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#REDIRECT ] |
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A '''ferromagnet''' is a piece of ] material such as ] containing tiny magnetized crystals, called ''domains'', that can be aligned by an external ] from another permanent ] or ] so that the piece itself becomes a permanent magnet. The name derives from the ] ''ferrum'', meaning iron (the most well-known ferromagnetic material). |
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{{Redirect category shell| |
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==Physical origin== |
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{{R from related word}} |
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}} |
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The property of ferromagnetism is due to the direct influence of two effects from ]: ] and the ]. |
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The spin of an unpaired ] has a magnetic ] moment and creates a ]. A ferromagnetic material has many such electrons, and if they are aligned they create a measurable macroscopic field. |
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However, according to classical electromagnetism, two nearby magnetic dipoles will tend to align in ''opposite'' directions (which would create an '''anti-ferromagnetic''' material). In a ferromagnet, however, they tend to align in the ''same'' direction because of the Pauli principle: two electrons with the same spin cannot lie at the same position, and thus feel an effective additional repulsion that lowers their electrostatic energy. This difference in energy is called the ''exchange energy'' and induces nearby electrons to align. |
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At long distances (after many thousands of ions), the Pauli exclusion is no longer significant, and the classical tendency for dipoles to anti-align takes over. This is why, in an equilibriated (non-magnetized) ferromagnetic material, the spins in the whole material are not aligned. Rather, they organize into ''domains'' that are aligned (magnetized) at short range, but at long range adjacent domains are anti-aligned. The transition between two domains, where the magnetization flips, is called a ''Bloch wall'', and is a gradual transition on the atomic scale (covering a distance of about 300 ions for iron). |
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Many other interesting phenomena, such as ] curves, are involved in the ] of ferromagnets. |
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====References==== |
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* Charles Kittel, ''Introduction to Solid State Physics'', (Wiley: New York, 1996). |
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