Magnetic "waves" discovered in uranium semiconductor open path to magnetic control of layered materials

Senior Scientist Hironori Sakai and Group Leader Yo Tokunaga of the Strongly Correlated Actinide Science Group, together with Group Leader Koji Kaneko of the Strongly Correlated Materials Research Group at the Nuclear Science Research Institute (NSRI), Japan Atomic Energy Agency (JAEA), have revealed that in single crystals of the layered uranium semiconductor α-UTe₃ (alpha-uranium tritelluride), the magnetic moments of uranium atoms not only align alternately in opposite directions but also form a modulated structure with periodic "waves" of varying strength depending on position. The finding is expected to contribute to the development of new quantum materials. The results were published in the August 7 issue of Physical Review B, published by the American Physical Society, and were selected as an Editors' Suggestion.

α-UTe₃ is a compound that forms layered structures through van der Waals interactions (weak forces) between Te atoms surrounding the uranium atoms. It is a van der Waals material with a crystal structure similar to graphite, which consists of stacked graphene layers.

In uranium compounds, electrons in uranium atoms can give rise to magnetic and superconducting properties not found in ordinary elements. In particular, few studies have been conducted on uranium-containing van der Waals materials, and little is known about how the magnetic moments of uranium atoms emerge and are arranged within these materials. The details were also unclear for α-UTe₃, although it was known that magnetic ordering appears at low temperatures.

In general, in magnetic semiconductors, the magnitude of the magnetic moment of each atom is nearly constant and the moments are arranged in a regular pattern. In contrast, in α-UTe₃ crystals, uranium atoms are regularly arranged at the vertices of isosceles triangles. It had been thought that the system exhibits magnetic frustration, in which neighboring uranium moments would need to align in opposite directions to satisfy antiferromagnetic interactions, but such an arrangement cannot be realized simultaneously.

To investigate this issue, the researchers carried out a detailed study of the physical properties of α-UTe₃ crystals. They first grew single crystals of α-UTe₃ and successfully obtained crystals large enough for a variety of analyses. Precise measurements of electrical resistivity, specific heat, and magnetization revealed that α-UTe₃ is a semiconductor with low electrical conductivity and exhibits antiferromagnetism below approximately 5 K.

The team then used nuclear magnetic resonance (NMR) to probe the magnetic properties of individual atoms (local magnetism) and neutron scattering measurements to determine the magnetic structure of the crystal as a whole. The NSRI houses an NMR system capable of measuring uranium compounds and the only neutron scattering instrument in Japan capable of determining the magnetic structure of uranium compounds.

As a result, the researchers found that α-UTe₃ exhibits an antiferromagnetic state in which the magnitude of the magnetic moments varies in a wave-like manner throughout the crystal, an extremely rare phenomenon in semiconductors. The magnitude of the magnetic moments is not constant but instead forms a modulated structure in which it periodically becomes stronger and weaker depending on position within the crystal.

Such magnetic modulation is typically observed in metals, where electrons move freely through the crystal, and is not normally seen in semiconductors. The phenomenon is believed to originate from magnetic frustration and the strong magnetic anisotropy associated with uranium 5ƒ electrons.

It is well known that graphite exhibits dramatically different magnetic and electrical properties when reduced to single-layer graphene. Similarly, α-UTe₃ may also exhibit new properties when thinned down to fewer layers.

Sakai commented: "Going forward, we hope to deepen our understanding of layered compounds, including α-UTe₃, using synchrotron radiation and other techniques. We would also like to investigate whether it is possible to control the period of the magnetic modulation and the temperature at which antiferromagnetism emerges by exfoliating the crystal into thin films or by inserting different elements between its layers. Through continued basic research, we hope to establish guiding principles for the development of next-generation magnetic materials."

Journal Information
Publication: Physical Review B
Title: Incommensurate antiferromagnetism with amplitude modulation in the semiconducting 5ƒ van der Waals magnet α-UTe₃
DOI: 10.1103/jls4-6s89