To the parent and its sibling comments: There is no atomic or subatomic current that can explain ferromagnetism in any approximation.
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.
> 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 the same way that a geodesist navigates using a reference ellipsoid defined by WGS-84, while a city commuter uses Cartesian coordinates on a flat map. The commuter's navigational tool will never work in geophysics research, and it doesn't need to be.
> To the parent and its sibling comments: There is no atomic or subatomic current that can explain ferromagnetism in any approximation. [...] 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.
I consider "circulating currents create ferromagnetism" to be as true as "an atom's structure is similar to a solar system." Both concepts break down when it's examined in details, so its use by research physicists is obviously unacceptable, but I consider it's nevertheless as an useful mental image in introductory discussions among non-physicists.
Would you consider Rutherford's original atom model to be a first approximation? Can it be considered a very oversimplified but useful heuristic, at least when people who know anything about atoms are first introduced to this concept? Alternatively, would you consider Rutherford's atom to be "an explanation attempt that fails spectacularly if you actually try to do the math (which gives an electron that collapses into the nucleus in picoseconds, as Rutherford's colleagues quickly found out)?
If you believe the latter case, everyone can stop this conversation right now. Because it means the entire disagreement is entirely down to what kinds of "metal images" are acceptable, rather than any factual, like "whether a full quantum treatment of ferromagnetism is necessary to completely explain ferromagnetism (of course it is)." The rest of us who don't solve research problems believe a toy model is still interesting, but don't deny (nor mention) better models. You, as a professional physicist, believe many "what if?" metal models from history are just not legitimate physics, and should not be mentioned at any circumstances to avoid poisoning the minds of youths - an approach known as Whig history, in which scientific progress marches from one victory to another, and all losers be damned - a perfectly valid approach for teaching physics to students who only care about pure physics science, instead of "who said what."
As a side note, I know some engineers who really hate the idea that electric circuits works due to an electron flow. The most extreme one I've seen of wanted to ban this concept in introductory textbooks, calling it a big lie (an explanation attempt that fails spectacularly if you actually try to do the math, which gives the speed of an electron 30 billion times slower than the speed of light in free space). As we all know, the steady-state electron flow was only a result of the transient propagation and reflection of electromagnetic waves in free space or dielectric materials. Thus, they believe the wave model should be the only interpretation in a science textbook, since "they're high-school teachers, I'm a design engineer who work with high-speed digital systems with 20 years of experience, and I know for sure that high-speed circuits and computers can't even be made functional if you ignore fields and transmission line effects." Meanwhile, I believe the electron flow model still works as an introductory mental image (although the field view perhaps needs to be mentioned earlier).
> 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.
The earlier "electron as a rotating ball" idea was considered by Ralph Kronig and Uhlenbeck-Goudsmit in 1925. Pauli personally never accepted it due to its unphysical flaws. Only in 1927 did Pauli publish a rigorous QM treatment. Thus, "electron spin using classical rotation as analogue" was still an intermediate step before establishing this concept in QM. It was a footnote in history since Pauli was a great physicist and already considered the problem himself earlier and found the solution before everyone else. Otherwise this intermediate step may last longer than 2 years.
> 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.
This, in comparison, is a more interesting criticism.
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.