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L. Stoleriu:
"The Effect of the Reversible Magnetisation Processes on the Magnetisation Mechanism of the Ferromagnetic Particulate System";
Supervisor, Reviewer: H. Hauser, Ph. Bissel; Department of Electricity, Alexandru Ioan Cuza-University, 2001.

Magnetic recording media is the one of the most important category of applications of magnetic materials. In order to develop particulate magnetic materials for applications such as magnetic information storage, it is important to be able to understand and to predict their behaviour so that their properties can be manipulated to improve their functionality. Their behaviour is complex and basically involves two interrelated issues:

- the magnetic properties of the individual particles and the statistical distribution of these properties over the system and

- the magnetic interactions between the particles.

At a general scale, the magnetisation processes can be divided in irreversible and reversible magnetisation processes. The irreversible processes are usually associated with the dissipation of energy and with the switching of the magnetic moment from one to another equilibrium position while the reversible magnetic processes are quasistatic processes. Both of these processes are involved in the magnetic recording process. The writing process is taking place then a magnetic head brings the medium into the desired magnetised state, which can be associated to irreversible magnetic changes in the medium. When the head passes, its magnetic field acts no more on the written medium and therefore, the magnetisation remaining after the removal of the field is more relevant for the recording performance than the magnetisation during the application of the field. This remanent magnetisation is attained mainly through reversible magnetisation processes.

This work will focus on the analysis of the reversible and irreversible magnetic processes of the small magnetic particles and on the way these processes influence the characteristics of a system of such particles - the statistical distribution of different magnetic properties over the system and the magnetic interactions between the particles.

The first chapter - Reversible and irreversible magnetization processes - presents some theoretical topics on the reversible processes of magnetic media using scalar, vector and energetic approaches. The reversible magnetisation processes are analysed starting from one of the simplest problems in magnetism - the Stoner-Wohlfarth model for coherent rotation of the magnetic moment of a single domain particle - to complex particles systems. It is shown that the magnetisation processes can be divided into reversible and irreversible changes together with apparent reversible changes and super-paramagnetism.

In order to reveal the effect of reversible part on the magnetisation processes of the recording media we analysed experimental procedures which are able to separate the reversible and the irreversible magnetisation processes. The second chapter - Experimental methods and devices of studying reversible magnetisation processes - presents such scalar and vector magnetic measurements. This chapter also describes methods of measuring the distribution of some properties of a particulate magnetic system, which are relevant for the reversible phenomena as easy axes and switching field distributions and discuss the efficiency of these methods.

The models for magnetic hysteresis can be categorised in physical models and phenomenological models. The physical models start from physical hypotheses while in the phenomenological models one uses mathematical functions to describe the experimental observations. Among the existing phenomenological models, the Preisach Model is one of the most flexible and most widely used. The third chapter - The reversible magnetisation in the models of magnetic hysteresis. Phenomenological models - starts by describing the evolution and the hierarchy of the scalar models. Some vector models are also presented together with some of the contributions of the author - the vector mixed model in its discrete and continuous forms.

The speed of calculations is the main advantage of the phenomenological models over physical ones. The remarkable last years increase in computer power made the physical models accessible for personal computers modelling and changed the ratio between the research focused on phenomenological and on physical models. The last chapter of this work - The reversible magnetisation component in the models of magnetic hysteresis. Physical models - describes the widest used physical model, the model based on the Landau-Lifshitz-Gilbert equation of the dynamic of the magnetic moment of a single domain particle. Personal contribution of the author are presented:

1. The LLG model was used to verify the physical basis of the Preisach-type phenomenological models. While the some of the hypotheses of the scalar models were in an excellent agreement with the results of the physical model, a few hypotheses of the vector models were infirmed.

2. The LLG model was used to simulate the complex magnetisation processes taking place during the magnetic recording. Writing and reading processes were simulated for small samples of particles which allowed studies of efficiency of the recording process and studies of the recording noise.