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. 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 anisotropie 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 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.