Permanent magnets still an open problem

[jostpuur]

Physicists often speak about permanent magnets as if there was a theory that explained why they exist, and this theory is often assumed to be the Ising model or something very much related to the Ising model.

However, everybody knows that the Ising model has a severe issue with the artificial assumption that the microscopic magnets (the spin states of the lattice points) would tend to point in the same direction with their neighbors, only being disturbed by randomness from the surrounding heat. This assumption is of course wrong under the ordinary assumptions of electromagnetism, because in reality small magnets tend to point in the opposite directions with their neighbors. That is how the magnetic energy is minimized.

The purpose of science is not only to produce as much knowledge as possible, but also to maintain a clear line between what is known and what is not known. Despite this, physicists usually seek to deceive others into believing that a theory for permanent magnets would exist. Since the fact is that the physicists don't have a theory for permanent magnets, wouldn't it be fully reasonable to just frankly admit it?

I wouldn't see it awfully embarrassing to admit that permanent magnets are still an open problem. You known, sometimes it happens that something looks simple at a first glance, but turns out to be more complicated when examined more closely.

 

[mrspeedybob]

in reality small magnets tend to point in the opposite directions with their neighbors.

This was not my experience playing with small magnets as a child.

 

[jostpuur]

This was not my experience playing with small magnets as a child.

This left me scratching my head a little bit, but I think I realized where the interpretation issue is coming from. I meant that if you try to put magnet bars side by side, like the lengthy sides close to lengthy sides, then they will tend to point in opposite directions, right? Were you thinking about the case where you put magnet bars into a train like configuration, making the chain look narrow and long like a train? I see that then they will tend to point in the same direction, yes. The Ising model explanation for permanent magnets will require that the small magnets would tend to point in the same direction also when side by side.

 

[mrspeedybob]

OK, I see what you're saying and I think we're on the same page.

What you are describing is why random chunks of iron do not spontaneously self-magnetize. If you put a piece of iron in a magnetic field, though, now some of those tiny little magnetic particles find a lower energy state by aligning with the external field then by opposing their neighbors, especially if you heat it up so that the particles can move around more. When you remove the external field, some particles flip back to their previous orientations and oppose their neighbors, but some get stuck. It is, after all, a solid. Particles cannot move around very freely at all. If enough get stuck to add up to a detectable macroscopic field, then you have a permanent magnet.

If you heat up a permanent magnet to the point where the individual magnetic particles can get unstuck, then you demagnetize it.
https://en.wikipedia.org/wiki/Curie_temperature

 

[Dale]

the artificial assumption that the microscopic magnets (the spin states of the lattice points) would tend to point in the same direction with their neighbors, only being disturbed by randomness from the surrounding heat. This assumption is of course wrong under the ordinary assumptions of electromagnetism

No, the assumption is correct for materials where the exchange interaction is stronger than the magnetic interaction.

While it is indeed important for scientists to admit our current theoretical and experimental knowledge, it is also important to not assume that your personal ignorance implies that the scientific community as a whole is also ignorant. I don't think that your characterization is correct in this case.

 

[jostpuur]

https://en.wikipedia.org/wiki/Exchange_interaction

Both bosons and fermions can experience the exchange interaction. For fermions, it is sometimes called Pauli repulsion and related to the Pauli exclusion principle. For bosons, the exchange interaction takes the form of an effective attraction that causes identical particles to be found closer together, as in Bose-Einstein condensation.

I had never before heard anyone claiming, that bosons would have a habit of attracting each others due to being bosons. I knew that there is an approximation which tells that fermions can be thought to behave like repelling each other, but that bosons would attract each others? How is that supposed to work? If particles are bosons, it means that (while ignoring all other interactions between them and while for approximation assuming that the bosons are somewhat independent) the bosons are allowed to occupy the same states defined by some background potential. But if they are allowed to occupy the same states, it doesn't mean that they would attract each others to the same states!

If two bosons happen to hit different states, that state of affairs alone is not going to increase any energy term, right? I mean that if one boson is on some energy level E_1, and another boson in one some higher energy level E_2, the total energy is not going to be E_1+E_2+\Delta_{\textrm{boson}}, where some \Delta_{\textrm{boson}} >0 would come from the particles being bosons, the energy is just going to be E_1+E_2? And if the energy is E_1+E_2 in the above situation, then, if the two bosons are on the same energy level E_1, similarly the total energy is going to be 2E_1, right? Not something like 2E_1-\Delta_{\textrm{boson}}?

The claim on that Wikipedia page almost sounds like some kind of \Delta_{\textrm{boson}} term should be present in the total energies, but I'm having difficulty believing that. I don't remember anything like that from ordinary quantum mechanics material.

 

[rumborak]

I may be entirely misunderstanding the topic here (especially since now bosons seem to have entered the picture, which I really don't see the connection to magnetism to), but I certainly have never seen any assumption of uniformity inside magnets.

In fact, one of the experiments in my Master's was to watch magnetic domain walls move under external magnetic field influence. A magnet is composed of myriad of those domains, and each of them points in a different direction.

https://en.m.wikipedia.org/wiki/Magnetic_domain