Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase in ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$

Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase in ${Cu}_{2} {OSeO}_{3}$

T. Adams, A. Chacon, M. Wagner, A. Bauer, G. Brandl, B. Pedersen, H. Berger, P. Lemmens, and C. Pfleiderer

Phys. Rev. Lett. 108, 237204 – Published 8 June 2012

Abstract

We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator ${Cu}_{2} O {SeO}_{3}$ . The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and ${Fe}_{1 - x} {Co}_{x} Si$ , i.e., binary isostructural siblings of ${Cu}_{2} O {SeO}_{3}$ that order helimagnetically. The temperature dependence of the specific heat of ${Cu}_{2} {OSeO}_{3}$ is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.

Received 21 February 2012

DOI:https://doi.org/10.1103/PhysRevLett.108.237204

Authors & Affiliations

T. Adams¹, A. Chacon^1,2, M. Wagner¹, A. Bauer¹, G. Brandl^1,2, B. Pedersen², H. Berger³, P. Lemmens⁴, and C. Pfleiderer^1,*

¹Technische Universität München, Physik-Department E21, D-85748 Garching, Germany
²Forschungsneutronenquelle Heinz Maier Leibnitz (FRM II), Lichtenbergstr. 1, 85748 Garching, Germany
³Ecole Polytechnique Federale Lausanne, CH-1015 Lausanne, Switzerland
⁴Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany

^*christian.pfleiderer@frm2.tum.de

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Vol. 108, Iss. 23 — 8 June 2012

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Figure 1
Magnetization of single crystal ${Cu}_{2} O {SeO}_{3}$ for various crystallographic directions. (a) Temperature dependence of the magnetization in the vicinity of $T_{c}$ . (b) Ratio $μ_{0} M / B$ versus temperature revealing the features characteristic of the transition to the $A$ phase. Panels (c) through (j): Magnetization as a function of field at various temperatures. Panels on the right-hand side show typical data just below $T_{c}$ , where a clear minimum in $μ_{0} d M / d B$ , calculated from the magnetization, is observed in the $A$ phase.Reuse & Permissions
Figure 2
Magnetic phase diagram of ${Cu}_{2} O {SeO}_{3}$ as a function of applied magnetic field for various orientations inferred from the magnetization. (a) Overview for field parallel $⟨ 111 ⟩$ . Panels (b) through (d): Phase diagram in the vicinity of $T_{c}$ for various orientations. Differences as a function of field are mostly due to demagnetizing effects; the brown shading indicates the regime of nearly critical spin fluctuations.Reuse & Permissions
Figure 3
Typical integrated small-angle neutron scattering rocking scans in ${Cu}_{2} O {SeO}_{3}$ . Data are shown as counts per standard monitor (stand. mon.). (a) Zero-field scattering pattern along $⟨ 100 ⟩$ , characteristic of helimagnetic order along $⟨ 100 ⟩$ . (b) Zero-field scattering pattern along $⟨ 110 ⟩$ , characteristic of helimagnetic order along $⟨ 100 ⟩$ . (c) Typical scattering pattern in the field range $B_{c 1} < B < B_{c 2}$ for $T ≪ T_{c}$ . (d) Scattering pattern in the $A$ phase for magnetic field perpendicular to the neutron beam. Panels (e) through (h): Typical scattering pattern in the $A$ phase for magnetic field parallel to the neutron beam for various orientations.Reuse & Permissions
Figure 4
Typical temperature and magnetic field dependencies of peak intensities recorded in SANS (see text for details). (a) Temperature dependence of the helical order, cf., Fig. 3a. (b) Field dependence of the helical and conical state, where triangles correspond the helical state, cf., Fig. 3b, and squares to the conical state, cf., Fig. 3c. Labels (1), (2), and (3) mark the sequence in which sweeps were recorded; arrows show the sweep direction. Panels (c) and (d): Temperature and field dependence in the $A$ phase, cf., Fig 3g. (e) Field dependence in the temperature range of the $A$ phase; helical order (triangles, marked h), conical order (squares, marked c) and $A$ phase (triangles, marked A), cf., Figs. 3b, 3c, 3d.Reuse & Permissions
Figure 5
Specific heat, $C$ , of ${Cu}_{2} O {SeO}_{3}$ . With increasing temperature the helimagnetic transition is characterized by peak at $T_{c}$ followed by a hump with a point of inflection at $T_{2}$ . The lack of field dependence at $T_{2}$ is known as Vollhardt invariance. The $B$ and $T$ dependence of $C$ is characteristic of nearly critical helimagnetic spin fluctuations.Reuse & Permissions

Physical Review Letters

Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase in Cu2OSeO3

T. Adams, A. Chacon, M. Wagner, A. Bauer, G. Brandl, B. Pedersen, H. Berger, P. Lemmens, and C. Pfleiderer

Phys. Rev. Lett. 108, 237204 – Published 8 June 2012

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Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase in ${Cu}_{2} {OSeO}_{3}$