An Investigation of the Magnetic Properties of Spin-Valves Using Transmission Electron Microscopy

The work presented in this thesis is primarily a study of the reversal mechanisms of the magnetic layers within spin-valve materials. Spin-valve materials display the phenomenon of Giant Magnetoresistance (GMR) and are strong candidates for use in future generation magnetoresistive read heads where predicted areal bit densities are beyond that which is presently recoverable using existing Anisotropic Magnetoresistance technology. Spin-valves typically consist of two ferromagnetic layers separated by a spacer layer. One of the ferromagnetic layers is exchange coupled to a layer of FeMn, TbCo or NiO which effectively shifts the hysteresis loop by a few hundred Oersted. External fields with a magnitude less than this have little effect on the magnetisation of the exchange biased layer but do switch the other ferromagnetic layer. Thus the magnetisation in the two ferromagnetic layers can be switched, by the application of a small field (?10 Oe), from a parallel low resistance state, which exists at zero field, to an antiparallel high resistance state. The majority of the work presented in this thesis is concerned with the reversal mechanisms of continuous spin-valves which are imaged using the Lorentz mode of transmission electron microscopy. Domain structures within fluxguides, which are suitable for use in thin film recording heads, are also studied, but using Kerr microscopy. Since this is a secondary topic only chapter 8 is given over to these results. The first chapter of this thesis reviews the basic concepts of ferromagnetism, magnetoresistance and magnetic recording, which are all relevant to this work. Since the dominant method of imaging used in this thesis is transmission electron microscopy chapter 2 concentrates on both the image formation theory of electron microscopy and the techniques available to reveal magnetic contrast. The theory of Kerr microscopy is also briefly reviewed with respect to FeNbSiN fluxguides. As a prelude to investigating spin-valve structures thin permalloy films are studied in chapter 3 where typical hard and easy axis reversal mechanisms are observed upon applying an external field. Magnetisation ripple is visible in all films studied and by considering images acquired using the Differential Phase Contrast (DPC) mode of electron microscopy it is apparent that the ripple structure is dependent on the thickness of the film. The ripple is partially quantified as a function of film thickness by analysis of the Fourier Transforms of the DPC images. Interactions between domain walls and inclusions are studied in some detail and in particular the mechanism for the creation of 36