Ferromagnetism and antiferromagnetism have long been known to scientists as two classes of magnetic ordering of materials. In 2019, researchers at the Johannes Gutenberg University Mainz (JGU) proposed a third class of magnetism, called altermagnetism. This altermagnetism has been the subject of heated debate among experts ever since, with some expressing doubts about its existence. Recently, a team of experimental researchers led by Prof. Hans-Joachim Elmers at JGU was able to measure for the first time at DESY (Deutsches Elektronen-Synchrotron) an effect considered to be a hallmark of ultramagnetism, thus its Evidence of existence is found. This third type of magnetism. The results of the research were published. Advances in science.

Altermagnetism – a new magnetic phase

While ferromagnets, which we all know from refrigerator magnets, have all their magnetic moments aligned in the same direction, antiferromagnets have alternating magnetic moments. Thus, at the macroscopic level, the magnetic moments of antiferromagnets cancel each other, so there is no external magnetic field — which is why refrigerator magnets made of this material easily fall off refrigerator doors. Alternating magnets have magnetic moments that vary according to their orientation. “Ultramagnets combine the advantages of ferromagnets and antiferromagnets. Their neighboring magnetic moments are always parallel to each other, as in antiferromagnets, so there is no macroscopic magnetic effect, but, with Heck, they exhibit a spin-polarized current — just like ferromagnets,” explained Professor Hans-Joachim Elmers, Head of the Magnetism Group at JGU’s Institute of Physics.

Moving in the same direction with uniform curvature

Electric currents usually produce magnetic fields. However, if one considers the altermagnet as a whole, integrating the spin polarization in the electronic bands in all directions, it becomes apparent that the magnetic field must be zero despite the spin-polarized current. If, on the other hand, attention is restricted to electrons that move in a particular direction, it follows that they must have uniform spin. “This alignment phenomenon has nothing to do with the spatial arrangement or where the electrons are located, but only with the direction of the electron’s trajectory,” Elmers added. Since velocity (v) times mass (M) equals momentum (P), physicists use the term “momentum space” in this context. This effect was predicted in the past by JGU theoretical groups led by Prof. Jairo Sinova and Dr. Lieber Smajkal.

Evidence was obtained using momentum electron microscopy.

“Our team was the first to confirm the effect experimentally,” Elmers said. The researchers used a specially adapted velocity microscope. For their experiment, the team exposed a thin layer of ruthenium dioxide to X-rays. The resulting excitation of electrons was sufficient to eject them from the ruthenium dioxide layer and detect them. Based on the speed distribution, the researchers were able to determine the speed of the electrons in the ruthenium dioxide. And using circularly polarized X-rays, they were also able to infer the directions of curvature.

For their speed microscope, the researchers changed the focal plane typically used for observation in standard electron microscopes. Instead of a large image of the surface of the ruthenium oxide film, their detector showed a spatial representation of the momentum. “Different momenta appear at different locations on the detector. More simply, the different directions in which electrons move in a layer are represented by corresponding dots on the detector,” Elmers said.

Altermagnetism may also be related to spintronics. This would involve using the magnetic moment of electrons instead of their charge in dynamic random access memory. As a result, storage capacity can be significantly increased. “Our results could be the solution to what is a major challenge in the field of spintronics,” suggested Elmers. “Taking advantage of the potential of ultramagnets will make it easier to read information stored in electronic bands based on spin polarization.”