Ferromagnetic domains

Ferromagnetic domains

Ferromagnetic domains are small regions in ferromagnetic materials within which all the magnetic dipoles are aligned parallel to each other. When a ferromagnetic material is in its demagnetized state, the magnetization vectors in different domains have different orientations, and the total magnetization averages to zero. The process of magnetization causes all the domains to orient in the same direction. The purpose of this chapter is to explain why domains occur, to describe their structure and the structure of their boundaries, and to discuss how they affect the properties of materials. As a preliminary, we will describe some experiments which allow us to observe domains directly with rather simple equipment.

Domains are usually too small to be seen using the naked eye. Fortunately there are a number of rather straightforward methods for observing them. The first method was developed by Francis Bitter in 1931 [29]. In the Bitter method, the surface of the sample is covered with an aqueous solution of very small colloidal particles of magnetite, Fe3O4. The magnetite deposits as a band along the domain boundaries,at their intersection with the sample surface. The outlines of the domains can then be seen using a microscope. Figure 7.1 is taken from Bitters original 1931 publication; the light-colored lines are magnetite deposits on a crystal of nickel at 16 times magnification.

As we will discuss later in the chapter, at the domain boundaries the directions of the magnetic dipole moments change, and poles are formed at the surface of the sample. A magnetic field originates at the pole, and this attracts the fine magnetic particles to it. Note therefore that the Bitter method actually observes the domain boundaries, rather than the domains themselves. The technique can also be used to observe domain-wall motion, because the magnetite particles follow the intersection of the wall with the surface. However the sample must first be carefully cleaned and polished so that the magnetite particles don;t get stuck in cracks or around impurities.

It is also possible to observe domains using polarized light. As a result of the magneto-optic effect (which we will discuss in detail in Chapter 16), the plane of polarized light is rotated when it either passes through, or is reflected from, magnetic material. The direction of rotation depends on the orientation of magnetization. Therefore, regions of the sample with opposite orientations of the magnetization will rotate the polarized light in opposite directions. This method was first used in the early 1950s ; in Fig. 7.2 we show photographs of domains in demagnetized silicon iron from an early application of the technique .

Note that both the Bitter and magneto-optic techniques are sensitive to the domain structure at the surface of the sample. The surface domain structure is sensitive to local details of flux closure on the surface, and can be more complicated than the basic domain structure running through the bulk of the sample.

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