Effects of disorder on the crystal field modulated phases of a magnetic model.

Author(s)

Nelson Alves Junior

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2001-09-26

Examining board members

Carlos Seihiti Orii Yokoi; Valdir Bindilatti; Sérgio Galvão Coutinho; Evaldo Mendonca Fleury Curado; Silvio Roberto de Azevedo Salinas

• Advisor: Carlos Seihiti Orii Yokoi

Abstract

In this thesis we present a mean-field study of the effects of the disorder of the single-ion anisotropy on the phase diagrams of the spin-l ANNNI model. Previous mean-field results for the model with constant single-ion anisotropy showed partially disordered phases in the phase diagrams of the model. H owever , the existence of such phases cannot be taken as granted, due to the fiuctuations not taken into account in the mean-field approximation. Thus, we studied the model with constant single-ion anisotropy by means of Monte Carlo simulations. Qur aim was to give credit to previous mean-field results and to get more confidence in the mean-field results showed in this thesis obtained for a random single-ion anisotropy. The results obtained from the simulations showed evidences that the configuration of the commensurate phase with period 6 displaying disordered planes, found at low temperatures, may undergo an internaI phase transition to a phase in which such planes are absent. This result is in agreement with previous mean-field studies. For a random single-ion anisotropy, we investigated the phase diagrams of the model within mean-field approximation. For a generic probability distribution of the anisotropy, we obtained the free-energy functional and the mean-field equations, as well as the expressions for the criticallines and tricritical points of the model. In order to investigate disorder effects, we obtained phase diagrams of the model by considering a double-delta and a Gaussian probability distribution of the single-ion anisotropy. (AU)