An investigation of the magnetic properties of spinvalves 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 360° walls, via this interaction, is looked at.
In chapter 4 spin-valve structures are studied for the first time in this thesis. The first samples discussed have co-linear easy axes and biasing directions. Magnetisation ripple is observed in both the free and biased layers of the spin-valves, though it appears to be substantially reduced. The reversal of spin-valves with a permalloy free layer differs markedly from that of an isolated permalloy layer with the most obvious difference being the large number of stable 360° walls involved. Unlike those in an isolated layer, these 360° walls are not necessarily associated with topographical contrast and often change direction several times along the length of the wall. The effect on the free layer reversal of the addition of thin GMR enhancing Co layers at the spacer layer interfaces is also studied.
In chapter 5 the free layer reversal mechanisms for spin-valves with orthogonal easy axis and biasing directions are considered. The main result from this chapter is the fact that coherent rotation of the free layer magnetisation in a spin-valve can be achieved by deliberately off-setting the applied field with the biasing direction by only a small angle. A perfect alignment however causes the rotation of adjacent regions of the free layer magnetisation in clockwise and anti-clockwise senses with the formation of 360° walls between the boundaries. High resolution profiles are made across these walls using DPC imaging.
In chapter 6 the pinning mechanism of the biased layer in a spin-valve is investigated by studying the response of the biased layer to external fields. In the case of FeMn the reversal of the biased layer proceeds by the nucleation of erratic domain structures which exist on a fine scale (typically a few microns). In the case of uniaxial TbCo the reversal is quite different and the phenomenon of magnetisation creep is prominent during the reversal process. The reversal is dominated by domain wall motion and due to there being relatively few nucleation sites the domain wall density is low. The walls which do exist are again erratic and typically change direction many times over a length scale of a few microns. Domain structures are observed in the biased layer of uniaxial TbCo spin-valves after the application of a vertical field. These structures are replicated in the free layer and double domain walls exist at the boundaries. DPC imaging is used to study the domain boundaries in fine detail and allows the in-plane component of magnetisation in the TbCo layer to be estimated. A model with two easy axes is postulated to account for domain structures in the remanent state.
Chapter 7 deals with the nucleation of a magnetic vortex which is stabilised in a buckled film by the presence of a vertical field. While the structure of the vortex is not fully understood the application of fields with an in-plane component reveals that it is confined to regions with exaggerated bend contours.
In chapter 8 the magnetic states of fluxguides, which are intended for thin film recording heads, are studied. These structures were deposited on both magnetic and non-magnetic substrates and the zero field states are studied as a function of the shape of the fluxguide. The study reveals what appears to be a transition from a multiple domain state to a single domain state, for fluxguides deposited on a ferrite substrate, at a fluxguide height of » 8m m. This may be responsible for an improvement in the noise characteristics which have been observed for prototype thin film heads with throatheights below » 8mm.