The Study Of Magnetic Properties And Magnetocaloric Effects In The Low-dimensional Frustrated Systems

Geometrical magnetic frustration leads to unusual phenomenon, such as superconductivity, spin glass, spin ice, spin liquid and so on. Frustration has changed the symmetry and the phase transition behaviors thoroughly. Therefore, magnetic frustration has attracted considerable attention.In this paper, the magnetic and thermodynamic properties are calculated for the systems with square, triangular and Kagome lattices. First, magnetic monolayer structure is established based on two-dimensional classical Heisenberg antiferromagnet, then anisotropic energy model and bond phonon model are simulated by using modified and standard Monte Carlo methods respectively. Magnetization, specific heat and magnetic entropy change are calculated in the models with anisotropic energy.This paper is divided into five parts. First, an overview of magnetic frustrated materials is introduced and the international achievements are reviewed. In the second chapter, the Monte Carlo method is introduced; the expressions of magnetization, specific heat, magnetic entropy change are deduced. In the third and forth chapters, for three kinds of lattice systems, the magnetization behaviors and magnetocaloric effects are studied. A summary is given in chapter5. The main conclusions are as follows:1. When the anisotropy is constant, isothermal magnetization curves are calculated for three kinds of lattice systems at different temperature. At low temperature, magnetization curves of frustrated systems show many magnetization plateaus. With the increase of temperature, the M=1/3plateau is observed and other plateaus has disappeared. Magnetization plateau at M=0.0is observed for square lattice system at low temperature. In different applied fields, the temperature dependence of magnetization is calculated. In frustrated systems, the magnetization decreases rapidly with the increase of temperature when the applied field is small; with the increase of applied field, the magnetization shows a trend of first increase and then decrease and a peak is founded. Indicating, the thermal fluctuation don’t prevent completely the magnetization, however, the magnetization is enhanced for the further increase of temperature.2. When the temperature is unchanged, isothermal magnetization curves are calculated for three kinds of lattice systems with different anisotropy. The locations of magnetization plateaus in frustrated systems change with the enhancement of anisotropy. Magnetization plateaus are also observed for the square lattice.3. In bond phonon model, isothermal magnetization curves also show many magnetization plateaus in the system with triangular lattice.4. When the anisotropy is constant, the magnetic entropy change of systems are calculated. The magnetic entropy change of frustrated systems is always negative, while the magnetic entropy change exhibits positive in the system with square lattice.