【作者】 顾明；

【导师】 刘公强；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2008， 博士

【摘要】 本论文工作分两部分。第一部分为强磁场下顺磁性物质的磁和磁光特性研究，第二部分是关于掺杂影响MnZn功率铁氧体材料磁性能的研究。近年来，实验研究发现某些顺磁性材料在低温、强磁场下呈现异常的磁和磁光特性。例如，其法拉第旋转角θ与外磁场强度之间存在非线性和互易性关系，而随着的增强，某些顺磁性介质的低温磁化强度eHeHM呈现异常的饱和现象。目前尚无相应的理论能够较好地解释上述实验现象。本文首先简要介绍了磁学和磁光学的基本概念及其相关理论的发展历程，以及物质的自旋玻璃态特性和理论。随后，本文理论研究和分析了顺磁性介质法拉第磁光效应的非线性和互易性，首先从法拉第效应的宏观理论出发，然后过渡到顺磁性介质法拉第效应的量子理论，推导出了顺磁性介质法拉第旋转角θ与之间关系。理论研究表明，顺磁性介质的法拉第旋转角eHθ与外磁场之间存在复杂的非线性关系。在高温和中、低磁场下，eHθ主要受的一次项影响，两者近似呈线性关系，但在低温和高场下必须考虑高次项的影响，因此eHeHθ表现出明显的非线性特性，且其互易性也不能忽略。本文同时还分析了顺磁性介质费尔德常数与磁化率比值Vp/χ的复杂温度特性。利用上述理论结果，本文拟合分析了顺磁性钕镓石榴石（Nd3Ga5O12）材料在轨道-自旋耦合作用、晶场交换作用、间接交换作用和外磁场共同作用下高场法拉第效应的实验数据。研究发现高场下其法拉第旋转角θ与外磁场之间存在明显的非线性关系，且的高次项系数强烈依赖温度eHeHT和入射光频率ω,而其费尔德常数V (θ/( H eL))也是的函数。理论分析同时表明,高场情况下顺磁性钕镓石榴石法拉第效应的互易性不可忽略。理论分析结果和实验数据吻合得很好。最后,本文中利用朗之万顺磁性经典理论,结合顺磁性物质中(间接)交换作用有效场的概念,研究了低温、高场下NdF3单晶的异常磁特性。发现并证明了NdF3单晶在低温下具有明显的自旋玻璃态特性,而在光辐照条件下,被冻结的自旋磁矩将会“融化”。与强磁性介质的自旋玻璃态特性相似,当顺磁性介质的自旋玻璃态特性与(间接)交换作用密切相关。与此同时,其磁特性与前者相比又存在明显的不同,例如,其自旋冻结程度与外磁场H e的大小有关,在低场区可逆磁化率χM随温度下降反而增大等;本文最后详细分析了导致这些特性的内在因素。本文第二部分是关于掺杂影响MnZn功率铁氧体材料磁性能的研究。由于高性能铁氧体对于烧结炉的温度和气氛控制精度有很高的要求,而目前国内铁氧体烧结设备的性能相对国外仍有一定的差距,因此立足现有设备,通过新的掺杂提高MnZn铁氧体的性能具有重要意义。本文首先研究了作为主配方原料之一的Mn3O4用作掺杂对于MnZn功率铁氧体材料性能的影响。研究发现,相对于将全部Mn3O4作为主配方原料使用的传统方法,将适量Mn3O4作为添加剂,在二次球磨时进行掺杂能够有效提高MnZn功率铁氧体材料的初始磁导率和饱和磁感应强度,同时降低功率损耗。实验结果表明,应用Mn3O4添加剂对进一步改善现有MnZn铁氧体材料的性能具有重要的潜在价值。除了探索Mn3O4作为新掺杂的可能性之外,本文对之前报道过的MoO3和TiO2掺杂从不同角度进行了研究,得到了一些新的结论。研究表明,以往主要作为助熔剂,用于提高铁氧体初始磁导率的MoO3在特定的含量下对材料功耗也有显著的改善,而少量的TiO2掺杂不仅并不影响铁氧体的初始磁导率等磁特性,还可进一步降低铁氧体功耗,并改善材料初始磁导率的温度稳定性。更多还原

