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# Illustration 27.5: Permanent Magnets and Ferromagnetism

Low temperature:

This is a simplified model of permanent magnets called the Ising model. In this Illustration you can change the temperature and background magnetic field to see how these variables affect the production of permanent magnets. Restart.

To turn an ordinary nail into a magnet, you can put it in a magnetic field. The iron will be magnetized and it will retain its magnetization even when the field is removed. This model illustrates how that is possible. The red and green represent regions within a material with magnetic moments that are lined up in one direction (red) and the other direction (green). After you push "play," notice that, to begin with, there are essentially equal areas of red and green regions. This is recorded on the graph as a magnetization of about 0. This means that inside our iron there is no organization of the magnetic moments. The thermal energy available in the material allows for the changing between red and green that you see.

Put this material in a magnetic field (push the "B > 0" button). What happens? Now the magnetic moments (little magnets inside the material) are lined up with the applied field. What do you expect will happen if you push the "B < 0" button? Why? Try it.

Now, what do you expect will happen if you push the "B = 0" button? Try that. What happens? Does the magnetization go to zero right away? Even when the magnetic field is no longer in place, the magnets want to stay lined up. It takes energy to disorganize them again. Over a long period of time, they can get randomly oriented again, but they will line back up quickly with an external field. Verify this.

Another way to get the magnets randomly oriented again is to increase the temperature (give them enough thermal energy to destroy the order). To simulate this, first magnetize the material (either red or green), set the field back to zero and then push the "increase temperature" button.

Illustration authored by Anne J. Cox.
Script authored by Wolfgang Christian and modified by Anne J. Cox.

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