Magnetostatic energy

Magnetostatic energy

A magnetized block of ferromagnetic material containing a single domain has a macroscopic magnetization. The magnetization causes the block to behave as a magnet, with a magnetic field around it. Figure 7.3(a) illustrates a magnetized block with its associated external field. It is apparent from the figure that the field acts to magnetize the block in the opposite direction from its own magnetization. For thisreason it is called the demagnetizing field, Hd. We will encounter demagnetizing fields again in Chapter 11 when we discuss shape anisotropy.

The demagnetizing field causes a magnetostatic energy which depends on the shape of the sample. It is this magnetostatic energy which allows the block to do work such as lifting another ferromagnet against the force of gravity. The magnet static energy can be reduced by reducing the external demagnetizing field; one way to do this is to divide the block into domains .Here the external field is lower, so that the block is capable of doing less work, and(conversely) is storing less magnetostatic energy. Of course the magnetic moments at the boundary between the two domains are not able to align parallel, so the formation of domains increases the exchange energy of the block.

To reduce the magnetostatic energy to zero, we need a domain pattern which leaves no magnetic poles at the surface of the block. One way to achieve this is shown in Fig. 7.3(c). Before we can decide whether this is a likely domain pattern, we need to understand a little about the magnetocrystalline and magnets strictive energy contributions.

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