Scientists Discover a Newly Identified Form of Magnetism: Altermagnetism

There is now a new addition to the magnetic family: thanks to experiments at the Swiss Light Source SLS, it has been possible to confirm the existence of altermagnetism. The discovery of this new class of magnetic materials is published in Nature and opens up new realms of physics for spintronics.

Magnetism is a lot more than just things that stick to the refrigerator door. This understanding came with the discovery of antiferromagnets nearly a century ago. Since then, the family of magnetic materials has been divided into two fundamental phases: The known for several millennia ferromagnetic branch and the antiferromagnetic branch.

The new type of magnetism called altermagnetism was discovered in the SLS, with the help of the PSI and the Czech Academy of Sciences.

The basic magnetic phases are characterized by the certain orientations of the magnetic moments, or electron spins, and of the atoms that host the moments in a crystal.

Ferromagnets are the type of magnets that stick to the fridge: here spins point in the same direction, giving macroscopic magnetism. In antiferromagnetic materials, spins are aligned in such a way that they do not exhibit any macroscopic magnetization and do not cling on the fridge. While other types of magnetism have been classified including diamagnetism and paramagnetism these are responses to external magnetic fields as opposed to self generated orderings in materials.

Specifically, the arrangement of spins and crystal symmetries in an altermagnet is different from that in the majority of conventional systems. The spins are antiparallel as in antiferromagnetic compound and no net magnetic moment thus there is no net magnetization. Instead, when interacting, the symmetries lead to electronic band structure with the strong spin polarization and its direction shifts as you cross the bands of the material—thus, it called altermagnets. This leads to enhanced properties more akin to ferromagnets and some properties entirely separate from them.

A new and useful sibling

This third magnetic sibling provides specific benefits for the emergent area of next-generation magnetic memory, called spintronics. Unlike electronics that uses only the charge of electrons, spintronics utilizes the spin state of electrons in processing information.

While spintronics has been hailed for the past few years as the next big thing in IT, it is still in its early stages. Normally, ferromagnets have been employed for such devices because they exhibit some highly desirable strong spin-dependent physical effects. However the macroscopic net magnetization that is useful in various other applications presents practical constraints in the miniaturization of these devices as it leads to crosstalk which is interfering information carrying elements in data storage known as bits.

Antiferromagnets have more recently been considered for spintronics because they are free from net magnetization and are ultra-scalable as well as energy efficient. However, the strong spin-dependent effects, which are advantageous in ferromagnets, are missing, thus limiting their practical use.

Here enter altermagnets with the best of both: Zero net magnetization coupled with the desirable strong spin-dependent effects that are characteristic of ferromagnets are two features that were believed to be fundamentally mutually exclusive.

‘That’s the magic about altermagnets,’ explained Tomáš Jungwirth from the Institute of Physics of the Czech Academy of Sciences, the study’s principal investigator. “This is how, something that people believed was impossible until recent theoretical predictions [showed it] is in fact possible. ”

The search is on

Murmurings that a new type of magnetism was lurking began not long ago: The new class of magnetic compound was discovered by Jungwirth with his theoretical partners working with Prague-based Czech Academy of Sciences and Mainz-based University of Mainz in 2019.

Therapists also established that altermagnetism would exist in the year 2022. They figured out that over two hundred altermagnetic candidates were found in insulators semiconductors metals and superconductors. In fact, a majority of these materials have been well researched and known in the past, yet people never knew they possessed the ability to alter magnetic properties. Since altermagnetism encompassed extensive research and application potentials, these predictions stirred much interest among researchers. The search was on.

X-rays provide the proof

To prove the existence of altermagnetism, it was crucial to achieve a direct experimental evidence of altermagnets’ spin symmetry differences. The proof was provided through spin- and angle resolved photoemission spectroscopy at the SIS (COPHEE endstation) and the ADRESS beamlines of the SLS. This technique enabled the team to visualize a tell-tale feature in the electronic structure of a suspected altermagnet: the splitting of the electronic bands belonging to various spin states and is referred to as Kramers spin degeneracy breaking.

This was in crystals of manganese telluride, a substance which is a simple two-element compound that has been extensively studied. From a historical perspective, the material has been classified as an archetypal antiferromagnet due to the opposite alignment of the Mn moments that results in a net magnetization of zero.

However, antiferromagnets should not have a lifted Kramers spin degeneracy by the magnetic order while ferromagnets or altermagnets should. When the scientists observed the Kramers spin degeneracy lifted with the net magnetization reduced to zero, they realized that they were dealing with an altermagnet.

“Thanks to the high precision and sensitivity of our measurements we could detect the characteristic alternating splitting of the energy levels corresponding to opposite spin states and thus demonstrate that manganese telluride is neither a conventional antiferromagnet or a conventional ferromagnet but it belongs to the new altermagnetic branch of magnetic materials”, explains the first author of the work Juraj Krempasky from the group of beamline optics in the Paul Scherrer Institute.

The beamlines that enabled this discovery are now disassembled, awaiting the SLS 2. 0 upgrade. After twenty years of successful science, the COPHEE endstation will be fully incorporated into the new ‘‘QUEST’’ beamline. “These experiments were conducted just using the last photons of light available at COPHEE, and that such a significant scientific discovery was made is very heartwarming for us,” Krempasky added.

“Now that we have brought it to light, many people around the world will be able to work on it.”

The researchers are optimistic that this new fundamental finding involving magnetism will expand the knowledge of condensed-matter physics and has implications in a variety of fields of study and applications. Besides its numerous benefits to the emerging field of spintronics, it may also serve as a promising model in which to search for exotic forms of superconductivity, namely, those that may appear in various magnetic materials.

When asked about Altermagnetism, Jungwirth responds, “It is not as complex as it seems. It is something fundamental that was right in our face for decades and we simply never saw it. ”“It is not one of those phenomena that is just described in a couple of rare publications. It was there in many crystals that people just kept in their drawers. This is why, now that we have given it so much publicity, many people around the world will be able to work on it, which promises the field a rather wide scope.

Reference: Juraj Krempaský, Altermagnetic lifting of Kramers spin degeneracy, Nature (2024). DOI: 10.1038/s41586-023-06907-7