Study Of Magnetism And Phase Transition For Heisenberg Spin Lattices

In this thesis,two kinds of Heisenberg spin lattices are studied,one is cubic lattices and the other is magnetic films.For the cubic lattices.The phase diagram and the phase-transition at finite tem-perature for different configurations of antiferromagnetic face-centered-cubic lattices are discussed.According to the magnetization and internal energy at zero tempera-ture,the quantum frustration in different configurations and the unusual quantum state at the phase boundary are analyzed.The relevant experimental results are discussed by using our theoretical model.In body-centered-cubic lattices,we also obtain the phase diagrams for different ferromagnetic and antiferromagnetic configurations.The reason of unsaturation magnetization of antiferromagnetic states at zero temperature is analyzed and different stability are obtained by only considering the nearest neigh-bor spin exchange or considering the next nearest neighbor spin exchange under the random phase approximation.We calculated the magnetic entropy of three kinds of body-centered-cubic lattices and discussed the inverse behavior of sublattice magnet-ic momentum under different temperature and different magnetic field.Finally,we compared the magnetothermal effects for different configurations.For the magnetic films.The parameters of anisotropy strength in each monolayer and inter-layer interaction in transition metal sulfide VS₂are studied.By calculating the Curie temperature and hysteresis loop for multilayers,and comparing that with the experimental results,the reasonable parameters are estimated.Our parameters are con-sistent with the other references that different structures of VS₂have different stability at different thickness and different temperature.For the ferromagnetic and antiferro-magnetic thin film systems that considering the free surface,the spin exchange constant at surface have a great effect on the magnetization and the transition temperature for each layer.There is a critical value of surface spin exchange,the system behaves as a normal phase while less than this value.At this time,the thicker the film is,the closer the transition temperature is to that of the block system.Above this critical value,the system behaves an abnormal phase which the transition temperature have the contrary behaviors.The transition temperature at the critical point corresponding to the transi-tion temperature of the block counterpart.This critical value is independent of the spin quantum number.There is another critical value of the spin exchange at surface,below this value the deeper films have the larger magnetization,while above this value it has the inverse order.The study of free surface effect of multilayer magnetic films plays an important role on studying the practical magnetic film materials.