Mar 31 2009
Dia, Para, Ferro, & Antiferromagnetism
From a non-quantum perspective, there are only two ways to produce magnetic fields: using electric currents and having time-varying electric fields. Electric fields inside matter generally don’t vary in time with high enough frequencies to make their contributions to the magnetic properties of matter all that important, so electric currents are the main cause for matter’s magnetic properties, other than quantum effects. The primary quantum cause of magnetism is spin.
Given the above, there are several kinds of magnetic phenomena in matter:
a) Diamagnetism: because electrons move around the nuclei of the atoms they’re associated with, there are tiny currents inside of every atom. These currents cause a weak local magnetic field, which averages to zero over the large number of atoms in any chunk of matter. However, when you apply an external magnetic field to a chunk of matter, the orbital motion of electrons changes, resulting in a non-zero (but small) net magnetic field, which always tends to repel the applied external field. As a result, every
b) Paramagnetism: in some materials, the distribution of electrons in the material’s atoms is such that these atoms end up having a net magnetic dipole moment, that is, the atoms are really very much like tiny magnets of their own. However, these are usually randomly oriented and, as a result, a chunk of paramagnetic matter has no net magnetic field. When an external magnetic field is applied, these dipole moments align with the external field, producing a non-zero net magnetic field which is then attracted
piece of matter is diamagnetic, and the response is stronger the stronger the external magnetic field is, being essentially zero when there are no external magnetic fields being applied. to the external field. Just as with diamagnetism, the stronger the externally applied field, the stronger the response of the material is.
However, the paramagnetic response, when it exists, is far stronger than the diamagnetic response (which always exists). Moreover, because it has to do with the alignment of otherwise randomly oriented magnetic dipoles, the paramagnetic response is very susceptible to the temperature of the material.
(c) Ferromagnetism: Ferromagnetic materials (such as iron) have electrons in quantum states such that the electrons’ spin magnetic moments are aligned over extended regions of the material (called magnetic domains), even in the absence of an externally applied magnetic field. As a result, these magnetic domains have net magnetic dipole moments of their own. However, they need not be aligned, so a ferromagnetic material may have a zero net magnetic moment. When an external magnetic field is applied, these domain dipoles align themselves with the external field, creating a net magnetic dipole moment which is attracted to the external field.
So far, this looks a little like paramagnetism. The main difference, however, is that if one now removes the external magnetic field, the domains remain aligned with one another, so the material’s net magnetic moment is not zero. The material is now a “permanent” magnet.
Just as in the case of paramagnetism, ferromagnets are sensitive to temperature. Increasing the temperature eventually randomizes the magnetic domains’ dipole moments, resulting in the material ceasing to behave like a permanent magnet. This happens at a particular temperature for each material, called the Curie temperature.
d) anti-ferromagnetism: this is similar to ferromagnetism, but neighboring electron spins aren’t aligned in the same direction. Instead, they’re aligned like this: up-down-up-down, etc, rather than up-up-up-up etc or down-down-down-down etc, which is the case in ferromagnetism.
The relative intensities are as follows, from strongest to weakest response:
ferro > antiferro > para >> dia
(> meaning “stronger than”, >> meaning “much stronger than”)
As pointed out already, all materials are diamagnetic, some are paramagnetic, and some are ferromagnetic or anti-ferromagnetic (but not both at the same time).
