摘要
层状磁性材料具有轻、薄、短、小以及直接成型等特点，能充分满足小型电机和特种磁器件等要求，已成为当今高新技术发展的关键材料之一。磁学理论的发展与磁性材料的技术应用息息相关。其中，磁性材料中的相变温度和补偿温度在电机和磁光记录方面具有重要应用。本文选取两类层状混自旋磁性材料AFeⅡFeⅢ(C2O4)3和[NiCr2(bipy)2(C2O4)4(H2O)2]H2O为研究对象，从量子力学的微观理论出发，采用MonteCarlo方法，建立双子格混自旋Ising模型，详细研究了两类层状磁性材料的磁性质和热力学性质、表面效应以及阶梯效应等物理性质。论文主要内容包括以下几个方面：
针对两类层状磁性块体材料，研究了磁晶各向异性、交换耦合作用、温度以及纵场等对系统补偿温度和相变温度、磁矩、初始磁化率、磁滞回线、内能和比热的影响。探讨了补偿温度产生的原因和影响因素，并获得了相图。研究结果表明：子格磁晶各向异性和铁磁性层间交换耦合作用的增强都会稳定其子格的磁矩；热力学涨落会破坏系统子格磁矩的稳定性，削弱系统的磁有序；纵场会在磁场方向上稳定系统磁矩；磁矩曲线类型的变化来源于上述几种相互作用之间竟争的结果。纵场能够削弱系统的补偿行为。当取某些特定参数时，系统会呈现两环或三环形状的磁滞回线。两类系统的内能随温度升高而升高，并且磁晶各向异性、交换耦合作用以及纵场的增大都会使内能减小，而比热在相变点处出现奇异现象，纵场使比热曲线变得平缓。
针对两类层状磁性薄膜材料，研究了表面参数和层厚对系统磁性和热力学性质的影响。发现了补偿温度、相变温度及矫顽力保持不变的表面参数条件，建立了三维相图，获得了系统存在多个补偿点的表面参数范围。通过与实验结果比对，验证了计算结果的可靠性。研究结果表明：对于亚铁磁混自旋2和5/2系统，表面层内交换耦合作用和表面层间交换耦合作用对补偿温度的影响相反。而对于相变温度来说，二者的影响基本一致。由于表面效应和尺寸效应的共同作用，表面子格磁晶各向异性的增大有利于补偿现象的发生，并且层厚的改变也会使系统会出现多个补偿点的现象。由于两类系统结构的不同，补偿行为和相变规律也有所差异。对于双层亚铁磁混自旋1和3/2系统，并没有发现补偿温度不变的表面参数条件。
基于层状亚铁磁混自旋2和5/2系统在低温下所表现出的阶梯效应，探究了磁滞回线产生阶梯效应及形状变化的原因。研究结果表明：系统磁滞回线会表现出2S+1个磁性阶梯，其数量的变化反映了系统在基态或低温下自旋组态的变化。温度会削弱系统的阶梯效应，使磁滞回线的磁能积、矫顽力和剩磁都减小。
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.
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