In an article published in Nature an international team of scientists breaks down the traditional idea of dividing magnetism into two branches – the ferromagnetic one, known for several millennia, and the antiferromagnetic, discovered about a century ago. Researchers have now succeeded in directly experimentally demonstrating a third altermagnetic branch theoretically predicted by researchers in Prague and Mainz several years ago.
We usually think of a magnet as a ferromagnet, which has a strong magnetic field that can keep a shopping list on the fridge or allows the electric motor in an electric car to function. The magnetic field of a ferromagnet is created when the magnetic field of millions of its atoms is aligned in the same direction. This magnetic field can also be used to modulate the electric current in IT components.
Limitations of the traditional magnetic branches for IT
At the same time, however, the ferromagnetic field poses principal limitations on the spatial and temporal scalability of these components. Thus, a significant research focus in recent years has been directed at the second, antiferromagnetic branch. Antiferromagnets are materials less known but much more common in nature characterized by directions of the atomic magnetic fields on adjacent atoms alternating like white and black color on a chessboard. Thus, antiferromagnets as a whole do not create undesirable magnetic fields, but unfortunately, they are so anti-magnetic that they have not yet been actively used in IT.
Altermagnets combine “incompatible” merits
“The recently predicted altermagnets can combine merits of ferromagnets and antiferromagnets, which were thought to be fundamentally incompatible, and also have other unique merits not found in the other branches," says Tomas Jungwirth of the Institute of Physics of the Czech Academy of Sciences. Altermagnets can be thought of as magnetic arrangements where not only the directions of the magnetic fields on neighboring atoms alternate, but so also does the spatial orientation of the atoms in the crystal (see the figure). However, the internal magnetic fields modulate the electric current in a similar way to ferromagnets. This combination of properties is potentially very attractive specifically for applications in future ultrascalable nanoelectronics.
In addition, altermagnetic candidates have been identified among more than 200 materials ranging from insulators and semiconductors to metals and superconductors. Research groups have investigated many of these materials in the past, but their altermagnetic nature has remained hidden from them.
The altermagnetic branch was theoretically predicted five years ago
Since 2019, the team from the Institute of Physics in Prague and University of Mainz has published a series of articles theoretically identifying unconventional magnetic materials. In 2021 theorists predicted, that these materials represent a third fundamental type of magnet, which they called altermagnets and whose crystal and magnetic structure is completely different from conventional ferromagnets and antiferromagnets (in the words of physicists, altermagnets have completely different symmetries than ferromagnets or antiferromagnets).
Since altermagnetism opens broad and unprecedented research and application opportunities, the theoretical prediction was almost instantly followed by an outburst of follow-up studies by research groups from all around the globe. The next question then was when direct experimental evidence of altermagnetism would be available.
Experimental evidence was carried out on material considered for decades to be a "classical antiferromagnet"
The international team has now provided such evidence. The scientists decided to examine crystals of a simple two-element altermagnetic candidate – manganese telluride (MnTe). Traditionally, this material has been considered one of the classical antiferromagnets because the magnetic fields on neighboring manganese atoms point in opposite directions, and so they do not create an external magnetic field around the material.
But now, writing in Nature, the researchers have directly demonstrated altermagnetism in MnTe for the first time. They used theoretical predictions to navigate which direction to "shine the light" on high-quality MnTe crystals in a photoemission experiment.
The team measured band structures (maps that physicists use to describe the properties of electrons in crystals) on a synchrotron, using which they were able to show that despite the absence of an external magnetic field, electronic states in MnTe are strongly spin-split. The scale and shape of the spin splitting exactly match the altermagnetic splitting predicted using quantum mechanical calculations. This is direct evidence that MnTe is neither a conventional antiferromagnet nor a conventional ferromagnet but belongs to a new altermagnetic branch of magnetic materials.
The study made use of the expertise of researchers from the Institute of Physics of the Czech Academy of Sciences in collaboration with scientists from the Paul Scherrer Institute in Switzerland, University of Mainz in Germany, University of West Bohemia and Charles University in Czech Republic, University of Linz in Austria, and University of Nottingham in Great Britain.
The discovery of altermagnetism has launched new directions in world research
“After the initial predictions and with the rapidly growing world-wide interest in altermagnetism, and given that many of the theoretically identified material candidates were well-known and broadly available, we knew that it was only a matter of time when the first direct experimental evidence will be discovered. We are glad that we could be part of and coordinate this initial work, which we have performed jointly with our colleagues from Czech, Swiss, Austrian, German and British laboratories”, says Tomas Jungwirth from the Institute of Physics of the Czech Academy of Sciences, and he adds: “The discovery of altermagnetism has launched new directions in global research into new physical and material principles for highly scalable and energy-efficient IT components.” As can be seen, the discovery of altermagnetism in MnTe is just the beginning of an exciting new direction in magnetism.
More about the authors of the paper:
Libor Šmejkal, a principle theory author on the Nature paper and a former Ph.D student in Prague and Mainz, is the Winner of the Falling Walls Science Breakthroughs of the Year 2023 for his theory work on altermagnetism and non-dissipative nanoelectronics
Dominik Kriegner, an experimental co-author from Prague, has received the Lumina Quaeruntur Fellowship of the Czech Academy of Sciences to develop the materials portfolio of altermagnetism.
Helena Reichlová, another Prague experimentalist contributing to the Nature paper, has established a Max Planck Dioscuri Center for spin-caloritronic and magnonic research in altermagnets.
Tomas Jungwirth has started his second ERC Advanced Grant focusing on applications of altermagnetism in spintronic IT. Altermagnetism is also among the central themes of the recently awarded 20MEUR project coordinated by Tomas Jungwirth of the EU-funds Programme Johannes Amos Comenius.
Description: In altermagnets on adjacent magnetic atoms, not only the directions of spin polarization alternate (shown in purple and blue), but also the shapes of the atoms themselves (shown by the tilt of the electron densities in two different directions).
References:
Altermagnetic lifting of Kramers spin degeneracy – Nature 626, p. 517–522 (2024)
J. Krempaský*, L. Šmejkal*, S. W. D'Souza*, M. Hajlaoui, G. Springholz, K. Uhlířová, F. Alarab, P. C. Constantinou, V. Strokov, D. Usanov, W. R. Pudelko, R. González-Hernández, A. Birk Hellenes, Z. Jansa, H. Reichlová, Z. Šobáň, R. D. Gonzalez Betancourt, P. Wadley, J. Sinova, D. Kriegner, J. Minár, J. H. Dil, T. Jungwirth