R. Masrour

STUDY OF CRITICAL BEHAVIOUR IN DILUTED FERROMAGNETIC: THIN FILMS AND SEMI-INFINITES FILMS

By using the high-temperature series expansion technique, we have analyzed the phase transition and the critical phenomena of a ferromagnetic thin film and ferromagnetic semi-infinite film, through three models: Ising, XY and Heisenberg. The critical reduced temperature τC(v) is studied as a function of the thickness of the film. In the case of the magnetic film and semi-infinite film, on the simple cubic lattice and the face-centered cubic lattice, τC(v) is studied as a function of the exchange interactions in the bulk, and on the surface. A critical value of the surface exchange interaction in the film above which the surface and the interface magnetism appears is obtained. The dependence of the reduced critical temperature on the thickness of the film has been investigated. These shifts of the critical temperatures TC(L) from the bulk value can be described by a power law. The obtained values for the simple cubic lattice and face centered cubic ferromagnetic thin film are in qualitative accordance with the universality class hypothesis. The critical exponent associated with the magnetic susceptibility is studied as a function of interactions. In a defined range of the exchange interactions, the obtained values for Heisenberg, XY and Ising models, for simple cubic thin film are comparable to the universal ones and are independent of the film thickness. The asymmetry of the structure and the competition of the effects of the exchange coupling, are important for the magnetic properties of the system. A critical value of the surface exchange interaction above which the surface magnetism appears is obtained. For the dependence of the critical parameter of surface reduced coupling as a function of the dilution x and the ratio of the exchange interaction between the surface and nearest neighbour layer to the bulk one R1 for the three studied models has been investigated. The magnetic phase diagrams are obtained for two structures. The percolation threshold is defined as the concentration xp at which τC=0. In the case of the thin film, the obtained values are xp≈0.2 in the bulk and xp≈0.4 at the surface. For the case of the semi-infinite film, the value obtained in the surface and the nearest layer at the surface is xp≈0.2.

CROSSOVER OF CRITICAL EXPONENTS INDUCED BY DILUTION VIA MONTE CARLO SIMULATIONS

The crossover of dilution in the critical exponent associated with the thermodynamic properties in the magnetic materials via Monte Carlo simulations is observed. The obtained results of critical exponent associated with the magnetic susceptibility, specific heat and correlation length for the ZnCr2xAl2-2xS4, Cd1-yCr2-2xIny+2xSe4 and Zn1-xMnxTe systems are comparable with those given by the experiment results.

Magnetic properties and Binder cumulants of a mixed spin-2 and spin-5/2 Ising diamond chain

The mixed spin-2 and spin-5/2 Ising diamond chain is studied using Monte Carlo simulation. The thermal variation of magnetization and magnetic susceptibilities are obtained in diamond chain. The values of transition temperatures of a mixed spin-2 and spin-5/2 Ising diamond chain have been obtained for different sizes of system N, for different crystal field and for different exchange interactions. The Binder cumulant of magnetization 〈M〉 is investigated. The magnetization with the crystal field and the exchange interaction has been established. The magnetic hysteresis cycle is determined for different values of the crystal field, exchange interactions and temperatures. The system presents the superparamagnetic behavior for a fixed crystal field and temperature value.

Magnetic properties of zigzag metallic chains: A Monte Carlo study

The magnetic properties of zigzag metallic chains have been calculated based on the Monte Carlo simulation. The reduced transition temperatures and the magnetization have been obtained for different exchange interactions and for different crystal fields. The hysteresis cycle is obtained for different exchange interactions, temperatures and crystal field. The superparamagnetic behavior is obtained for a specific value of temperatures and crystal field. The second-order phase transitions are also observed. The obtained results are comparable with some given by experiment results and theoretical calculations.

STUDY OF MAGNETIC PROPERTIES OF DILUTED FERRIMAGNETIC SPINELS SYSTEMS BY: HIGH TEMPERATURE SERIES EXPANSIONS

The ferrimagnetic spinels systems CoxFe1-x Cr2S4 has been studied by high-temperature series expansions method in the range 0 ≤x ≤1. The exchange interactions and exchange energies are calculated by using the probability law. The high-temperature series expansions have been applied in the spinels systems CoxFe1-xCr2S4, combined with the Pade approximants method, to determine the magnetic phase diagram, i.e., TC versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced.