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Revision as of 04:21, 10 June 2003 by Stevenj (talk | contribs) (link)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)A ferromagnet is a piece of ferromagnetic material such as iron containing tiny magnetized crystals, called domains, that can be aligned by an external magnetic field from another permanent magnet or electromagnet so that the piece itself becomes a permanent magnet. The name derives from the Latin ferrum, meaning iron (the most well-known ferromagnetic material).
Physical origin
The property of ferromagnetism is due to the direct influence of two effects from quantum mechanics: spin and the Pauli exclusion principle.
The spin of an unpaired electron has a magnetic dipole moment and creates a magnetic field. A ferromagnetic material has many such electrons, and if they are aligned they create a measurable macroscopic field.
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.
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).
Many other interesting phenomena, such as hysteresis curves, are involved in the phase transitions of ferromagnets.
References
- Charles Kittel, Introduction to Solid State Physics (Wiley: New York, 1996).