Electron microscopy studies of spin-valve materials

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.

Perspectives of giant magnetoresistance

“…The microstructure affects strongly not only the transport properties but also the magnetic characteristics. 236 The correlation between the microstructure and micromagnetics is a challenging issue, which requires further investigations. In particular, we need to study the influence of defects on the creation of domain walls in the process of switching and the magnetization ripple in nanoscale films.…”

Atomic-scale analysis of CoFe/Cu and CoFe/NiFe interfaces

“…5,6 The importance of being able to study each interface separately is shown by Laidler et al, who observed that GMR changes on annealing a Co/Cu multilayer with a Au cap resulted not from changes in the degree of interfacial interdiffusion in the multilayer stack, but from a change in the resistivity of the Au cap following mixing with the underlying Cu layer. 7 To date, one of the most commonly used techniques to study multilayer structures has been transmission electron microscopy, 8 which can be used to study both the microstructure and compositional profile ͑in cross section͒ and the magnetic-domain structure ͑in plan view͒. However, a limitation of transmission electron microscopy is that it is a projection technique ͑i.e., it produces a two-dimensional image of a three-dimensional specimen͒, and integration through the specimen thickness can make it difficult to distinguish interfacial mixing and roughness.…”

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

“…The experimental loops saturate at higher magnetic fields than the theoretical curves for the free NiFe layers, which can be attributed to the formation and propagation of domain walls in the free FM layers [2]. The t eff1 /t eff2 , obtained from the simulations (much higher than the nominal ratio of 1.8), decreases with the increase of t. This can be originated from the variation of the thickness of a magnetically dead layer at Cu/NiFe and/or at NiFe/FeMn interfaces.…”