Materials and Condensed Matter Physics

Magnetisation reversal behaviour in advanced magnetic films

Multilayer magnetic thin films have attracted much attention both in the scientific community and the magnetic recording industry due to their commercial value. The work presented in this thesis is mainly a study of the magnetisation reversal mechanism of the free layer of the spin-valve. A spin-valve is a metallic multilayered structure that exhibits giant magnetoresistance. It consists of two soft ferromagnetic layers separated by a thin nonmagnetic layer. The magnetisation of one of the ferromagnetic layers is fixed by an adjacent antiferromagnetic layer by exchange bias coupling. The magnetisation direction of the other ferromagnetic layer can be rotated by applying a small external field. Hence, it is called the free layer.
This thesis begins in Chapter 1 with an introduction to the basic concept of ferromagnetism, including various magnetic energies. The origin of giant magnetoresistance and spin-dependent scattering phenomena are also discussed. Various spin-valve structures are described such as the top and bottom spin-valves and spin-valves with a synthetic antiferromagnet. The modified Stoner-Wohlfarth model is also discussed in Chapter 1. This model is used throughout the thesis to analyse the experimental observation. Various reversal modes are derived from the model, for instance coherent rotation, discontinuous jumps and symmetrically split rotation.
Chapter 2 is devoted to a discussion of the instrumentation and experimental techniques employed. Some magnetic thin film characterisation techniques such as vibrating sample magnetometry, B-H looper and magnetostriction tester, are described briefly. This is followed by an introduction to transmission electron microscopy. Magnetic imaging techniques using the transmission electron microscope are presented. An overview of Fresnel imaging, Low Angle Diffraction and Differential Phase Contrast is given. Fresnel imaging is used extensively in this thesis. A brief introduction to magnetic force microscopy is included.
Electron beam lithography, which is used to fabricate small structures, is explained in Chapter 2. This includes fabrication processes like spin-coating, exposure, developing and deposition.
The free layer magnetisation reversal process for a range of spin-valves is presented in Chapter 3. The spin-valves discussed in this chapter have different magnetostriction coefficients, magnetocrystalline anisotropies and coercivities. Some are top spin-valves and some are bottom spin-valves. They also have different exchange biasing structures, which are the antiferromagnet and the synthetic antiferromagnet. Marked differences in free layer reversal mode are apparent for these spin-valves. Reversal could be by simple magnetisation rotation or by rotation combined with complex domain processes. However, experimental results showed that a simple reversal process was often associated with films with low magnetostriction and magnetocrystalline anisotropy. The texture of the film was found to have no significant effect on the observed free layer reversal mechanism.
The effect on the free layer reversal process of replacing the antiferromagnet with a synthetic antiferromagnet is discussed in Chapter 4. Three bottom spin-valves with different synthetic antiferromagnetic structures were studied. The experimental results showed no significant difference in free layer reversal process in these spin-valves.
Domain structures were studied for a series of bottom spin-valves annealed in different magnetic fields. Magnetic imaging showed a significant improvement in films annealed at 20 000 Oe. No difference in magnetic domain structures for films annealed in 250 Oe and 10 000 Oe was evident.
Asymmetric magnetisation reversals processes were seen on occasion. Insight into why the reversal mode varied in the way it did was obtained using a modified Stoner-Wohlfarth model. The model provided a good description of the various modes and there was reasonable agreement between predicted and observed fields at which key stages of the reversals took place. Even though a single variable parameter model of the kind used cannot describe a multi-domain state, its use in inferring the nature of domain configurations that arise is discussed as are its other strengths and weaknesses.
The effect of temperature on free layer reversal process is discussed in Chapter 5. It was found that the spin-valve with the thinner synthetic antiferromagnetic layer was more susceptible to temperature changes. A change in the reversal process was observed between room temperature and 200oC. The process changed from a simple coherent rotation to an asymmetrical reversal process. No such changes were observed for the spin-valve with the thicker synthetic antiferromagnetic layer.
The studies of magnetisation reversal of magnetic thin films were extended to patterned magnetic elements in Chapter 6. The design and construction of a new in-situ magnetising rod is described in Chapter 6. The rod allows a pulsed field to be applied to the specimen. My work involves mainly the characterisation of the rod.
The rod was used to study the angular dependence of switching field for a rounded ends Ni80Fe20 element. The experimental results showed a slight increase in the switching field with field orientation relative to the long axis of the element for most of the elements. The data was analysed using the Stoner-Wohlfarth and Kondorsky models. The overall results indicate that the angular dependence of switching field shows a Kondorsky-type behaviour at small field orientation tending toward Stoner-Wohlfarth-type behaviour at large field orientation.
The final experimental chapter discusses the fabrication work on a micro-electromagnet. The aim of fabricating a micro-electromagnet was to serve as an alternative magnetising method to study patterned magnetic elements. The micro-electromagnet acts as a source of local field, which allows magnetising of a few elements in an array. A fabrication technique was developed and the completed device was shown to be able to generate magnetic field. However, the strength of the field has yet to be calibrated.
This thesis ends with general conclusions and discussion on further future work in Chapter 8.