Newswise – Now there is an addition to the magnetic family: thanks to experiments at the Synchrotron Light Source Switzerland SLS, researchers have proven the existence of alter magnetism. The experimental discovery of this new branch of magnetism is reported in Nature and means new fundamental physics with major implications for spintronics.
Magnetism is much more than just things sticking to the fridge. This understanding arose with the discovery of antiferromagnets almost a century ago. Since then, the family of magnetic materials has been divided into two basic phases: the ferromagnetic branch, known for several millennia, and the antiferromagnetic branch. Experimental evidence of a third branch of magnetism, called altermagnetism, was provided at the Synchrotron Light Source Switzerland SLS through an international collaboration led by the Czech Academy of Sciences together with the Paul Scherrer Institute PSI.
The fundamental magnetic phases are defined by the specific spontaneous arrangement of magnetic moments – or electron spins – and of atoms that carry the moments in crystals. Ferromagnets are magnets that stick to the refrigerator: here the spins point in the same direction, which leads to macroscopic magnetism. In antiferromagnetic materials, the spins point in alternating directions, so that the materials have no macroscopic net magnetization – and therefore do not stick to the refrigerator. Although other types of magnetism, such as diamagnetism and paramagnetism, have been categorized, these describe specific responses to externally applied magnetic fields rather than spontaneous magnetic orders in materials.
Alter magnets have a special combination of spin arrangement and crystal symmetries. The spins alternate like antiferromagnets, resulting in no net magnetization. But instead of simply canceling out, the symmetries result in an electronic band structure with strong spin polarization, the direction of which changes as it passes through the material’s energy bands – hence the name alter magnets. This leads to extremely useful properties more similar to those of ferromagnets, as well as some entirely new properties.
A new and useful sibling
This third magnetic brother offers significant advantages to the emerging field of next-generation magnetic storage technology known as spintronics. While electronics only uses the charge of electrons, spintronics also uses the spin state of electrons to transport information.
Although spintronics has been promising to revolutionize IT for several years, it is still in its infancy. Typically, ferromagnets have been used for such devices because they offer certain highly desirable, highly spin-dependent physical phenomena. But the macroscopic net magnetization, useful in so many other applications, poses practical limitations 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 because they benefit from having no net magnetization and therefore offer high scalability and energy efficiency. However, the strong spin-dependent effects that are so useful in ferromagnets are missing, which in turn hinders their practical applicability.
This is where alter magnets come into play, which have the best of both: zero net magnetization along with the sought-after strong spin-dependent phenomena typically found in ferromagnets – advantages that have been viewed as fundamentally incompatible.
“This is the magic of alter magnets,” says Tomáš Jungwirth from the Institute of Physics of the Czech Academy of Sciences, principal investigator of the study. “Something that people believed was impossible until recent theoretical predictions is actually possible.”
The search is ongoing
Rumors that a new type of magnetism was lurking arose not long 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 with a spin structure that does not fit into the classical ones Descriptions fit of ferromagnetism or antiferromagnetism.
In 2022, the theorists published their predictions about the existence of alter magnetism. They discovered more than two hundred altermagnetic candidates in materials ranging from insulators and semiconductors to metals and superconductors. Many of these materials were well known and extensively studied in the past without any awareness of their agemagnetic nature. Due to the enormous research and application possibilities offered by altermagnetism, these predictions caused great excitement in the community. The search was on.
X-rays provide proof
To obtain direct experimental evidence for the existence of alter magnetism, it was necessary to demonstrate the unique spin symmetry properties predicted for alter magnets. The evidence was provided using spin- and angle-resolved photoemission spectroscopy at the SIS (COPHEE end station) and ADRESS beamlines of the SLS. This technique allowed the team to visualize a telltale feature in the electronic structure of a suspected alter magnet: the splitting of electronic bands corresponding to different spin states, known as Kramers spin degeneracy cancellation.
The discovery was made in crystals of manganese telluride, a well-known simple material made up of two elements. Traditionally, the material is considered a classic antiferromagnet because the magnetic moments of neighboring manganese atoms point in opposite directions, producing a vanishing net magnetization.
However, antiferromagnets should not have a Kramers spin degeneracy enhanced by the magnetic order, whereas this should be the case with ferromagnets or alter magnets. When scientists saw the cancellation of the Kramers spin degeneracy, accompanied by the vanishing 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 splitting of energy levels corresponding to opposite spin states, thereby showing that manganese telluride is neither a conventional antiferromagnet nor a conventional ferromagnet, but belongs to the new altermagnetic branch of magnetic materials says Juraj Krempasky, beamline scientist in the Beamline Optics Group at PSI and lead author of the study.
The beamlines that made this discovery possible are now dismantled and 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. “We carried out these experiments with the last photons of light at COPHEE. The fact that they have achieved such an important scientific breakthrough is very emotional for us,” adds Krempasky.
“After we bring it to light, many people around the world can work on it.”
The researchers believe that this new fundamental discovery of magnetism will enrich our understanding of condensed matter physics, with implications for various areas of research and technology. In addition to its advantages for the developing field of spintronics, it also provides a promising platform for exploring unconventional superconductivity through new insights into superconducting states that can appear in various magnetic materials.
“Altermagnetism is actually not something very complicated. It is something completely fundamental that we have had in front of our eyes for decades without noticing it,” says Jungwirth. “And it’s not something that only exists in a few unknown materials. It exists in many crystals that people just had in their drawers. In that sense, once we bring it to light, many people around the world will be able to work on it, which has the potential for widespread impact.”