Structural analysis of high-pressure phase for skyrmion-hosting multiferroic ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$

Rapid Communication

Structural analysis of high-pressure phase for skyrmion-hosting multiferroic ${Cu}_{2} {OSeO}_{3}$

E. Nishibori, S. Karatsu, C. Terakura, N. Takeshita, M. Kinoshita, S. Ishiwata, Y. Tokura, and S. Seki

Phys. Rev. B 102, 201106(R) – Published 9 November 2020

Abstract

${Cu}_{2} {OSeO}_{3}$ is known as a unique example of insulating multiferroic compounds with a skyrmion spin texture, which is characterized by a chiral cubic crystal structure at ambient pressure. Recently, it has been reported that this compound shows a pressure-induced structural transition with a large enhancement of magnetic ordering temperature. In the present Rapid Communication, we have investigated the detailed crystal structure in the high-pressure phase, by combining a synchrotron x-ray diffraction experiment with a diamond anvil cell and an analysis based on the genetic algorithm. Our results suggest that the original pyrochlore Cu network is sustained even after the structural transition, while the orientation of the ${SeO}_{3}$ molecule as well as the position of oxygen in the middle of the Cu tetrahedra are significantly modified. The latter features may be the key for the reported enhancement of $T_{c}$ and the associated stabilization of the skyrmion phase at room temperature.

Received 16 September 2020
Accepted 27 October 2020

DOI:https://doi.org/10.1103/PhysRevB.102.201106

Physics Subject Headings (PhySH)

Crystal structure Skyrmions Structural phase transition Structural properties

Multiferroics

Pressure techniques X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

E. Nishibori ¹, S. Karatsu¹, C. Terakura², N. Takeshita³, M. Kinoshita⁴, S. Ishiwata ⁵, Y. Tokura^2,4,6, and S. Seki ^2,4,7,8

¹Faculty of Pure and Applied Sciences and Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, Tsukuba 305-8571, Japan
²RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
³National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan
⁴Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
⁵Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan
⁶Tokyo College, University of Tokyo, Tokyo 113-8656, Japan
⁷Institute of Engineering Innovation, University of Tokyo, Tokyo 113-8656, Japan
⁸PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan

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Vol. 102, Iss. 20 — 15 November 2020

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Figure 1
(a) Crystal structure of ${Cu}_{2} {OSeO}_{3}$ at ambient pressure. (b) and (c) Schematic illustration of helical and skyrmion spin textures, respectively. (d)–(f) Pressure dependence of magnetic ordering temperature $T_{c}$ , lattice constants, and volume of the crystallographic unit cell, respectively. The detailed magnetic structure in the high-pressure phase (denoted as “X”) has not been resolved at this stage. In (f), the red solid line represents a linear fitting result for the data points in the $P 2_{1} 3$ phase, which is extended into the $P 2_{1}$ phase as the red dashed line. The data points in the latter phase show a considerable deviation from this line, suggesting the further suppression of the unit cell volume upon the structural phase transition. Note that the existence of an intermediate phase with space group $P 2_{1} 2_{1} 2_{1}$ has also been suggested for the narrow $P$ region between the $P 2_{1} 3$ and $P 2_{1}$ phases in Ref. [25], while this phase was not identified in the present experiments, possibly due to the limited number of pressure data points or the difference in the pressure medium.
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Figure 2
(a) Temperature dependence of ac magnetic susceptibility $χ^{'}$ , measured for the ${Cu}_{2} {OSeO}_{3}$ single crystal under zero magnetic field with various amplitudes of hydrostatic pressure $P$ . (b) and (c) Fitting results of Rietveld refinements for synchrotron x-ray powder diffraction data at (b) 2.7 GPa and (c) 10.3 GPa, i.e., before and after the structural phase transition. Here, the blue line, red $+$ symbol, light blue line, and green line represent the experimental profile, the fitted profile, the background profile, and the deviation between the experimental and fitted ones, respectively. The * symbol indicates the peak from the steel gasket. The measurement was performed at room temperature. The optical microscope image of the sample taken at each condition is also indicated.
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Figure 3
The network of Cu ions and ${SeO}_{3}$ molecules at (a) ambient pressure and (b) 10.3 GPa (i.e., the high-pressure phase). Other oxygen atoms are not shown here for clarity. Orange, blue, green, and pink balls represent the Cu1, Cu2, Se, and O atoms, respectively.
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Figure 4
(a) A pair of corner-shared Cu tetrahedra with the oxygen atoms located at their center and (b) the network of Cu ions viewed from the [010] direction, which are deduced from a structural analysis at ambient pressure. (c) and (d) The corresponding ones for $P = 10.3$ GPa (i.e., the high-pressure phase). The color of the triangular plaquette in (d) is common with the one in (c).
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Physical Review B

covering condensed matter and materials physics

Structural analysis of high-pressure phase for skyrmion-hosting multiferroic ${Cu}_{2} {OSeO}_{3}$

E. Nishibori, S. Karatsu, C. Terakura, N. Takeshita, M. Kinoshita, S. Ishiwata, Y. Tokura, and S. Seki

Phys. Rev. B 102, 201106(R) – Published 9 November 2020

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Physical Review B

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Structural analysis of high-pressure phase for skyrmion-hosting multiferroic Cu2OSeO3

E. Nishibori, S. Karatsu, C. Terakura, N. Takeshita, M. Kinoshita, S. Ishiwata, Y. Tokura, and S. Seki

Phys. Rev. B 102, 201106(R) – Published 9 November 2020

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Structural analysis of high-pressure phase for skyrmion-hosting multiferroic ${Cu}_{2} {OSeO}_{3}$