Scientists from Masaryk University’s Faculty of Science have made progress in the study of altermagnets

That magnetism is useful for recording information is a well-known fact; after all, everyone today knows what a hard disk or a bank card is. Current technologies are based on ferromagnets, and information encoded in the orientation of their magnetic moment is easily read. However, while this is an advantage on the one hand it is an Achilles heel on the other. Ease of reading is not always desirable; moreover, such a record can be destroyed by a magnetic field. An alternative is a antiferromagnet, in which the magnetic moments of individual atoms are arranged so that they compensate for each other, meaning that, externally, such a material appears non-magnetic. A fundamental problem with antiferromagnets is how to detect the orientation of atomic moments and thus read the information stored on it. The solution appears to be altermagnets, which are superficially non-magnetic, just like antiferromagnets, but at the same time exhibit phenomena that can be used to determine the direction in which atomic moments are pointing.

One such phenomenon that has been investigated by physicists from Brno and Osaka is magnetic circular dichroism (MCD). This refers to the difference in absorption of right- and left-polarised light by magnetic material. Following last year's theoretical prediction of MCD in the altermagnetic material manganese telluride [MnTe], the team of Atsushi Hariki and our own Jan Kuneš put forward a prediction on the intriguing behaviour of altermagnets with rutile structures.

The magnitude of the MCD observed will depend on the direction of the magnetic moments and the direction radiation strikes the sample. From a practical point of view, it is most interesting to find the direction for which MCD is at its maximum, which will vary considerably for different material types. For ordinary ferromagnets, it is the direction in which the magnetic moments of the atoms point. If we rotate them in space, by using a magnetic field for example, the position of the maximum rotates in the same direction. On the other hand, in manganese telluride, the maximum always lies in the same direction and rotation of the atomic moments leads to changes in the magnitude and sign of MCD.

Now, in a paper published in npj Quantum Materials from the Nature group, Jan Kuneš and his colleagues show that MCD behaves differently in altermagnets with rutile structures. In this case, when the magnetic moments are rotated, the magnitude of the effect does not change, similar to ferromagnets; however, the MCD maxima does not follow the direction of the moments but rotates in the opposite direction. “We expected that MCD would depend strongly on the orientation of the magnetic moments. However, that the shape and amplitude of the MCD spectra does not change, only the direction of the maximum, and in a way that is easy to represent geometrically, came as quite a surprise to us”, said Jan Kuneš. Their published calculations provide a detailed prediction of MCD behaviour in nickel fluoride [NiF2], which only now requires experimentation to verify it.

While determining the direction of magnetic moments in materials like manganese telluride requires a combination of several different methods, the work presented in npj Quantum Materials shows that, in the case of materials with rutile structures, circular dichroism alone is sufficient to do the job. This will not only facilitate the experimental study of such materials, it could potentially influence future technologies for reading altermagnetic recording media.

Sources:
https://www.nature.com/articles/s41535-025-00753-8

DOI: 10.1038/s41535-025-00753-8

Additional resources:
https://www.science.org/content/article/breakthrough-2024
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.176701