Altermagnetism: Researchers detect new magnetic phase in manganese telluride

Almost everyone is familiar with the two typical types of magnetism: ferromagnetism, in which all magnetic moments are aligned (as in a fridge magnet), and antiferromagnetism, in which the magnetic moments cancel each other out. At the beginning of 2024, researchers discovered a third form between these two – altermagnetism ("other magnetism"). Scientists from the University of Nottingham have now been able to detect this new magnetic phase in the material manganese telluride (MnTe) and observe it directly using high-resolution methods, as they report in the December issue of the scientific journal Nature. They carried out experiments at the "Max IV" laboratory in Sweden.

Prof. Peter Wadley from the University of Nottingham summarizes the special features as follows: "Altermagnets consist of magnetic moments that are aligned antiparallel to their neighbors. However, each part of the crystal that houses these tiny moments is twisted regarding its neighbors. It's like antiferromagnetism with a twist! But this subtle difference has far-reaching effects".

Due to the special arrangement of the magnetic moments (spins), the material shows no external magnetization, but internally, it exhibits magnetic properties that are interesting for new technical applications. According to the scientists, the spin polarization of alter-magnetic materials is similar to that of high-temperature superconductors. This property makes it possible to observe novel effects such as the anomalous Hall effect without the need for an external magnetic field. As these materials are largely insensitive to external magnetic fields, they are ideal for use in modern electronic devices that use not only the electrical charge but also the intrinsic angular momentum (spin) of the electrons to process information – called spin electronics or spintronics.

Another significant advantage is the wide range of possible materials that can exhibit electromagnetic properties: from insulators and semiconductors to metals and superconductors. In principle, the possible applications therefore range from the development of new types of electronic components to the integration into spintronic devices to improve their efficiency and robustness to the research and use of topological phenomena in quantum information technology. The fact that alter magnetism can occur in such different classes of materials makes this field of research particularly promising for future technological developments.

Experimental visualization

To visualize the age-magnetic order in MnTe, Wadley's team combined two X-ray methods: circular magnetic X-ray dichroism (XMCD) and linear magnetic X-ray dichroism (XMLD). Together, these methods make it possible to determine the orientation of the so-called age-magnetization vector L, which characterizes the age-magnetic order, with a resolution of up to 10 nanometres, according to the Nature article.

The researchers produced the 30-nanometer-thin MnTe layers using molecular beam epitaxy, a process for the controlled deposition of crystalline materials. By specifically structuring the layers into hexagonal and triangular shapes, they were able to control the formation of different alter-magnetic textures. According to their own statements, they could also use special cooling processes in magnetic fields to specifically adjust the direction of the L-vector in individual domains and create microstructures with just one magnetic domain.

Possible application in memory elements

Such structures could form the basis for novel storage elements in spintronics based on alter magnetism. Similar to ferromagnets, altermagnets exhibit strong spin-current effects that enable information to be read and written. According to the researchers, faster switching times than with conventional magnetic memories are also conceivable. "Our experimental work has built a bridge between theoretical concepts and their practical implementation, which hopefully points the way to the development of age-magnetic materials for practical applications," says Oliver Amin, one of the co-authors of the publication.

Altermagnetism was experimentally detected almost simultaneously in three different materials in early 2024, after being predicted in 2020 by theorists led by Libor Šmejkal. The imaging and controlled generation of alter-magnetic states now demonstrated by Wadley and colleagues paves the way for further research into the phenomenon and its technological use in the coming years.

(vza)