Nanomagnetic and Nanostructural Studies of Thin Film Magnetic Systems

The work described in this thesis involves the study of advanced thin film magnetic systems. All the systems investigated have proposed commercial applications and are examined to determine both their physical properties on the nanometre scale and their magnetic properties to a resolution of » 20nm. Most of the work was carried out on the highly modified Philips CM20 transmission electron microscope (TEM) at the University of Glasgow. Use was also made of the JEOL 2000FX TEM at the University of Glasgow, an alternating gradient field magnetometer (AGFM) at the University College of North Wales at Bangor and a vibrating sample magnetometer (VSM) at the University of Twente in Enschede, the Netherlands.

In chapter 1 the basic concepts of thin film magnetism along with the thin film systems which are to be investigated are introduced. In chapter 2 TEMs and magnetometers are described with particular reference to the instruments used. This is followed by a description of the processes of image formation and collection and the other techniques used in the study.

The experimental techniques developed to apply magnetic fields to the samples are described in chapter 3. They allow a microscopist to apply a changing field of up to several thousand Oersteds to a thin film sample while it is being imaged magnetically. This allows details of the micromagnetic structure to be determined that would otherwise be very difficult to deduce. These types of experiment form a large part of the work contained in this thesis, and the results are discussed in chapter 4 to 6.

In chapter 4, Co/Cu and CoCu/Cu multilayers with giant magnetoresistance (GMR) properties are described. These thin films have proposed applications as position sensors. The magnetisation reversal processes of these films are described and are compared with a model. The latter suggests that there is an anisotropy present in the films where the thickness of the Cu spacer layer is at the 1st antiferromagnetic maximum (AFM). The model further indicates that the uniaxial anisotropy constant for the film with alternating magnetic layer thicknesses |K|» 1.25|J/t| where J is the ferromagnetic coupling constant and t is the thickness of the thinner magnetic layer and the uniaxial anisotropy constant for the film with magnetic layers of equal thickness, |K|» |J/t|. This anisotropy is not detected in all the samples examined, and since it was not introduced deliberately, its presence is surprising.

Chapters 5 and 6 both involved the study of magnetic information storage media. In chapter 5 CoNi/Pt multilayers and NdTbFeCo amorphous alloys with attractive properties for use as magneto optic (MO) media at low laser wavelengths are described. Two characteristic hysteresis loop shapes for perpendicularly magnetised media are considered and their micromagnetic reversals are contrasted. To aid the understanding of the reversal mechanisms, they are compared with results from computer model predictions for samples with the same physical parameters. From these comparisons the following estimates are made: a wall energy (s w) of 1.8x10-3 Jm-2 and an activation volume (Vact) of 0.015 of the crystallite volume. The validity of these figures is then discussed.

Proposed in-plane longitudinal recording media are described in chapter 6. The samples are CoPt alloys deposited under varying conditions, to different compositions and thicknesses and onto two different types of substrate. The crystallite structure of the samples are shown to be independent of the thickness of the sample but are found to vary as the deposition conditions vary. The micromagnetic properties are shown to be dependent on the thickness of the samples and the crystallite structure of the samples. Two different reversal mechanisms are identified and described in detail. They involve domain walls moving through the sample in one case and the nucleation of many independent reverse domains in the other. Bits, which have been written on the sample deemed most suitable, are then examined and related to the reversal mechanism. This showed that the domain size during reversal and the transition width are comparable.

Chapter 7 contains conclusions drawn from the discussions in chapters 4 to 6 and proposals for future work.