Formation mechanism of helical Q structure in Gd-based skyrmion materials*

Recently, short-period skyrmion phases have been observed in Gd-based compounds[1-3]. Using the ab initio local force method, we investigate the formation mechanism of the helical spin structure in GdRu₂Si₂ and Gd₂PdSi₃[4]. We calculate the paramagnetic spin susceptibility and find that the Fermi surface nesting is not the origin of the incommensurate modulation, in contrast to the naive scenario based on the Ruderman-Kittel-Kasuya-Yosida mechanism. We then decompose the exchange interactions between the Gd spins into each orbital component, and show that spin-density-wave type interaction between the Gd-5d orbitals is ferromagnetic, but the interaction between the Gd-4f orbitals is antiferromagnetic. We conclude that the competition between these two interactions, namely, the inter-orbital frustration, stabilizes the finite-Q structure.
[1] T. Kurumaji et al., Science 365, 914 (2019).
[2] M. Hirschberger et al., Nat. Commun. 10, 5831 (2019).
[3] N. D. Khanh et al., Nat. Nanotechnol. 15, 444 (2020)
[4] T. Nomoto, T. Koretsune, and R. Arita, Phys. Rev. Lett. 125, 117204 (2020).