Photon Factory Activity Report 2019 #37 (2020)

Skyrmion phase on a frustrated breathing kagomé lattice

BL-3A/2015S2-007

Max Hirschberger1, Taro Nakajima1, Shang Gao1, Licong Peng1, Akiko Kikkawa1, Takashi
Kurumaji1, Markus Kriener1, Yuichi Yamasaki2,3, Hajime Sagayama4, Hironori Nakao4, Kazuki
Ohishi5, Kazuhisa Kakurai1,5, Yasujiro Taguchi1, Xiuzhen Yu1, Taka-hisa Arima1,6, and Yoshinori
Tokura1,61
RIKEN Center for Emergent Matter Science, 2National Institute for Materials Science, 3Japan
Science and Technology Agency, 4High Energy Accelerator Research Organization, 5Comprehensive
Research Organization for Science and Society, 6The University of Tokyo
1 Abstract
Recently, our group has experimentally realize [1-3]theoretical predictions [4,5] of a new class of skyrmion
host compounds with centrosymmetric space groups. In
contrast to skyrmions stabilized by Dzyaloshinskii-Moriya
interactions [6], skyrmions here emerge due to frustrated
interactions [4] or due to higher order terms included in theKondo Hamiltonian describing local moment magnets
coupled to a Fermi sea [5]. Gd3Ru4Al12 is a good metal
with Heisenberg-type Gd3+ moments, where competinginteractions are realized due to the symmetry of the lattice.
We have experimentally observed the skyrmion phase andcompeting magnetic orders using resonant elastic x-ray
scattering (REXS) at the Gd-L2 edge using beamline BL3A [2].
2 MainCentrosymmetric materials with relatively high(hexagonal or tetragonal) symmetry are an emerging focus
in the hunt for topological spin textures [1-3]. Here,
competing Heisenberg-like interactions or four-spin terms(e.g. in the RKKY–expansion of the Kondo Hamiltonian)
can realize complex magnetic orders such as spirals andskyrmion vortices. As compared to skyrmions incentrosymmetric materials (e.g. MnSi [6]) or at interfaces[7], Dzyaloshinskii-Moriya interactions (DMI) are absent
or cancel out globally. Characteristically, spin textures incentrosymmetric materials condense at much shorter
length scales ( ~2 − 3 nanometers) as compared to thetypical DMI case (~10 − 300 nm), and generate a verylarge emergent magnetic field 𝐵𝐵𝑒𝑒𝑒𝑒 . The latter is thought todrive large transport and magneto-optical responses [8], aswell as new functionality [9].
Gd3Ru4Al12 crystallizes in a quasi-layered structure ofP63/mmc space group, with perfectly planar sheets of Gd3+
magnetic ions in a distorted (‘breathing’) Kagome lattice(Fig. 1). The Ru sublattice, forming a triangular net, islargely responsible for electronic conduction. As compared
to another compound, Gd2PdSi3, the present material ismore metallic and boasts a qualitatively different phase
diagram, where a transverse conical state competes withthe skyrmion lattice at low temperature T (Fig. 2). This
indicates that manipulation of magnetic anisotropy isimportant in the control of topological spin textures incentrosymmetric magnets.

Figure 1: (a) crystal structure of Gd3Ru4Al12 in
P63/mmc space group. (b) Illustration of the ‘breathing
Kagome’ rare earth sublattice dominating the magneticproperties. Excerpt from Ref. [2].
We studied REXS at BL-3A of Photon Factory usinglarge, polished single crystals in reflection geometry inside
the 8 Tesla superconducting magnet [2]. With the aid ofpolarization analysis, scattering from the q = (0.23, 0, 0)
reflection was observed in several phases. At the lowest
temperature (T = 2.4 K), we identified helical order in zeromagnetic field (H = 0), followed by a transverse conicaland a fan-like state as H is increased. At higher temperature(T ~ 8 K), within the boundaries of the SkL phase as
determined from bulk characterization, transport, andLorentz-transmission electron microscopy [2], weobserved REXS intensity consistent with two volume
fractions: a helical-type state (the skyrmion lattice)
coexists with fan-like order. As the helical nature of theskyrmion state cannot be tested in LTEM, REXS provided
very important input for the interpretation of our combined
experimental results.

Photon Factory Activity Report 2019 #37 (2020)

Figure 2: (a-c,j) Illustration of magnetic orders in various phases.(d-i) REXS with polarization analysis, HK0 scattering
plane, Gd-L2 edge for phases in (a-c). 𝜋𝜋 − 𝜋𝜋 ′ (red) and 𝜋𝜋 − 𝜎𝜎 ′ (blue) components of the magnetic satellite at q = (0.23,0, 0) were extracted and associated with in-plane and out-of-plane components of the ordered moment. (k-m) same forthe skyrmion lattice. The 𝜋𝜋 − 𝜋𝜋 ′ intensity was found to be comparable for three reflections, consistent with multi-q
magnetic order [inset of (m)]. The magnetic field was applied parallel to the c-axis
References[1] T. Kurumaji et al., Science 365, 914-918 (2019).
[2] M. Hirschberger et al., Nat. Comms. 10, 5831 (2019)[3] N.D. Khanh et al., to appear in Nat. Nano. (2020)[4] T. Okubo, S. Chung, and H. Kawamura, Phys. Rev. Lett. 108, 017206 (2012).
[5] S. Hayami, R. Ozawa, and Y. Motome, Phys. Rev. B 95, 224424 (2017).
[6] S. Mühlbauer et al., Science 323, 915 (2009).
[8] R. Shindou and N. Nagaosa, Phys. Rev. Lett. 87, 116801 (2001)[9] N. Nagaosa, Jap. J. App. Phys. 58, 120909 (2019)* arima@k.u-tokyo.ac.jp