Study Of The Physical Properties Of Layered Mixed-spin Magnetic Materials By Monte Carlo Simulation

Layered magnetic materials have many characteristics such as light, thin, short, smalland direct forming so that they can fully meet small motors and special magnetic devices,and have become the key to the development of today’s high-tech materials. Developmentof magnetism theory is closely linked with the technology application of magneticmaterials. The existences of the transition temperature and the compensation temperaturein magnetic materials have potential applications in electric motors and optical-magnetorecording. Selecting two kinds of layered mixed spin magnetic materials AFeⅡFeⅢ(C2O4)3and [NiCr2(bipy)2(C2O4)4(H2O)2]H2O, we set up the twin lattice mixed spin Ising modelsto study their magnetic and thermodynamic properties, surface effect and step effect byMonte Carlo simulation from the microscopic theory of quantum mechanics in thedissertation. The study contents are mainly as follows:For two kinds of layered magnetic solid materials, the effects of the anisotropy, theexchange coupling, the temperature and the longitudinal magnetic field on thecompensation temperature and the transition temperature, the magnetization, the initialsusceptibility, the internal energy, the specific heat, the hysteresis loop and the coercivityof two systems are discussed. The causes and influence factors of the compensationtemperature are discussed, and the phase diagrams are obtained. The studied results showthat both the anisotropy and the interlayer ferromagnetic exchange coupling can stabilizethe magnetization of sublattice. The thermodynamic fluctuation makes the magnetic orderweakened, destroying the stability of the sublattice magnetization. The longitudinalmagnetic field can stabilize the longitudinal magnetization of the system. Suchmagnetization behaviors of the system are originating from the competition among abovethese effects. The longitudinal magnetic field can make the compensation behaviourweakened. For certain parameters, the double and triple hysteresis loops appear in the system. For two systems, the internal energy increases as the temperature increases, andthe anisotropy, the exchange coupling and the longitudinal magnetic field all can decreasethe internal energy. The specific heat exhibits the discontinuity at the transitiontemperature and become flatter for the larger longitudinal magnetic field.For two kinds of magnetic thin film materials, the effects of the surface parametersand the layer thickness on the magnetic and thermodynamic properties of two systems arestudied. The surface parameter conditions where the compensation temperature, thetransition temperature and the coercivity remain constant are found, and thethree-dimensional phase diagrams are obtained. The surface parameter scopes of themultiple compensation points are discovered. Comparing with the experimental results, thereliability of our results is verified. The studied results show that: for the ferrimagneticmixed spin-2and spin-5/2Ising system, the effect of the surface intralayer exchangecoupling on the compensation temperature is opposite to that of the surface interlayerexchange coupling, whereas both effects on the transition temperature are same. Due to thecombined effects of surface and size, increasing the surface anisotropy can contribute tothe compensation behavior, and changing the layer thickness can also make the systemdisplay the multiple compensation points. Because of the differenent structure of two kindsof materials, the compensation behavior and phase transition rule are also different. For thedouble layer ferrimagnetic mixed spin-1and spin-3/2system, we have not found thesurface parameter condition where the compensation temperature remains constant.Based on step effect in the layered ferrimagnetic mixed spin-2and spin-5/2system atlow temperature, the evolution and the change of the shape of the hysteresis loop arediscussed. Research results show that the2S+1magnetic plateaus appear in the hysteresisloop, which reveals the variation of spin configuration of the system at the ground state orlow temperature. The temperature weakens step effect and decreases magnetic energyproduct, the coercivity and the remanence of the hysteresis loop.