【Abstract】 The main work in this doctoral dissertation consists of two parts. One is the theoretical investigation on the magnetic and magneto-optical properties in paramagnetic media under high field and at low temperatures, the other is the study on the effect of doping on the magnetic properties of MnZn ferrites.Recently, it has been found in experiment that some paramagnetic media present novel magnetic and magneto-optical characteristics under high applied magnetic field and at low temperatures. For example, there is a nonlinear and reciprocal relationship between the Faraday rotationθand the applied field H e, and with the increase of the applied field, the magnetization M of some paramagnetic media presents abnormal saturation characteristics in the low temperature range. Till now, there are no corresponding theoretical progresses which keep up with the above experimental findings.In this paper, firstly, it is briefly introduced the basic conceptions regarding magnetism and magneto-optics as well as the development of relative theories. Also is described a general picture of the spin-glass properties and theories.Then, the nonlinearity and reciprocity of the Faraday effect in paramagnetic media are theoretically studied and the results analyzed. It is started from the classical theory of the Faraday effect, and then transited into the quantum theory of the Faraday effect in paramagnetic media. It is worked out the relationship between the Faraday rotationθof paramagnetic media and the applied field. Theoretical study reveals a complicated nonlinear relationship between eHθand . Specifically, under middle or low field and at high temperatures, eHθis mainly decided by the first order term of , therefore, they can be considered as linearly dependent; but in the case of high field and low temperatures, the higher order term of must be taken into account, as a result, ieHieHθdemonstrates adistinguishing nonlinear property, meanwhile its reciprocity can not be neglected. Also theoretically analyzed is the complicated temperature dependence of the Verdet constant over the magnetic susceptibility (Vp/χ) in paramagnetic media.Based on the above conclusions, a theoretical fitting and analysis on the experimental data of the Faraday effect in paramagnetic neodymium gallium garnet (Nd3Ga5O12) is presented by taking account of the Spin-Orbit interaction, the Crystal field interaction, the super-exchange interaction and the effect of the applied field. It is found that under high applied field, the Faraday rotationθof Nd3Ga5O12 is strongly nonlinear with , and the coefficients of the higher order term of deeply dependent on the frequency of the incident light and the temperature. Moreover, the Verdet constant (eHieHVeH/θ) is also a function of . Meanwhile, the theoretical analysis shows that the reciprocity of the Faraday effect in paramagnetic NdeH3Ga5O12 can not be neglected under high applied field. The theoretical results are in good agreement with the experimental data. Finally, with Langevin theory of paramagnetism and introducing the conception of the effective (indirect) exchange field, a deep and comprehensive study is conducted on the experimental magnetic and MO data of NdF3 single crystal under high applied field and in the low temperature range. It is found and proved that typical spin-glass characteristics does exist in NdF3 single crystal at low temperatures, and the“frozen”spins will be“melted”under light irradiation. Like the case of the spin-glass states converted from ferromagnetic or ferrimagnetic ones, the spin-glass characteristic of the paramagnetic media are closely connected with the (indirect) exchange interaction. At the same time, the latter also presents some magnetic characteristics quite different from the former. For example, the degree of“spin freezing”is affected by the applied field, and in the low field range, the magnetic susceptibilityχM increases with the decrease of temperature. Their mechanisms are analyzed in details at the end of this paper.The second part of this doctoral dissertation is about the effect of doping on the magnetic properties of MnZn power ferrites. Synthesizing high performance MnZn ferrites demands highly accurate control on temperature and oxygen concentration of sintering furnace. Take into consideration that the performance of sintering furnaces at home lags quite behind those abroad, therefore, it is of great significance to improve the performance of MnZn ferrites via basing new kind of doping on the current equipment.The effect of Mn3O4, one of the three raw materials of MnZn ferrites, as a dopant on the properties of MnZn power ferrites is firstly studied. It is found that remaining suitable amount of Mn3O4 to add as a dopant within the second time ball milling, in contrast with the traditional method of adding all the Mn3O4 powders as the raw material, can effectively raise the initial permeability and saturated magnetic induction of MnZn power ferrites, while reduce its power loss. Therefore, adding Mn3O4 as a dopant is of great potential in further improving the performance of current MnZn ferrites.In addition to the exploring the possibility of Mn3O4 as a kind of new dopant, the effect of MoO3 and TiO2 addition on the properties of MnZn power ferrites is also studied from a new aspect, and some interesting conclusion is made. It is found that suitable amount of MoO3, which is long considered as a melting agent to raise the initial permeability, can also greatly reduce the power loss of MnZn power ferrites. Besides, small amount of TiO2 addition does not deteriorate the initial permeability of MnZn power ferrites, instead, it can further reduce the power loss, and improve the temperature stability of the initial permeability.更多还原