THERMODYNAMICS OF DISORDERED FERROMAGNETS AND SPIN GLASSES

Abstract

The theoretical investigations aimed at understanding the thermodynamic properties of disordered and diluted magnetic alloys have been presented in the thesis. The contents have been divided in two parts. First part of the thesis deals with the calculations of thermodynamic properties of disordered and diluted ferromagnets and second part contains calculations on low temperature properties of spin glass systems. To study the properties and behaviour of disordered ferromagnets in intermediate temperature range, we have calculated the average free energy using the cluster expansion method, proposed by Strieb, Callen and Horwitz. Our calculations include contributions to the.free energy up to two site spin clusters in the case of Heisenberg ferromagnet and three site spin clusters in the case of Icing ferromagnet. Using these free energy expressions, we have obtained the expressions for. Curie temperature, specific heat and susceptibility. The method is capable of tackling fairly general kinds of disorder. Numerical calculations on Curie temperature, specific heat and susceptibility for bond disorder and site disorder cases have been performed. We have also specialised our calculations to consider the cases of bond dilution and site dilution. The value of critical concentration, necessary to sustain ferromagnetism has been calculated. The interesting aspect of our approximation is the variation of Curie temperature with concentration. We find that there is a discontinuity in the value of the critical temperature at the critical concentration in contrast to the earlier results of cluster theories....