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May22-06, 06:17 AM
P: 2,056
Why does higher temperature and mechanical stress have a negative effect on the magnet's strength? Think of a collection of atoms as representing the magnets. They all have arrows on them pointing different ways, because each atom is a little magnet that works to create a bigger one. In a more perfect magnet, all the arrows are pointing the same way. In a not-magnet, the arrows are pointing all different ways. When a ferromagnet (permanent magnet such as on your refrigerator), the arrows jumble up and point different ways. Because heat is actually the microscopic shaking of atoms and molecules, it has the same effect as the macroscopic shaking of dropping the ferromagnet. Oh look, a picture.
The main implication of the domains is that there is already a high degree of magnetization in ferromagnetic materials within individual domains, but that in the absence of external magnetic fields those domains are randomly oriented. A modest applied magnetic field can cause a larger degree of alignment of the magnetic moments with the external field, giving a large multiplication of the applied field.

These illustrations of domains are conceptual only and not meant to give an accurate scale of the size or shape of domains. The microscopic evidence about magnetization indicates that the net magnetization of ferromagnetic materials in response to an external magnetic field may actually occur more by the growth of the domains parallel to the applied field at the expense of other domains rather than the reorientation of the domains themselves as implied in the sketch.