# User:Brian P. Josey/Notebook/2010/01/20

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## More on Magnetism

I read some more of the Hitchhikers Guide to Magnetism over the break and today. I still haven't finished it all the way through yet, but it's a work in progress. I can have a hard time keep track of all the different terms, especially when they are longer and are composed of the same couple of morphemes. So the next couple of entries will just be vocab lists that hint at the major ideas of the paper.

### Anisotropy

• Magnetic Anisotrophy- the dependency of magnetic properties on a particular direction. There are several types, below, and they all strongly affect the shape of hysteresis loops an coercivity and remanence.
• Magnetocrystalline anisotropy-the energy necessary to deflect the magnetic moment in a single crystal from a "easy" to "hard" direction. These directions represent how well you can magnetize a sample in a particular direction relative between the sample and the magnetic field. It depends on the structure of the crystal lattice. Obviously, if you try to magnetize a sample in the "easy" direction, it reaches the maximum quicker. It is an intrinsic property of all ferrimagnets.
• Stress Anisotropy- depends on the applied and residual stresses on the sample.Magnetostriction a result of the strains depending on the anisotropy constants. When a previously demagnetized crystal is magnetized, it experiences a strain that can be measured as a function of the applied field along the axes. This will change the samples dimensions.
• Shape Anisotropy- depends on the shape of the sample. A magnetized sample produces magnetic charges, poles, at its surface. Alone, this distribution will create another magnetic field, the demagnetizing field that will act in opposition to the magnetization that produced it. Larger bodies will favor magnetocrystalline anisotropy, while magnetite that is smaller than about 20 microns depends on shape anisotropy.
• Isotropic point- the point where the sign of Kl is negative below, and positive above. For magnetite, this is ruffly 118K. In grains wher the remanence is controlled by shape anisotropy or stress anisotropy, no remanence should be lost upon cooling through the isotropic point.

### Domain Theory

• Domain- a subdivision of the magnetic material that has its magnetization oriented in a particular direction. A magnetic material can have one, several or very many domains.