Catherine (Katia) Pappas (Delft University of Technology) Phase Transformations In Chiral Magnets

All archetype cubic chiral magnets show similar phase diagrams composed of helical spiral, conical spiral, and the skyrmion lattice phase, which sets-in in the so-called A-phase, just below the magnetic ordering temperature. A notable exception to this universality is the Mott insulator Cu₂OSeO₃, where an instability of the conical spiral state and a novel skyrmion phase have been reported [1-5]. These new phases occur where they are least expected, at low temperatures, where thermal fluctuations are suppressed, and at magnetic fields strong enough to align all spirals along their direction. Both the instability of the conical spiral state, which can be considered as a re-entrance into the helical state, and the skyrmion phase are sensitive to the direction of the magnetic field and occur only when the field is applied along the [001] easy crystallographic axis. According to theory, the tilted spiral state originates from an interplay of competing anisotropies, which are generic to chiral magnets and have the potential to stabilize new skyrmion states in a wide range of magnetic fields and temperatures, beyond the A-phase.

These theoretical findings are qualitatively consistent with the outcome of our small angle neutron scattering experiments. The latter reveal that, depending on the magnetic history, extremely robust skyrmionic states can be produced over large areas of the magnetic phase diagrams, from the lowest temperatures up to the A-phase. These skyrmions are thermodynamically stable or metastable depending on the orientation and strength of the magnetic field. Furthermore, the metastable skyrmions are surprisingly resilient to high magnetic fields and persist even in the field polarized state.

In order to further understand the aforementioned behaviour we have proceeded to a quantitative and detailed comparison between experiment and theory. This process leads us to the conclusion that the anisotropy constants, which are the driver behind the observed behaviour, exhibit a pronounced temperature dependence. This explains the evolution of the thermodynamically stable phases between high (weak anisotropy) and low (strong anisotropy) temperatures. In particular we find that at low temperatures the cubic anisotropy exceeds a critical value and becomes strong enough to induce a qualitatively new behavior, as seen both in the tilted spiral phase and the enhanced stability of skyrmions [6,7].

The implications of this work certainly go beyond the case of Cu₂OSeO₃. Indeed, the resulting quantitative comparison between experiment and theory marks an important step towards an in-depth understanding of chiral magnets in view of tailoring their properties for future applications.

[1] F. Qian, L. J. Bannenberg, H. Wilhelm, G. Chaboussant, L. M. DeBeer-Schmtt, M. P. Schmidt, A. Aqeel, T. T. M. Palstra, E. H. Bruck, A. J. E. Lefering, C. Pappas, M. Mostovoy, and A. O. Leonov; Sci. Adv. 4, eaat7323 (2018).

[2] A. Leonov, arXiv:1406.2177; [3] A.O. Leonov, C. Pappas; Phys.Rev.B. 99, 144410 (2019)

[4] A. Chacon, L. Heinen, M. Halder, A. Bauer, W. Simeth, S. Mühlbauer, H. Berger, M. Garst, A. Rosch, and C. Pfleiderer, Nat. Phys. 14, 936 (2018).

[5] L. J. Bannenberg, H. Wilhelm, R. Cubitt, A. Labh, M. Schmidt, E. Lelievre-Berna, C. Pappas, M. Mostovoy, and A. O. Leonov, NPJ Quantum Mater. 4, 11 (2019).

[6] A.O. Leonov and C. Pappas, Phys. Rev. Research 4, 043137 (2022).

[7] M. Crisanti, A. O. Leonov, R. Cubitt, A. Labh, H. Wilhelm, Marcus P. Schmidt, and C. Pappas submitted to Phys. Rev. Research.