| MadSci Network: Physics |
Dear Matt,
Well, you certainly know how to ask a very difficult question! The theory of magnetism is quite complex, and indeed, different materials exhibit different magnetic behavior, which, as you say, has to do with the way spins align in the material. Now, a real formally correct description of magnetism is so horribly complicated I'm not going to try to explain it here, also because I don't quite know myself. But we can look at the basic physics that causes magnetism, namely how these spins align in various materials and see what forms of magnetism result from that. But first, let's take a look at how electron spin causes magnetism in the first place.
An atom consists of a core that is surrounded by electrons1. These electrons have a property called spin. You can think of this as the electron spinning around its axis. Now, the funny thing about this spin is that it always has the same magnitude. The electrons, however, are not allowed to spin freely-the axis of spin is the same for all the electrons in an atom, and they can spin clockwise ("down") or counterclockwise ("up") around this axis. There is another rule these electrons have to obey: all but a few are paired, meaning that a spin down and a spin up electron act as a pair that has a net spin contribution of zero2. The core has a spin too, but the effect of this spin is a lot smaller3. Here, too, the individual protons and neutrons align in a way similar to the way electron spins line up. Although this certainly has applications, for instance in MRI3, this is not the type of magnetism that I will discuss here, mainly because the effect drowns in the effect of the electron spins. It turns out that the strength of the magnetic field of a particle such as an electron or a proton is inversely proportional to its mass. Hence, the light electrons produce a much larger magnetic effect than the heavy protons and neutrons in the core.
As said, in most substances, nearly all the spins align. Now, imagine a substance in which all electrons are neatly paired, one by one4. The net magnetic field produced by the spin of the electron is now zero. However, the electrons not only spin, they also orbit around the nucleus of the atom, and this produces its own very small magnetic effect. You can think of them as little loops of electric current-just like an electromagnet5. If an external magnetic field is applied to this material, these little currents will tend to align in such a way that the magnetic field they produce is opposite to the external field. (You can think of this as a result of Lenz' law, only on an atomic scale6). This effect is called diamagnetism.
Examples of diamagnetic materials include water, nitrogen, hydrogen, gold, mercury, and antimony. These are materials you might have thought are nonmagnetic. This is because diamagnetism is a very, very weak effect, typically hundreds of thousands times weaker than the strength of the external field. The strongest diamagnetic effects are seen in bismuth and pyrolytic graphite4. By the way, a superconductor is a perfect diamagnet. This means that many materials can be made into diamagnets, provided you cool them enough. A consequence of this diamagnetism is that magnetic fields cannot penetrate a superconductor. This is called the Meissner effect.
In 1997, researchers from Nijmegen University7 took a very, very strong magnet and put a tiny frog in it. Because the magnetic field of the big magnet was so strong, the weak diamagnetic field of the frog (or rather, the water in the frog) was strong enough to levitate the frog. You can see a picture here8. I wonder what the little frog was thinking. One implication of this is that the "anti-grav" of science fiction is possible-if you have a very, very strong magnet.
In paramagnetism, not all the spins of the materials line up-there are unpaired electrons9. These unpaired spins act as little magnets. In most materials, the little magnets move around chaotically, so the net magnetic field of the material is zero. However, when an external magnetic field is applied, the magnets align, this makes the paramagnetic material have a magnetic field of itself. The strength of the paramagnetic field is typically proportional to the strength of the external field that causes it10.
Now, paramagnetism is a pretty weak effect too, typically about a hundred times stronger than diamagetism. Common paramagnetic materials include oxygen (you can do very cool things with liquid oxygen and a magnet), aluminum, platinum and barium9.
The most impressive form of magnetism is ferromagnetism. It is only exhibited by a few pure materials (iron, nickel, cobalt, gadolinium, dysprosium), and several alloys, in particular of the rare earths. In a ferromagnet, there exists a long-range ordering which causes the spins of many electrons to line up. This gives small islands of material that have the same orientation of the magnetic field. These islands are about a millimeter in size, and are called magnetic domains. Now, while these islands have a directed magnetic field, in a normal chunk of iron, there are many such islands, and again, the net magnetic field cancels out. However, if a small magnetic field is introduced, the magnetic domains align, and a strong magnetic force results. What is unique about ferromagnets is that the produced field is actually much, much larger than the field that is causing it, often thousands of times stronger. This is in contrast to dia- and paramagnetism, where the field produced in the material is much smaller than the external field.
For this ordering to exist, a ferromagnet needs to have a temperature that is below a certain threshold called the Curie temperature13. If the temperature is higher, the effect is ruined by the thermal motion in the material. Because this temperature is well below the melting point of the ferromagnets, there are no liquid or gaseous ferromagnets-although you can make a ferromagnetic "liquid" by creating a suspension of ferromagnetic powder in a liquid.
Permanent magnets are magnets in which the magnetic domains are permanently aligned in a way that gives a net magnetic field. They can for instance be made by putting a ferromagnet in a magnetic field; depending on the material, it might retain some or all of the ferromagnetic field. An alloy called Neodymium Iron Boride is particularly good for making permanent magnets, and it can retain a very strong magnetic field after being magnetized.
In short, it's not only the electron and nuclear spins that determine magnetism, but especially how they are ordered. Depending on this ordering, various materials, some in other ways quite alike, may produce very different magnetic effects14. I hope this sheds some light on your question.
Regards,
Bart Broks.
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