Institut National Polytechnique de Grenoble (France) (INPG)
Centre National de Recherche Scientifique (CNRS).
Laboratoire Louis Neel.

"Image Processing and Magnetic Patterns"

Abstract

The study of magnetic patterns requires many observations techniques. Image Processing plays an important role in the quantitative analysis of such patterns and in the interpretation of their spatial arrangements. In the thesis we are interested in 3 different magnetic systems : cellular structures, thickness modulation in a ferrofluid film and different states of an array of compass needles. These structures have in common the presence of dipolar interactions which leads to the existence of a number of metastables states and hysteresis phenomena.

In a first part we give an overall presentation of image processing with special emphasis place on the system which we studied.

The second part is devoted to cellular magnetic structures. These are obtained from an array of magnetic bubbles in a garnet thin film and present numerous analogies with soap bubbles arrays. We have developed image analysis procedures which enables these evolutions of cellular structures to be characterised. Special contribution has been done under the influence of of magnetic field cycles. In these case the structure relaxes towards an equilibrium state. We have characterised this relaxation process and have tried to analyse and relate it to the local evolution process, i.e., the collapse of small five sided bubbles and reorganisation of vertices.

For the case of ferrofluids we have studied the thickness of modulation which results from a competition between magnetic energy and stabilising terms such as surface energy and gravity. Image processing enables us, on the one hand, to analyse the observed structure, and on the other hand, to determine the form of the modulation from a measure of light absorption, both as function of applied magnetic field. One can thus investigate the energy terms which stabilise the interfaces.

Finally, for the case of the array of compass needles, image processing was used to measure the orientation of the needles which therefore enables all parameters which quantify the system to be obtained : domain sizes, magnetisation, total energy,...

Key-words

image processing, magnetic domains, cellular structures, irreversible processes and absorption measurements.