clear correlation between the resistance valu es and the domain structure of the element. Such facilities have also resul ted in a better understanding of the behaviour of various SV elements under real operating conditions. In particular, the effect on the reversal mecha nism of the current density, the stray-field coupling at the edges of the e lement for different shapes of elements and the current direction through t he element have been carefully studied.