Study On The Magnetic And Magneto-optical Properties Of Paramagnetic Media Under High Field And At Low Temperatures And On The Effect Of Doping On The Magnetic Properties Of MnZn Power Ferrites

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.