Altermagnetism proves its place on the magnetic family tree

Magnetism is more than just things that stick to the fridge. This understanding came about a century ago with the discovery of antiferromagnets. Since then, the family of magnetic materials has been divided into two main phases: the ferromagnetic branch known for several millennia and the antiferromagnetic branch. Experimental evidence of a third branch of magnetism, called altermagnetism, was made at the Swiss Light Source SLS, through an international collaboration with the Paul Scherer Institute PSI, led by the Czech Academy of Sciences.

The fundamental magnetic phases are defined by the magnetic moments — or electron spins — and the specific self-organizing arrangements of the atoms that carry the moments in the crystal. Ferromagnets are the type of magnets that stick to fridges: here they spin in one direction, giving macroscopic magnetism. In antiferromagnetic materials, the spins point in alternate directions, resulting in the material having no macroscopic net magnetism — and thus not sticking to the fridge. Although other types of magnetism are classified, such as diamagnetism and paramagnetism, these describe specific responses to externally applied magnetic fields rather than spontaneous magnetic ordering in the material.

Electromagnets have a special combination of spin and crystal symmetry configuration. The spin alternates, as in antiferromagnets, resulting in no net magnetization. Yet, instead of simply canceling, the symmetry gives an electronic band structure with strong spin polarization that flips in direction as it passes through the material’s energy band — hence the name altermagnet. This results in highly useful properties closely resembling those of ferromagnets, as well as some completely new properties.

A new and useful brother

This third magnetic sibling offers distinct advantages for the burgeoning 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.

Although spintronics has promised to revolutionize IT for a few years now, it is still in its infancy. In general, ferromagnets have been used for such devices, as they exhibit some of the most desirable strong spin-dependent physical phenomena. Yet macroscopic net magnetization, which is useful in many other applications, imposes practical limits on the scalability of these devices because it causes crosstalk between bits—the elements that carry information in data storage.

More recently, antiferromagnets have been investigated for spintronics, as they benefit from no net magnetization and thus offer ultra-scalability and energy efficiency. However, the strong spin-dependent effects that are so useful in ferromagnets are lacking, which again hinders their practical application.

Enter altmagnets with the best of both worlds: zero net magnetization combined with the coveted strong spin-dependent phenomena typically found in ferromagnets — qualities that were thought to be fundamentally incompatible.

“That’s the magic about the altermagnet,” says the study’s principal investigator, Tomáš Jungwirth from the Institute of Physics of the Czech Academy of Sciences. “Something that people believed was impossible until recent theoretical predictions were actually possible.”

The search is on.

The rumblings that a new type of magnetism was lurking started a while ago: in 2019, Jungwirth, together with theoretical colleagues from the Czech Academy of Sciences and the University of Mainz, identified a class of magnetic materials in which spin had a structure that did not fit the classic descriptions. of ferromagnetism or antiferromagnetism.

In 2022, theorists published their predictions of the existence of ultramagnetism. They uncovered more than two hundred electromagnetic 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 noticing their alternating magnetic nature. Because of the vast research and application opportunities that altermagnetism poses, these predictions created great excitement in the community. The search was on.

X-rays provide evidence.

In order to obtain direct experimental evidence for the existence of altermagnetism it is necessary to demonstrate the unique spin symmetry properties predicted in altermagnets. The evidence came using spin- and angle-resolved photoemission spectroscopy at the ADRESS beamlines of SIS (COPHEE End Station) and SLS. The technique enabled the team to visualize a tell-tale feature in the electronic structure of the suspected altermagnet: the distribution of electronic bands corresponding to different spin states, known as the lifting of Kramers spin degeneracy.

The discovery was made in crystals of manganese telluride, a well-known simple two-element material. Traditionally, the material is regarded as a classic antiferromagnet because the magnetic moments on neighboring manganese atoms point in opposite directions, creating a vanishing net magnetism.

However, antiferromagnets should not exhibit Kramer’s spin degeneracy according to magnetic order, whereas ferromagnets or altermagnets should. When scientists saw the Kramers spin degeneracy disappear, along with the vanishing net magnetism, they knew they were looking at an altermagnet.

“Thanks to the high precision and sensitivity of our measurements, we can detect the characteristic alternative distribution of energy levels corresponding to anti-spin states and thus show that manganese telluride is neither a conventional antiferromagnet And not traditional ferromagnets, but belongs to a new branch of magnetic materials”, says Joraj Krempski, beamline scientist in the beamline optics group at PSI and first author of the study.

The beamlines that enabled the discovery have now been decommissioned, awaiting the SLS 2.0 upgrade. After twenty years of successful science, the COPHEE end station will be fully integrated into the new ‘QUEST’ beamline. “It was with the last photon of light at COPHEE that we did these experiments. That they made such an important scientific breakthrough is very emotional for us,” adds Krempasky.

“Now that we’ve made it public, many people around the world will be able to work on it.”

The researchers believe that this new fundamental discovery in magnetism will improve our understanding of condensed matter physics with implications for diverse areas of research and technology. Along with its advantages in the developing field of spintronics, it also offers a promising platform for exploring unconventional superconductivity, through new insights into the superconducting states that arise in various magnetic materials. does.

Jungwirth says, “Altermagnetism is actually not a very complicated thing. It’s a completely basic thing that’s been right in front of our eyes for decades, without us noticing it.” “And it’s not something that’s just in a few obscure materials. It’s in a lot of crystals that people just had in their drawers. In that sense, now that we’ve brought it out, all over the world. Many people will be able to work on it, providing the potential for a wider impact.”