Amimer, Zakariya2022-06-222022-06-222021-07-05salle des thèsesDOC-530-78-01https://dspace.univ-tlemcen.dz/handle/112/18644In order to study the nature of phase transitions in magnetic systems, the aim of our research is to explore the S=1 and σ=3/2 states where the ferromagnetism as well as the influence of different crystal fields will be studied by the Monte Carlo method of The Ashken-Teller model which is a coupling of two Ising models with a four-component parameter. We have given some theoretical definitions related to the phase transition as well as the types of magnetism also the spin models and the simulation method. First, we identify the most stable phases in the phase diagrams at temperature T=0 by energy minimization. Then For higher temperatures, we find phase diagrams rich in ordered phases: a 3/2 Baxter phase and a 1/2 Baxter phase as well as a <S> phase that appears neither in spin 1 nor in spin 3/2 ATM, and finally, a <σ> = 1/2 phases with 1st and 2nd order.frIsing model, Ashkin-Teller model, spin-1, spin-3/2, Monte CarloModèle d'Ising, Modèle d'Ashkin-Teller, spin-1, spin-3/2, Monte Carlo.Thesis