Spin excitations in the skyrmion hostCu2OSeO3

Abstract

We have used inelastic neutron scattering to measure the magnetic excitation spectrum along the high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound Cu 2 OSeO 3 . The majority of our scattering data are consistent with the expectations of a recently proposed model for the magnetic excitations in Cu 2 OSeO 3 , and we report best-fit parameters for the dominant exchange interactions. Important differences exist, however, between our experimental findings and the model expectations. These include the identification of two energy scales that likely arise due to neglected anisotropic interactions. This feature of our work suggests that anisotropy should be considered in future theoretical work aimed at the full microscopic understanding of the emergence of the skyrmion state in this material. [8], and in the polar magnetic semiconductor GaV 4 S 8 [9]. To understand the formation and the microscopic origin of these skyrmion phases, one needs a multiscale approach that covers the macroscopic domain of the skyrmion as well as the quantum scale of the local spins. This, however, breaks down in the above-mentioned metals because the low-energy delocalized electrons and magnetic degrees of freedom are mixed, intrinsically involving multiple energy and spatial scales.Among cubic helimagnets, Cu 2 OSeO 3 is the only insulator with magnetoelectric properties in the ground state [8,[10][11][12][13][14]. It offers an ideal laboratory to explore the microscopic ingredients that lead to skyrmion formation in a quantitative manner, since its Bloch-type ground-state properties and low-energy excitations are fully governed by the magnetic interactions between localized spins and are not affected by the presence of itinerant carriers. Exchange pathway considerations, susceptibility measurements, and ab initio calculations reveal that two magnetic energy scales divide the system into weakly coupled Cu 4 tetrahedra [15]. These Cu 4 "molecules," with an effective spin of S = 1, are the elementary magnetic building blocks of Cu 2 OSeO 3 instead of the single Cu ions. The effective spins of the Cu 4 tetrahedra are ferromagnetically coupled and form a trillium lattice, just as the Mn and Fe ions do in the B20 structure of the metallic skyrmion compounds MnSi and FeGe.Prior to the undertaking of the present work, previous studies of the magnetic excitation spectra of Cu 2 OSeO 3 were conducted using Raman scattering [16] and microwave resonance absorption [17], i.e., techniques that are sensitive only to excitations in the center of the Brillouin zone. In contrast, inelastic neutron scattering (INS) is able to measure at finite momentum transfer and is therefore uniquely suited to probe the magnetic excitation spectra of Cu 2 OSeO 3 throughout reciprocal space. The additional information afforded by INS therefore provides more rigorous tests of theoretical models aimed at describing the excitation spectra of Cu 2 OSeO 3 .Single crystals of Cu 2 OSeO 3 (cubic P 2 1 3 space group, a = 8.82Å) were gro...

Low-energy magnons in the chiral ferrimagnet Cu2OSeO3 : A coarse-grained approach

“…The high symmetry points are defined as: Γ(h, k, l); X(h, k, l + 1 2 ); M (h, k + 1 2 , l+ 1 2 ); and R(h+ 1 2 , k + 1 2 , l+ 1 2 ) with h, k, and l integers. While broadly consistent with the prior work [15], our high-resolution data reveal important new features: (1) A splitting at the R point ∆ R = 1.6(2) meV between the two modes with dominant intensity(previously reported by Tucker et al [18]), whereas the Heisenberg model of Romhányi et al [15] implies four-fold degeneracy. A third mode between 6 meV and 8 meV can also be observed at R points for high momentum transfer.…”

“…A third mode between 6 meV and 8 meV can also be observed at R points for high momentum transfer. Consistent with [18], we identify this mode as a phonon ( Fig. 2(k)) based on the |Q| 2 dependence of the integrated intensity [25] (2) Near the X point there is a dramatic broadening of the lower branch (between 4 and 8 meV in Fig 1(e)), where the Heisenberg model [15] calls for two-fold degeneracy.…”

“…In Table. C we compare our fit parameters to previous studies [17][18][19]45]. The microscopic parameters J AF w , J FM w , J AF o.o and the DM interaction on these bonds can be transformed into FM exchange and DM interaction in the effective spin-1 cluster picture under the assumption |J AF s , J FM During the processing of MACS data, we identified and subtracted Bragg spurions that arise when neutrons at the energy E f = 2.4 meV reach the sample due to a diffuse process at the monochromator and Bragg diffract from the sample.…”

“…The structural unit cell has 16 magnetic Cu 2+ spin-1/2 ions which makes a microscopic description at the level of individual spins rather complex and impractical. Romhanyi et al [15][16][17][18] Fig. 1(a).…”

Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3

“…Such a full quantum treatment has been performed by Janson et al 29 and Romhányi et al 44 , while corresponding neutron scattering 42,43 , high field THz ESR 36 , and Raman spectroscopy 35 experiments reveal a striking agreement between the theoretical and experimentally observed excitation spectrums.…”