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COMPLAINT DEPARTMENT

You goofed again. Better crank up the insults to mask your incompetence--or enlist the aid of a real physicist. Aluminum does contain tiny magnets [September 7]. The reason it doesn't stick to a magnet is that the tiny magnets are jiggling around too much due to thermal motions. If the temperature were low enough, or if the magnet were strong enough, aluminum would be attracted just as strongly as iron.

The deal with iron and other ferromagnetic materials is that the "tiny magnets" are held in alignment by internal electric forces, so thermal jiggling doesn't disturb the alignment so much. There are some materials that are not attracted to magnets, but aluminum isn't one of them. Good luck and get smart! --Ted, Takoma Park, Maryland

It's always the way. Here I am, trying to ease my readers' burden by cleansing their lives of extraneous details, and some nitpicker comes along and says you forgot to talk absolute zero, Brownian motion, and the influence of William Faulkner. I figured I could skip a discussion of the paramagnetic materials, which are what you're talking about. As you know, all materials in some respects can be thought of as containing tiny magnets. (If nothing else, they contain electrons, the mother of all magnetism.) What I should have said, perhaps, was that aluminum doesn't contain tiny magnets of any practical consequence.

I did take simplification too far in one respect, though. I singled out unfilled inner electron shells as the key to iron magnetism. Important as these are, they're only half the story. The other half, however, has to do with quantum mechanical effects that even specialists have a difficult time explaining clearly. I figured if I kept mum about them nobody would notice. Silly me. So let's try again--and remember, you asked for this.

In a filled electron shell, the electrons spinning in one direction are paired up with electrons spinning in the opposite direction. Since magnetic polarity is determined by spin direction, "north-is-up" magnetism cancels out "north-is-down" magnetism and your overall magnetism is zip.

Iron and the other ferromagnetic materials, in contrast, have unfilled inner electron shells. The magnetism of the electrons doesn't cancel out and each atom as a whole is magnetic. But certain nonferromagnetic materials (chromium and manganese, for instance, although not aluminum) also have unfilled inner electron shells. Each atom of these substances is magnetic, but the substance as a whole is not. Why? Well, in chromium and manganese, each atom with "up" magnetism is paired with an atom of "down" magnetism, canceling out the magnetism of the substance as a whole. In iron, however, all the atomic magnets point in the same direction, so it does (or can) have magnetism overall.

What keeps all the iron atoms pointed in the same direction? It's something known as "exchange interaction." The details of this are still being debated, but here's one plausible view of what goes on: The laws of quantum mechanics require that electrons near each other have opposite spin. In iron the distance between atoms is such that the following occurs. Let's say the inner shell or "local" electrons of Atom A are spinning in such a way that they have "up" magnetism. The local electrons cause the nearby loose electrons floating around in the metal (the "conduction" electrons) to have opposite or "down" magnetism. The conduction electrons in turn cause the local electrons of neighboring Atom B to have "up" magnetism. Result: all the atomic magnets point up and the iron is potentially magnetic.

So why are chromium, manganese, et al different? It turns out the atoms in manganese and chromium are sufficiently close together that the local electrons of Atom A force the local electrons of neighboring Atom B to orient themselves in the opposite direction. Thus each "up" atom is paired with a "down" atom, and the material has no magnetism overall. Hope this covers matters to your satisfaction.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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