Neodymium magnets are the main type used in modern motors. Why are they good? Are there any good alternatives?

review of ferromagnetism

Magnetic fields contain energy. In an inductor, that energy comes from an increase in voltage when current is first applied. When a magnetic core is added to an inductor and a stronger field is produced from the same added energy, that extra energy has to come from somewhere.

The energy that ferromagnetic cores add to magnetic fields comes from their crystal structure fitting together better in a magnetic field. This implies that ferromagnetic cores should spontaneously magnetize to some extent, and they actually do; it's just that the spontaneously generated magnetic fields are curled into microscopic 3d loops. The microscopic internal field strength is approximately the saturation field of a ferromagnetic material, which is often greater than the field generated by a Nd magnet. Applying an external magnetic field causes those microscopic magnetic loops to partly unroll.

The actual field is generated by unpaired electrons of atoms; individual electrons are very magnetic. But ferromagnetism isn't a property of atoms, it's a property of crystals; without particular crystal structures that favor magnetic fields, those unpaired electron spins of iron atoms would just cancel out. For example, stainless steels contain a lot of iron, but most aren't ferromagnetic.

Atoms of crystals fitting together better in a magnetic field implies that iron cores slightly change shape when a magnetic field is applied. This effect is responsible for the humming noise transformers make, and has been used for eg sonar.

common misconceptions

Fucking magnets, how do they work? And I don't wanna talk to a scientist. Y'all motherfuckers lying, and getting me pissed.

— Insane Clown Posse

The Insane Clown Posse is sort of right there: a lot of explanations of magnets given to people by teachers and media scientist-figures have been partly wrong.

Magnetic flux was originally thought to be a flow of something like electric current, with ferromagnetic materials having lower resistance for that flow than air. It's even still taught that way sometimes. But no, it's a complex emergent phenomenon.

I remember being taught that "iron is magnetic because it has an unpaired electron". But again, ferromagnetism is a property of crystal structures, not atoms or elements.

A lot of people think the magnetism of neodymium magnets comes from the neodymium, but the actual magnetism comes from the iron in them.

The title of the quoted song is "Miracles". The physical constants that allow for the complex emergent phenomenon of ferromagnetism are the same physical constants that allow for the complex emergent phenomenon of life; most values of them wouldn't do either. The universe having values allowing for those is indeed a miracle that nobody really knows the reason for; thanks for reminding us of that, ICP.

neodymium magnets

In permanent magnets, the crystal structure is such that the magnetic fields of crystals can't rotate around to form closed loops very well.

Neodymium magnets (Nd2Fe14B, "Nd magnets") are the strongest permanent magnets currently available. Looking at the crystal structure we can see rings of iron atoms with Nd in the middle and some boron at the 3-way vertices. When a magnetic field is applied through that (tetragonal) pattern, the atoms fit together better. You can see how the magnetic field would be unable to smoothly rotate through directions.

Strong Nd magnets are made by cooling inside a strong magnetic field, so that the crystal structures are aligned in one direction.

alternatives

An obvious idea is using the same structure but replacing the neodymium with a different element. That's obvious enough that people tried everything, and neodymium is the best option.

Or, you can go for a different crystal structure. You need something that's different in 1 direction than the other directions. Considering the common crystal structures and the goal here, tetragonal and hexagonal structures are the obvious options.

One of the best permanent magnets besides Nd magnets is SmCo, which is hexagonal. SmCo magnets are weaker at room temperature, but stronger at higher temps, so they're better for a few applications. They require a lot of samarium so they're even more expensive than Nd magnets.

FeCo isn't a good permanent magnet, but has a really high saturation field. Perhaps FeCo with some sort of microscopic structure that prevents field rotation could be a good permanent magnet? Yes, actually; the magnetism of Alnico magnets comes from needle-shaped FeCo nanoparticles in a non-magnetic nickel-aluminum matrix. But of course, Nd magnets are much stronger.

If FeCo in a non-magnetic matrix is a decent magnet, then logically, a magnetic matrix would be better. So some people have worked on FeCo / Nd magnet nanocomposites, and the results seem similar to Nd magnets, with less Nd needed. But manufacturing such structures is currently impractical.

There's been some research interest in iron nitride magnets recently. Fe16N2 is quite magnetic, noteable for its high saturation flux. But it's also metastable, with the desired structure decomposing around 212 C. To compact it into fairly-dense magnets at 200 C, you need extremely high pressures - impractical pressures, like 10,000+ atmospheres. And the result is still worse than Nd magnets. If the goal is to reduce costs, that doesn't seem very good, but the lab-scale results have been good enough to get funding.

So, is something new going to replace Nd magnets? I'm not optimistic about that at this time. If they do get replaced, I think it'd involve a new manufacturing technique producing some good microstructure, rather than discovery of some magic ratio of elements that naturally produces better results with an existing process. In the meantime, what I expect to happen is more mining of neodymium for magnets for motors.

alternate alternatives

If developing good permanent magnets that don't need Nd isn't feasible, another option is finding ways to avoid using permanent magnets.

High-performance electric motors these days are all using permanent magnets, but switched reluctance motors aren't bad. There are also hybrid designs using some permanent magnets but less than current motors. Research into electric motor configurations is still ongoing in 2024, believe it or not. Anyway, switched reluctance motors aren't currently quite as good, but they work well enough. (On the other hand, induction motors are now obsolete, and Tesla Motors was dumb for using them.)

Superconductors are good at producing persistent magnetic fields, so they can be used instead of permanent magnets. But of course, they're expensive, and they have to be kept cold, and they have some losses when their current changes. See also my previous post on flux pumping. Maybe that will end up being a viable option for very large electric motors. Or maybe someone will find a better way to make YBCO wire that research has missed so far, but that seems less likely.