Researches On Demagnetizing Field Theory Of Magnetic Materials

Magnetic material is one of the most important parts of function materials, and has many applications. Magnetic materials are either spontaneous magnetization or technical magnetization. Magnetic domain theory is theoretical basis of the magnetizing mechanism of magnetic materials. The formation of magnetic domain is the important research field of magnetic domain theory. Reduction of demagnetizing energy is the cause of the magnetic domain formation, and then demagnetizing field and demagnetizing energy play a major role in the formation of the magnetic domain, that cannot be replaced. In applications, when external field is applied, material is magnetized and demagnetizing field is produced. The magnetic properties of materials depend directly on the demagnetizing field formed in magnetizing process.Firstly, an introduction about the microscopic origin and the classification of magnetic material are given. Then a summation about physical phenomenon of the ferromagnetic material and theories of spontaneous magnetization, as well as technical magnetizations are made. When material is magnetized, magnetic charges are caused on material surface and in the material. The magnetic field in magnetic material produced by the magnetic charges is called demagnetizing field. Demagnetizing energy related to demagnetizing field is unfavorable to homogeneous magnetizing, but is helpful to magnetic domains' forming. If there were no control of exchange action energy and magnetic crystal anisotropic energy, the magnetic domain's splitting would continue without limitation. Thus, all kinds of energy in magnetic material decide the size of magnetic domain and the shape of magnetic domain wall. In other words, exchange action energy only control microscopic properties, distribution of magnetization intensity in crystal is always decided by demagnetizing energy. Thus demagnetizing field must be fully discussed. Demagnetizing field is often found by the classical interaction of dipoles. It can also be obtained from the Maxwell's equations.Theoretical computation of demagnetizing field of arbitrarily shaped material is difficult. In this thesis, the analytical expressions of demagnetizing field and demagnetizing factors for cylinders and ellipsoids are obtained. Conclusions as following have been got by theoretical analysis and numerical calculation.1. From the point of view of magnetic charge, the demagnetizing field and demagnetizing factors of uniformly magnetized cylinder material along its axis are obtained. They are functions of field point coordinates and ratio of length to radius of cylinder. The distributions of demagnetizing factors on the cross-sections, on the cross-sections of axis and on the axis of cylinder are analyzed.2. In ellipsoidal coordinate system, the magnetizing problem of ellipsoid material in uniform magnetic field is solved. The analytical expressions of magnetic scalar potential and magnetic field in the entire space, as well as demagnetizing field and demagnetizing factors in the material, are all obtained.(1) The demagnetizing field in the ellipsoid material is uniform. The demagnetizing field is not always parallel with (along or opposite to) the magnetizing field. The angle between the direction of demagnetizing field and magnetizing field is relate to direction of magnetizing field and the shape of ellipsoid material ,as well as material susceptibility.(2) It has been proved that in ellipsoid axis coordinate system, the demagnetizing factor tensor is symmetric. The tensor's trace equals one. It is also found that the demagnetizing factor along the longer axis of ellipsoid is less than that along the shorter one. If the magnetizing field along the ellipsoid axis or the material is sphere, the demagnetizing field is parallel with (along or opposite to) the magnetizing field.