Rewrite the textbooks: New type of magnetism confirmed (Patreon)

[This is a transcript with links to references.]

As everyone knows, there are three different types of magnetism.  So I was a bit surprised to learn of a new, third type of magnetism.  Didn’t we already have three? What’s new? I had a look.

 The magnets that we all know and like are what physicists call ferromagnets.  Materials with these properties include most famously iron,  which the name “ferromagnet” derives from, as well as nickel, cobalt  and many alloys.

When it comes to the alloys it’s apparent already just why magnetism is so confusing.  Take steel as an example. Depending on the crystal structure and fabrication some types of steel are magnetic, others not, and some are magnetic in some places.  Though none of them will stick to your forehead after vaccination.

Contrary to popular belief, magnets also don’t stop working if you put them into water,

though I’m sure what Trump had in mind there was electromagnets which I indeed recommend you don’t throw into water, or if you do, at least don’t sit in the water at the same time.

Where does magnetism come from? Magic! Well, the electromagnetic interaction is one of the fundamental forces of nature.  It doesn’t come from anything, at least not that we know, it’s rather that other things come from it, for example, fridge magnets. Or, as Richard Feynman put it  

But we can understand, to some extent, how magnetism in materials comes about from this fundamental magnetic force. It’s that atoms all have electrons hovering around them,   and that makes each atom a tiny little magnet, where the strength depends on the type of atom.  The question is then how these individual magnets combine in materials. In ferromagnets what happens is that the magnetic moments of neighbouring atoms like to align, and they hold onto this alignment. This is why you can “magnetize” these materials.

In most materials that doesn’t work because the orientation of the tiny atomic magnets won’t stay aligned with the neighbours.  But even ferromagnetic materials will loose their magnetisation if you heat them up too much. That’s because then the atoms shake around too much basically. So maybe Trump was speaking of boiling water? I’m sure that’s what he must have meant.

In any case, that’s ferromagnetism,  which most of you will be familiar with. The second kind of magnetism is called diamagnetism. Diamagnetic substances react to magnetic fields by weakly repelling them.

An interesting example of a diamagnetic substance is water.  You can see this in this little demonstration in which the magnet seems to attract the air bubble, though actually it repels the water. Diamagnetism had its moment in the sun when a group of scientists demonstrated it by making a frog levitate. Since humans like frogs are basically big bags of water, I supposed you could levitate humans the same way if only the magnetic field was strong enough. You know I think if CERN said they want to build a magnet strong enough to levitate humans I’m pretty sure they’d have an easier time getting money.

So we have ferromagnetism and diamagnetism  and then there’s paramagnetism.  Paramagnetism is similar to diamagnetism in that it’s a response to magnetic fields, but paramagnets are attracted to rather than repelled by the magnetic field. Some paramagnetic materials are oxygen, aluminium, and platinum.

That makes ferromagnetism, diamagnetism, paramagnetism.  By my count that’s three.

And then I read this press release which says that a group of experimentalists in Switzerland  has experimentally proved “a third branch of magnetism, termed altermagnetism.”  Alright, but we already have three. Right?

I read the paper and they count,  ferromagnetism, anti-ferromagnetism, and altermagnetism. Oh dear! Don’t despair, we can sort it out.

Remember that in an ferromagnet the atomic magnets react strongly to each other and they like to align.  In an anti-ferromagnetic material, they also react strongly to each other, but they don’t want to all align, so you get a mix of one and the opposite direction. This happens for example in chromium. An anti-ferromagnetic material then has no total magnetization and won’t hold stuff to your fridge,  but they’re good at holding orientations, which is why they’re used in computing for storage and for other things.
 The altermagnetism now is a new type of magnetism in which the directions of the atomic magnets alter in periodic configurations, creating a local polarization of the field, but no total magnetisation.

So these altermagnets might appear to be anti-ferromagnetic, but the point is that they have these interesting stable structures in the polarization. What they did in the new papers is that they confirmed for the first time that some types of crystals actually have this altermagnetism.

It's been there all along, basically, it’s just that no one knew it existed.

Why is this interesting? Well first of all it’s fascinating  that they discovered a completely new class of materials. And it’s not even stuff that needs to be synthesized in complicated ways. One of the compounds they used is Ruthenium oxide which is maybe not exactly on your friend list,  but popular enough to have a Wikipedia page and be  available for order online.

But besides the general interest these altermagnets  might be useful because their alternating patterns can be used to encode and transport information, and that could be handy for electronics, maybe even for quantum computing.

And just in case you though you understood it, there’s a thing called ferrimagnetism…