A new form of magnetism could allow for more powerful memory devices

New research reveals there are three types of magnetism, not two, and the new one could be in high demand.

To the first users of compasses, magnets must have been a form of magic. Just when we started to think we had mastered the traditional form known as ferromagnetism, a new one was discovered, antiferromagnetism. Now it seems that our descriptions of these also fall short with evidence of a third type, altermagnetism. To the question: “Magnets, how do they work?” scientists have not lied, but their answers are incomplete.

Magnetism, if not caused by large-scale electric currents or changing fields, is a consequence of electron spins. Spinning here does not literally mean that electrons rotate like planets, but refers to subatomic behavior of the same name. Orbiting electrons have magnetic moments caused by their spins, but these are usually randomly arranged. With unimaginably large numbers of electrons in even a small item, everything falls away, or becomes so close that it doesn’t matter.

However, in rare cases the magnetic moments are aligned enough to produce a large-scale magnetic field. This is only possible in a few elements, most famously iron, but when it happens, the well-known attraction of the magnet on the (magnetic) north pole or of the magnet on the refrigerator is created.

Antiferromagnets, discovered in 1933, usually require low temperatures and involve patterns in which atoms have magnetic spins opposite those of their neighbors. Without an external magnetic field, antiferromagnets behave like any other material. However, when exposed to this, the atoms’ magnetic moments lie in the opposite direction of their neighbors, rather than randomly or in a common direction like ferromagnets. This does not lead to a net contribution to the field of the material, but can lead to sharp changes in conductivity under varying conditions, which may have applications.

If materials can fall in line with the commands of magnetic fields, reject them by changing direction, or ignore them completely, what’s left?

Surprisingly, alter magnets provide an answer. At first glance, alter magnets appear to be just another form of antiferromagnets. Their internal spins also go in the opposite direction of their neighbors, producing no net effect. The patterns repeat through rotational symmetry rather than translational symmetry, meaning that instead of neighboring atoms canceling each other out, they create a spin polarization that can lead to alternating bands.

The resulting properties could provide a middle ground between the two previously known types of magnets, and could prove very useful in making magnetic memory recorders much more powerful than existing ones.

This would be done through what is known as spintronics – which uses the spin states of electrons, as well as their charges, to transport information. Spintronics has been worked on for years, and sometimes also hyped, using ferromagnets. However, the bulk magnetism offered by ferromagnets makes it difficult to scale up spintronic devices, because the bulk magnetic field disrupts the spin of its own components and that of any neighbors. Antiferromagnets avoid this, but their lack of spin-dependent effects prevents some of the behavior sought.

Theories proposing alter magnets led physicists to wonder whether they could represent the Goldilocks zone of spintronics, and to seek confirmation that they do indeed exist.

Now that confirmation has been found, some are excited enough to use another M-word. “That is the magic of alter magnets,” said Professor Tomáš Jungwirth of the Czech Academy of Sciences in a statement. “Something that people thought was impossible until recent theoretical predictions are actually possible.”

In 2019, Jungwirth helped identify a class of magnetic materials that did not fit into existing categories and transport internal electric currents without an external magnetic field. Three years later, he was part of a team that revealed more than 200 candidates for altermagnetism in various material classes. Nevertheless, there is a wide gap between suspicion and proof, and confirmation of altermagnetism was lacking at the time.

However, after bathing crystals of manganese telluride, previously considered an antiferromagnet, in X-rays, Jungwirth and coauthors announced that it consists of electronic bands with alternating spin states, the key characteristic of an altermagnet.

Technically, the X-rays revealed that manganese telluride exhibits “Lifted Kramers Spin Degeneracy” of the magnetic order. Despite sounding like the levitation ritual of a pseudo-medieval cult, this is actually a heavily studied feature of ferromagnets that was expected in altermagnets but does not appear in antiferromagnets.

Comparison of ferromagnetism, antiferromagnetism and altermagnetism and when the nature of each was explained.  The difference between translational and rotational symmetry may seem abstract but it produces a fundamental difference between antiferromagnetism and altermagnetism

Comparison of ferromagnetism, antiferromagnetism, and altermagnetism, and when the nature of each was explained. The difference between translational and rotational symmetry may seem abstract, but it yields a fundamental difference between antiferromagnetism and altermagnetism.

Image credits: Libor Šmejkal

In addition to advancing our understanding of magnetism and the potential applications it could yield, Jungwirth and colleagues think their work could provide new insights into superconductivity.

‘Altermagnetism is actually not something very complicated. It is something completely fundamental that lay before our eyes for decades without noticing it,” says Jungwirth. “And it’s not something that only exists in a few obscure materials. It exists in a lot of “crystals that people just had in their drawers. In that sense, now that we’ve brought it to light, a lot of people around the world will be able to work on it, which has the potential for broad impact.”

The research will be published open access in Nature.

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