Observation of distorted tilted conical phase at the surface of a bulk chiral magnet with resonant elastic x-ray scattering

TL;DR

This study probes surface spin textures in the insulating chiral magnet Cu2OSeO3 using resonant elastic X-ray scattering at the Cu L-edge with a magnetic field applied along [001]. The authors reveal a robust distorted tilted conical (TC) surface phase that exhibits higher-harmonic satellites and can coexist with a multidomain skyrmion lattice; this surface state persists through field cycling and reorients under a tilted field, indicating a surface-specific twist not evident in the bulk. The distorted TC phase has a long real-space period of about 240 nm and shows a field- and angle-dependent reorientation and modulation-vector behavior, implying the influence of surface anisotropy and Dzyaloshinskii-Moriya interactions. Overall, the work demonstrates the importance of surface-specific chiral spin textures in Cu2OSeO3 and showcases resonant X-ray scattering as a powerful tool for engineering and characterizing such textures on bulk crystals.

Abstract

We report on various magnetic configurations including spirals and skyrmions at the surface of the magnetic insulator Cu$_2$OSeO$_3$ at low temperatures with a magnetic field applied along <100> using resonant elastic X-ray scattering (REXS). We observe a well-ordered surface state referred to as a distorted tilted conical spiral (TC) phase over a wide range of magnetic fields. The distorted TC phase shows characteristic higher harmonic magnetic satellites in the REXS reciprocal space maps. Skyrmions emerge following static magnetic field cycling and appear to coexist with the distorted TC phase. Our results indicate that this phase represents a distinct and stable surface state that does not disappear with field cycling and persists until the field strength is increased sufficiently to create the field-polarized state.

Figures (10)

• Figure 1: a) Illustration of the magnetic phase diagram for Cu$_2$OSeO$_3$ for magnetic field applied along $\hkl<001>$. HTS, LTS, and TC indicate high-temperature skyrmion, low-temperature skyrmion, and titled conical spiral phases. The rectangle highlights the field region where distorted TC spirals (magenta circles) are observed, at $T\approx$ 8 K. The blue arrows indicate the magnetic field sweep direction. b) Shows a schematic representation of two differently oriented hexagonal skyrmion lattices (blue and red) featuring Bloch-type skyrmions. c),d),e), and f) show the magnetic configurations of helical, conical, TC, and distorted TC spirals, respectively.
• Figure 2: (a) Schematic representation of the experimental setup. The X-ray beam is scattered off the sample satisfying the 2$\theta=$96.5$^\circ$ diffraction condition and is captured by a CCD camera. $\phi$ represents the rotational angle of magnetic field w.r.t to the sample normal ($\hkl<001>$) b) and c) highlight the coordinate system of field orientation relative to crystallographic directions of the sample.
• Figure 3: Comparison of the REXS intensity profile for helical (blue) and distorted tilted conical spirals (red). The figure highlights a single peak ($\text{q}_\text{h }$) from the Friedel pair of the helical spirals at 0 T, and four peaks (q, 2q, 3q, and 4q), each from an equally spaced Friedel pair of the distorted tilted conical at 45 mT, applied at an angle of $\phi = 6^\circ$ to the $\hkl[001]$ direction. The intensity peak corresponding to the structural Bragg peak is manually removed.
• Figure 4: (a) and (b) Evolution of the distorted tilted conical spiral phase under different applied magnetic fields with REXS at 8 K. (a) and (b) show the REXS intensity patterns obtained for two different magnetic field sweep directions. $\text{q}_\text{h }$ indicates the helical propagation vector defined at 0 T. The circles highlight the positions of higher-order peaks of the distorted tilted conical spiral phase. Note that the measurements shown in (a) and (b) have been carried out for two different magnetic field orientations with respect to the [001] axis represented as $\phi$.
• Figure 5: (a) and (b) show the change of both propagation vector and intensity of the first and second-order magnetic peaks (q and 2q) as a function of the applied magnetic field for ramping the magnetic field from zero to the field polarized state (a) and ramping the magnetic field to zero (b).