الفهرس 
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Abstract
We present, in this thesis, a statistical mechanicsbased model to calculate magnetic properties of systems of cubic, uniaxial and mixedanisotropy in a wide range of particle size, temperature and field. We start at evaluating the classical partition function of a given system using the appropriate forms of the anisotropy and zeeman energies. The anisotropy energy function depends on the symmetry of the system under consideration and involves both uniaxial and biaxial anisotropy in ultrathin ferromagnetic films, for example. The calculated magnetic quantities include magnetization curves, magnetic susceptibility and probability of the angular distribution of the magnetization vector. These quantities show size, temperature and fielddependencies. Magnetization discontinuities were observed, at certain magnetic fields, in ultrathin films where the growth techniques led to two different anisotropies in these systems. Nanosized particles demonstrate the interesting phenomenon of superparamagnetism. In the first chapter we present, in short, the fundamentals of magnetism needed in this thesis. The second chapter is devoted to the models used in our study. A relatively detailed analysis of the cubic and mixed anisotropies for the three major axes in cubic crystals i.e. [100], [110] and [111] is given. In addition to introducing our model we presented models for calculating the magnetization of superparamagnetic systems and magnetic hysteresis and used them, as well, for calculating the dependence of magnetic properties on size /shape of particles. The results of this thesis are presented and discussed in chapter 3. In the discussion we tried to link the magnetization behavior with the probability distribution for a wide range of particle sizes. Probability plots show sharp (broad) angular distribution for large (small) particles. The probability peaks close to the easy magnetic direction of the system under study. We calculated magnetic properties of Fe particles, Co thin films, ultrathin ferromagnetic films and a SmFe2 single crystal. Our model proves to be satisfactory in calculating properties of these systems on which experimental data is available. For one example, Fe particles of about 2nm in radius showed a superparamagnetic behavior even down to liquid nitrogen temperature. Another example is the change of the easy magnetization direction in a cubic SmFe2 crystal at about 190K from [110] to [111] direction. Our calculated magnetization curves at 77 and 300K show this behavior.