Domain Processes of Four Magnetic Thin Film Systems
This thesis is mainly concerned with the domain processes observed during in-situ magnetising experiments carried out on four ferromagnetic thin film systems. Three of these systems have been developed with commercial applications in mind and the aim of studying them was to evaluate their suitability for these applications. The fourth system is a novel thin film system, which has been developed in order to test some fundamental suppositions of the theory of magnetism. Most of the experiments carried out on these samples were performed on the modified JEOL 2000FX and the modified Philips CM20 at the University of Glasgow. Use was also made of the JEOL 1200 at the University of Glasgow.
Chapter one deals with the deposition and growth of thin films and the theory of magnetism in thin films. The four thin film systems investigated in this thesis are also introduced in this chapter. Chapter two gives an overview of electron microscopes in general and the specific instruments used in this study. Image formation, in-situ magnetising and energy dispersive X-ray spectroscopy are also discussed here.
In chapter three, the results of experiments on novel thin films known as domain wall junction (DWJ) systems are presented and discussed. DWJs were suggested as model systems in which to study the propagation of domain walls in magnetic systems from 0K to their Curie temperature. The type of DWJ studied here is an ultra-thin amorphous TbFe film, providing a potential energy barrier to the propagation of a planar domain wall, sandwiched between two thin amorphous GdFe films. The domain processes of two DWJs of this type were studied, both at room temperature and at temperatures approaching 110K. The observations were compared to the model developed from magnetometer measurements. The results of the in-situ magnetising experiments, in part, support the interpretation of the magnetometer measurements and also provide information on the scale at which the domain wall propagates over the energy barrier.
Chapter four presents results from magnetoelastic Ag/FeCo multilayers for novel thin film sensors, which are compared with single layer FeCo thin films. Three different FeCo layer thicknesses were studied, while the Ag layer thickness was kept constant. The thickness of the Ag layers means that they form islands instead of continuous films, the critical thickness for forming a continuous layer being about 6nm (depending on deposition conditions). One film, with a FeCo layer thickness below this, showed a Ag concentration that fluctuated widely, suggesting that the Ag distribution in this film was random, while in the others it was uniform. The magnetic structure of this film appeared to be linked to this morphology in that the initial domains were small enough to correspond to this grain structure. Both magnetic measurements and in-situ magnetising experiments carried out on these films show that they did not have the desired well-defined uniaxial magnetic anisotropy desired for sensor applications.
Chapter five deals with the basic microstructural characterisation of CoCr-based thin films for magnetic recording media and touches on the magnetic properties of such films. The CoCr lattice is hexagonal and as such has a well-defined uniaxial anisotropy that makes it ideal for magnetic recording applications. By inducing a preferred orientation of the grains within the sample, the film may be used either as longitudinal or perpendicular media. This preferred orientation can be induced by a crystalline substrate, Cr for the longitudinal media and Ti for the perpendicular media. Under the correct deposition conditions, CoCr grains also have a tendency to preferentially orientate on an amorphous substrate, making them similar in microstructure to films deposited on Ti. The preferential orientation of grains within CoCr/Cr and CoCr on Ti thin films was studied by electron diffraction and the CoCr/Ti films were compared with CoCr films of the same CoCr thickness grown on amorphous C. The CoCr on Cr grains were found to have one of two preferred orientations, these orientations being combined within each sample. On the other hand, the CoCr on C and CoCr on Ti films possessed one preferred orientation, but with some grains being randomly orientated. In general, those films deposited on Ti had a smaller volume of randomly orientated grains than those deposited on C. Magnetising experiments on these films were not possible, but magnetic imaging of domain structure showed that, in the case of the CoCr/Cr films, the domain size decreases as the CoCr thickness increases. The CoCr/C films are dominated by an in-plane component of magnetisation that makes it difficult to discern any underlying structure arising form the out-of-plane component. No magnetic structure could be discerned in the CoCr/Ti films.
Chapter six gives results from experiments carried out on three spin-valves, the orientation of one of the ferromagnetic layers (Co) being pinned by an antiferromagnetic NiO layer. Electron diffraction showed that each component of the systems (NiFe, Cu, Co, NiO) was face centred cubic in structure. In-situ magnetising experiments carried out on the three spin-valves, one a bottom spin-valve with the NiO layer deposited first, the others top spin-valves with the NiO layer deposited last, showed that all three exhibited a spin-valve response to some degree. However, it is probable that the NiO layer interacts with the applied magnetic field resulting in stress that manifests a curvature of the films, a magnetoelastic effect. This curvature makes the experimental results difficult to interpret.
Chapter seven presents summaries and conclusions from chapters three to six and suggests possibilities for future work on these systems.