For over a century, scientists have understood ferromagnetism and antiferromagnetism as the two primary classes of magnetic order in materials. Ferromagnets, like those found on refrigerator doors, have all their magnetic moments aligned in the same direction, creating a strong external magnetic field. Antiferromagnets, on the other hand, feature alternating magnetic moments that cancel each other out, resulting in no macroscopic magnetic effect. 

In 2019, researchers at Johannes Gutenberg University Mainz (JGU) proposed a third class of magnetism known as altermagnetism. This concept has sparked significant debate among experts, with some questioning its very existence. Recently, groundbreaking experimental evidence from DESY (Deutsches Elektronen-Synchrotron) has provided the first empirical support for altermagnetism, marking a significant milestone in the field of magnetism.

Understanding Altermagnetism

Altermagnets combine characteristics of both ferromagnets and antiferromagnets. Like antiferromagnets, altermagnets have neighboring magnetic moments that are antiparallel, meaning they cancel out at the macroscopic level and don’t produce an external magnetic field. 

 

However, similar to ferromagnets, altermagnets exhibit spin-polarized currents. Professor Hans-Joachim Elmers, head of the Magnetism group at JGU's Institute of Physics, explains, "Altermagnets combine the advantages of ferromagnets and antiferromagnets. Their neighboring magnetic moments are always antiparallel to each other, as in antiferromagnets, so there is no macroscopic magnetic effect, but, at the same time, they exhibit a spin-polarized current—just like ferromagnets."

The Unique Properties of Altermagnets

Typically, electric currents generate magnetic fields, but in altermagnets, the spin polarization across the entire material results in a net zero magnetic field. When focusing on electrons moving in a specific direction, it becomes clear that these electrons have a uniform spin. This phenomenon is related to the electron velocity direction vs. their spatial arrangement. This effect, predicted by theoretical groups led by Professor Jairo Sinova and Dr. Libor Šmejkal at JGU, emphasizes the importance of "momentum space" in understanding altermagnetism.

Experimental Verification at DESY

The major breakthrough came when a team of researchers, led by Professor Elmers, conducted experiments using a specially adapted momentum microscope. By exposing a thin layer of ruthenium dioxide to X-rays, they excited electrons sufficiently to observe their emission and velocity distribution. Circularly polarized X-rays allowed the researchers to infer the spin directions, providing crucial data for verifying the altermagnetism effect.

 

Their momentum microscope differed from standard electron microscopes by focusing on momentum space rather than the material's surface. This innovative approach showed differing electron momenta as distinct positions on the detector, effectively mapping the directions in which electrons moved within the ruthenium dioxide layer.

Major Implications for Spintronics

Altermagnetism holds significant potential for spintronics, a field that aims to use the magnetic moment of electrons vs. their charge for data storage and transfer. Spintronics promises increased storage capacity and faster, more efficient memory systems. "Our results could be the solution to what is a major challenge in the field of spintronics," Elmers suggests. Using altermagnets could simplify reading stored information based on spin polarization, potentially revolutionizing the tech behind data storage.

 

The discovery and experimental validation of altermagnetism mark an exciting new chapter in magnetic research. As the scientific community continues to explore and understand this new magnetic phase, its potential applications, particularly in spintronics, could lead to major advancements. Altermagnetism not only enhances our understanding of magnetic materials but also cracks open the door to innovative solutions, promising a fascinating future for technology.

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