Discovering new fundamental physics – Experiments prove the existence of a new type of magnetism

Experiments on a Swiss light source SLS Prove the existence of a new type of magnetism, with wide-ranging implications for technology and research.

There is now a new addition to the magnetic family: thanks to experiments conducted at the Swiss Light Source SLS, researchers have proven the existence of alternative magnetism. The experimental discovery of this new branch of magnetism was reported in nature It signifies new fundamental physics, with major implications for spintronics.

Magnetism is much more than just things that stick to your refrigerator. This understanding came with the discovery of antimagnets nearly a century ago. Since then, the family of magnetic materials has been divided into two basic stages: the ferromagnetic branch known for thousands of years and the antimagnetic branch. Experimental evidence for a third branch of magnetism, called alternative magnetism, has been provided at the Swiss Light Source (SLS), through an international collaboration led by the Czech Academy of Sciences with the Paul Scherrer Institute (PSI).

The fundamental magnetic phases are determined by specific spontaneous arrangements of the magnetic moments – or electron spins – and of the atoms carrying the moments in the crystals. Ferromagnets are the type of magnet that sticks to your refrigerator: here the spins point in the same direction, giving microscopic magnetism. In antiferromagnetic materials, the spins point in alternating directions, resulting in the material not having a macroscopic magnetization network — and therefore not sticking to the refrigerator. Although other types of magnetism have been classified, such as magnetism and paramagnetism, they describe specific responses to externally applied magnetic fields rather than spontaneous magnetic arrangement in materials.

Discovery and properties of substitute magnets

Replacement magnets have a special combination of spin arrangement and crystalline symmetries. The spins alternate, as in antimagnets, resulting in no net magnetization. However, instead of simply canceling out the symmetries, the symmetries give an electronic band structure with strong spin polarization that fluctuates in direction as you pass through the energy bands of the material – hence the name substituent magnets. This results in very useful properties similar to ferromagnets, as well as some completely new properties.

In the journal Nature, researchers report the discovery of a new type of fundamental magnetism, called “alternative magnetism.” Here, Juraj Krembaski, a scientist at PSI and first author of the publication, stands at the Swiss Light Source SLS where experimental evidence of alternative magnetism is presented. Source: Paul Scherrer Institute / Mahir Dzambigovic

Implications for Spintronics

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

Although spintronics has been promising for many years to revolutionize information technology, it is still in its infancy. Typically, ferromagnets have been used for such devices, because they offer some highly desirable spin-dependent strong physical phenomena. However, the net, macroscopic magnetization useful in many other applications imposes practical limitations on the scalability of these devices because it causes interference between bits—the information carrying elements in data storage.

Recently, antimagnets have been studied for spintronics, as they take advantage of the lack of net magnetization and thus provide superior scalability and energy efficiency. However, the strong spin-dependent effects that are so useful in ferromagnets are absent, which again hampers their practical applicability.

Here we enter the field of alternative magnets that have the best of both: net zero magnetization, plus the strong spin-dependent phenomena typically found in ferromagnets – advantages that were considered incompatible in principle.

“This is the magic of alternative magnets,” says Thomas Jungwirth of the Institute of Physics of the Czech Academy of Sciences, the study's lead researcher. “Something that people thought was impossible until recent theoretical predictions is actually possible.”

Theoretical predictions and experimental verification

Rumblings about the existence of a new kind of magnetism began not so long ago: in 2019, Jungwirth and fellow theorists at the Czech Academy of Sciences and the University of Mainz identified a class of magnetic materials with a spin structure that does not fit classical descriptions. Ferromagnetism or antimagnetism.

In 2022, theorists published their predictions for the existence of alternative magnetism. They have discovered more than two hundred candidates for alternative magnetism in materials ranging from insulators to SemiconductorsFor metals and superconductors. Many of these materials were well known and extensively explored in the past, without their alternative magnetic nature being noted. Given the enormous research and application opportunities presented by alternative magnetism, these predictions have caused great excitement within the community. The search was ongoing.

Obtaining direct experimental evidence for the existence of alternative magnetism requires elucidation of the unique spin symmetry properties expected in alternative magnets. The evidence came using rotational and angular optical emission spectroscopy at SIS (COPHEE's terminus) and the ADRESS beamlines of SLS. This technique enabled the team to visualize a clear feature in the electronic structure of a suspected alternative magnet: the splitting of electronic bands corresponding to different spin states, known as Cramer spin degeneracy lift.

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

However, antimagnets should not show high Cramer spin degeneracy in the magnetic order, while ferromagnets or substituent magnets should. When scientists saw a rise in Cramer spin decay, accompanied by a vanishing net magnetization, they knew they were looking at an alternative 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, thus proving that manganese telluride is neither a classical antiferromagnet nor a classical ferromagnet but belongs to the new branch of alternative magnetism,” says Juraj Krembaski, a beamline scientist in the group Beamline Optics Group at PSI and first author of the study: “From Magnetic Materials.”

The beamlines that enabled this discovery have now been dismantled, awaiting an SLS 2.0 upgrade. After twenty years of successful science, the final COPHEE station will be fully integrated into the new “QUEST” beamline. “We performed these experiments with the last photons of light at COPHEE. They have made such an important scientific breakthrough and it is very impressive for us,” adds Krempaski.

Conclusion and future directions

The researchers believe that this fundamental new discovery in magnetism will enrich our understanding of condensed matter physics, while impacting diverse areas of research and technology. In addition to their advantages in the developing field of spintronics, they also provide a promising platform for exploring unconventional superconductivity, through new insights into the superconducting states that can arise in different magnetic materials.

“Alternative magnetism is actually not something very complicated. “It's something quite fundamental that has been in front of our eyes for decades without us noticing,” Jungwirth says. “And it's not something that only exists in a few obscure materials. It is found in many crystals that people kept in their drawers. In this sense, now that we've brought this idea to light, many more people around the world will be able to work on it, opening up the potential for widespread impact.

Reference: “Magnetic levitation of Cramer spin decay” 14 February 2024, nature.
doi: 10.1038/s41586-023-06907-7