الفهرس 
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Abstract
In this thesis, we are interested in calculating the temperature and field-dependence of specific magnetic and magneto-thermal properties for the following compounds , and , using a simple model based on the fundamentals of classical statistical mechanics.
It is known that, the classical partition function includes the total energy of a magnetic system. Therefore, the first step in constructing the partition function is to choose an appropriate energy model for describing the energy of the magnetic. In this investigation, we apply the rigid model to a single-domain-particle system with a single sublattice, instead of the known two-sublattices model, to the uniaxial hexagonal and tetragonal systems under study. This choice demands that only both of the magnetocrystalline anisotropy energy and the Zeeman energy are taken into account in this single-domain-particle approach.
The constructed partition function was used to derive, and therefore to calculate the temperature and/or field dependence of a host of physical properties either along or perpendicular to the c-axis. Examples of these properties are: magnetization, magnetic heat capacity, magnetic entropy, magnetic susceptibility, probability angular distribution of the magnetization vector, and the associated angular dependence of energy.
Our calculations demonstrate a correlation between the energy of the
system, its magnetization behavior and the most probable angular coordinates of the magnetization vector. For example, a first-order phase transition, from the hard to easy axis, takes place in for a specific field at low temperature. On the other hand, the single crystalline is an easy-axis system in the temperature range , but switches to an easy plane system at . This transition is also supported by both of the temperature dependence of the magnetic heat capacity, which develops a peak at a temperature , and the probability landscape which shows, in zero magnetic field, a prominent peak in the basal plane at . The angular location of the most prominent peak in the probability landscape for this case is at independent of .
In the case of an anisotropic system, we correlate the orientation of the magnetization vector, relative to a specific crystallographic axis, to the angular coordinates of the most probable location in a probability landscape. Further, we correlate the probability distribution to the associated magnetic entropy. The field dependence of the canting angle, either off the c-axis or off the basal plane, is also discussed in particular for critical magnetic fields at which first-order magnetization process takes place. Specific features of magnetization curves, at certain temperatures, are discussed in the light of results based on models involving crystal field and/or exchange interaction effects.
Comparing our results with the available theoretical and experimental data, we found out a good agreement for the systems studied.
We divide the thesis into three chapters: chapter 1, chapter 2 and chapter 3, in addition to conclusion, list of figures and tables. In chapter 1, under the title of background, we introduced a review of some important concepts in magnetism associated with our method. The theoretical model and the equations used in our calculations are presented in chapter 2. Finally, the results of our calculations are presented and discussed in the light of our model and some available theoretical and experimental works in chapter 3.