Skyrmionic insulator Cu₂OSeO₃ at the nanoscale and in ultra-high magnetic fields

The situation changed with the discovery of skyrmions in the correlated insulator Cu2OSeO3 [1]: the
localized nature of S=1/2 moments on Cu2+ makes this material amenable to microscopic modeling.
Using first-principles calculations, we estimated five independent exchange interactions, and showed
that the key element of the magnetism is S=1 tetrahedra [2]. This model directly explains the origin of
an extremely broad and robust 1/2-magnetization plateau. Remarkably, an effective model of these
“strong’’ tetrahedra reproduces the experimental radius of skyrmions [2]. Later this model has been
successfully used for interpretation of high-field ESR and inelastic neutron scattering data.
In this talk, I will discuss two recent findings in this fascinating material. As known from several reports,
the magnetic phase diagram of Cu2OSeO3 can be significantly altered at the nanoscale. In our joint
experimental and theoretical study, we report the discovery of a new trigonal polymorph, observed
exclusively in nanoparticles [3]. This polymorph has a layered trigonal structure, supporting Néel-
type skyrmions that are qualitatively different from Bloch-type skyrmions found in bulk Cu2OSeO3.
The second story is the ultra-high-field measurement of magnetization in bulk Cu2OSeO3: by inspect-
ing the Faraday rotation angle it was possible to observe the closing of the 1/2-plateau at 180 Tesla and
even reach the saturation at 300 Tesla [4]. Yet the main surprise was the behavior in-between these
two fields: instead of a monotonic increase, the Faraday rotation angle unexpectedly developed a
broad maximum. Our analysis revealed that this nonmonotonic behavior correlates with electric po-
larization, and the phase in-between the plateau and full saturation is a magnon condensate [4]. Un-
like other magnets showing a Bose-Einstein condensation, the formation of the condensate here is
driven by the closing of the triplet-quintuplet gap in the strong S=1 tetrahedra.

[1] S. Seki, X. Z. Yu, S. Ishiwata, and Y. Tokura, Observation of skyrmions in a multiferroic material,
Science 336, 198 (2012).
[2] O. Janson, I. Rousochatzakis, A. A. Tsirlin, M. Belesi, A. A. Leonov, U. K. Rößler, J. van den Brink,
and H. Rosner, The quantum origins of skyrmions and half-skyrmions in Cu2OSeO3, Nat. Com-
mun. 5, 5376 (2014).
[3] A. Arakcheeva, P. R. Baral, W. H. Bi, C. Jandl, O. Janson, and A. Magrez, Cu2OSeO3 turns trigonal
with structural transformation and implications for skyrmions, Sci. Rep. 15, 31945 (2025).
[4] T. Nomura, I. Rousochatzakis, O. Janson, M. Gen, X.-G. Zhou, Y. Ishii, S. Seki, Y. Kohama, and Y.
H. Matsuda, Quintuplet condensation in the skyrmionic insulator Cu2OSeO3 at ultrahigh mag-
netic fields, Phys. Rev. Lett. 136, 076703 (2026).

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