A new type of magnetism, with broad implications for technology and research

A newcomer is now added to the magnetic family: thanks to experiments at the Swiss Light Source SLS, researchers have proven the existence of altermagnetism. The experimental discovery of this new branch of magnetism is reported in Nature and signifies new fundamental physics, with major implications for spintronics.

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

Experimental proof of a third branch of magnetism, called altermagnetism, was carried out at the Swiss Light Source SLS, as part of an international collaboration led by the Czech Academy of Sciences in collaboration with the Paul Scherrer Institute PSI .

Fundamental magnetic phases are defined by the specific spontaneous arrangements of magnetic moments – or electronic spins – and atoms that carry the moments in crystals.

Ferromagnets are the type of magnets that stick to the refrigerator: here the rotations point in the same direction, giving macroscopic magnetism. In antiferromagnetic materials, the spins point in alternating directions, so the materials possess no net macroscopic magnetization and therefore do not stick to the refrigerator. Although other types of magnetism, such as diamagnetism and paramagnetism, have been classified, these describe specific responses to externally applied magnetic fields rather than spontaneous magnetic orders in materials.

Altermagnets have a special combination of spin arrangements and crystal symmetries. The spins alternate, as in antiferromagnets, resulting in no net magnetization. Yet rather than simply canceling out, the symmetries result in an electronic band structure with strong spin polarization that changes direction as you pass through the energy bands of the material, hence the name altermagnets. This results in very useful properties that are more similar to ferromagnets, as well as completely new properties.

A useful new brother

This third magnetic sibling offers distinct advantages for the developing field of next-generation magnetic memory technology, known as spintronics. While electronics only uses the charge of electrons, spintronics also exploits the spin state of electrons to carry information.

Even though spintronics has promised to revolutionize computing for several years, it is only in its infancy. Typically, ferromagnets have been used for such devices because they offer certain highly desirable, strongly spin-dependent physical phenomena. Yet macroscopic net magnetization, useful in many other applications, poses practical limits to the scalability of these devices because it causes crosstalk between bits, the information-carrying elements in data storage.

More recently, antiferromagnets have been investigated for spintronics, as they benefit from the absence of net magnetization and thus provide ultra-scalability and energy efficiency. However, the strong spin-dependent effects so useful in ferromagnets are lacking, again hampering their practical applicability.

Here enter altermagnets with the best of both: zero net magnetization as well as the highly coveted spin-dependent phenomena typically found in ferromagnets – merits that were considered primarily incompatible.

“This is the magic of altermagnets,” says Tomáš Jungwirth of the Institute of Physics of the Czech Academy of Sciences, lead researcher of the study. “What people thought was impossible until recent theoretical predictions (showed it) is in fact possible.”

The search is launched

Rumors that a new type of magnetism was lurking began not so long ago: in 2019, Jungwirth, together with fellow theorists from the Czech Academy of Sciences and the University of Mainz, identified a class of magnetic materials with a spin structure that did not fit classical descriptions. ferromagnetism or antiferromagnetism.

In 2022, theorists published their predictions about the existence of altermagnetism. They discovered more than two hundred altermagnetic candidates in materials ranging from insulators and semiconductors to metals and superconductors. Many of these materials have been well known and widely explored in the past, without their altermagnetic nature being noticed. Due to the enormous research and application opportunities that altermagnetism offers, these predictions have generated great enthusiasm within the community. The search was on.

X-rays are the proof

To obtain direct experimental proof of the existence of altermagnetism, it was necessary to demonstrate the unique spin symmetry characteristics predicted in altermagnetisms. The proof was obtained using spin- and angle-resolved photoemission spectroscopy on the SIS (COPHEE final station) and ADRESS beamlines of the SLS. This technique allowed the team to visualize a telltale feature of the electronic structure of a putative altermagnet: the splitting of electronic bands corresponding to different spin states, known as lifting of Kramer spin degeneracy. .

The discovery was made in crystals of manganese telluride, a well-known simple two-element material. Traditionally, the material has been considered a classical antiferromagnet because the magnetic moments on neighboring manganese atoms point in opposite directions, generating a net disappearing magnetization.

However, antiferromagnets are not expected to exhibit high Kramers spin degeneracy depending on magnetic ordering, whereas ferromagnets or altermagnets are expected to do so. When the scientists noticed the disappearance of the Kramer spin degeneracy, accompanied by the disappearance of the net magnetization, they knew they were dealing with an alter-magnet.

“Thanks to the high precision and sensitivity of our measurements, we were able to detect the characteristic alternating division of energy levels corresponding to opposite spin states and thus demonstrate that manganese telluride is neither a classical antiferromagnet nor “A classic ferromagnet but belongs to the new altermagnetic branch of magnetic materials,” says Juraj Krempasky, beamline scientist in the Beamline Optics group at PSI and first author of the study.

The beamlines that made this discovery possible are now dismantled, awaiting the SLS 2.0 upgrade. After twenty years of scientific success, the COPHEE final station will be completely integrated into the new “QUEST” beamline. “It is with the last photons of light at COPHEE that we carried out these experiments. That they gave rise to such an important scientific advance is very moving for us,” adds Krempasky.

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

Researchers believe this fundamental new discovery in the field of magnetism will enrich our understanding of condensed matter physics, impacting various areas of research and technology. In addition to its benefits for the developing field of spintronics, it also offers a promising platform for exploring unconventional superconductivity, thanks to new insights into the superconducting states that can occur in different magnetic materials.

“Altermagnetism is actually not something very complicated. It’s something quite fundamental that was in front of us for decades without us realizing it,” explains Jungwirth. “And it’s not something that exists in just a few obscure materials. It exists in many crystals that people just had in their drawers. In that sense, now that we’ve brought it to light, many people in the world will be able to work on it, which gives the possibility of having a large impact.