| You said, > Ferromagnetism has nothing to do with currents This is why I said ferromagnetism is circulating current in the sense of "to a first approximation" and "heuristically". Wiktionary defines "heuristic" to be: > a practical method [...] not following or derived from any theory, or based on an advisedly oversimplified one. I think that if you ask Feynman, he would probably agree or sympathize with the naive idea of "atomic currents" as a heuristic argument in the introduction of this topic... which is nothing new anyway, and has been a heuristic argument used in electromagnetism for a long time, at least before QM. In Feynman's own words, > These days, however, we know that the magnetization of materials comes from circulating currents within the atoms—either from the spinning electrons or from the motion of the electrons in the atom. It is therefore nicer from a physical point of view to describe things realistically in terms of the atomic currents [...] sometimes called “Ampèrian” currents, because Ampère first suggested that the magnetism of matter came from circulating atomic currents. You said, > Spin is a type of intrinsic angular momentum that is not associated with any spatial motion. Yet the concept of spin in quantum mechanics was originally developed using macroscopic rotations as an analogy, although today we know that spin is an intrinsic property of subatomic particles (thus the joke, "Imagine a ball that is spinning, except it is not a ball and it is not spinning.") In the same sense that Ampère's concept of "atomic currents" was developed using circulating electric current as an analogy. > The Feynman lecture you linked to is an explanation why currents fails to explain ferromagnetism. You need to read the next chapter. Of course, "The actual microscopic current density in magnetized matter is, of course, very complicated." This is surely explained in the next chapter. I could've mentioned "atomic currents" without citing any link, but I included it to allow anyone who's interested to read the whole thing in context. |
You read some Wikipedia pages and Feynman lectures of physics. I'm a physicist who has done well over a decade of research in magnetic materials.
In understanding of ferromagnetism, many incorrect theories have been proposed. By connecting ferromagnetism to circulating currents (i.e, paramagnetism and diamagnetism), you just repeated the same mistake.
You're trying to bend the words to avoid being wrong. Physics is not philosophy or debate club. There is no approximation in physics in which electron is a ball with some radius, or its spin is due to a circulating current in physics. Any such explanation attempt fails spectacularly if you actually try to do the math (which gives an electron surface that is moving faster than speed of light, as Uhlenbeck/Goudsmit who proposed this incorrect idea quickly found out), so it doesn't even work as an approximation of any kind.
> Yet the concept of spin in quantum mechanics was originally developed using macroscopic rotations as an analogy,
Who developed this theory in quantum mechanics, where and when? Pauli, who first introduced it into quantum mechanics and the namesake of spin 1/2 matrices, insisted that it is purely quantum mechanical with no classical analogue. And regardless of who said what over 100 years ago, today, it is well understood that spin has nothing to with electric charges that move or rotate in space.
More importantly, the reason ferromagnetism develops in the first place is due to exchange interaction (as I wrote above) between magnetic moments, which is due to Pauli exclusion principle and also has nothing to do with movement of charges.
Furthermore, such magnetic moments (called magnetic impurities in that context) ruin the superconducting order by breaking the time-reversal symmetry, so trying to make a connection to ferromagnetism in the context of superconductivity is even worse.