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.
> 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.