Jan 31, 2020; Colloquium
SFB colloquium Ishiwata
(Osaka University, Japan)
Topological and helical spin structures in centrosymmetric perovskite-type oxides
Topological spin texture consisting of a multiple-q spin spiral is of great interest for novel quantum transport phenomena and spintronic functions. A recent interesting example is a magnetic skyrmion, which is a topologically stable spin texture discovered in noncentrosymmetric systems with Dzyaloshinskii-Moriya (DM) interaction [1]. A cubic perovskite SrFeO3 is a promising candidate of the centrosymmetric compound hosting a novel type of topological spin texture in the absence of the DM interaction. While the magnetic ground state has been believed to be a simple proper-screw-type spin order for long time, we have found that the magnetic phase diagram of SrFeO3 hosts a rich variety of helimagnetic phases, two of which show novel topological helimagnetic orders [2].
In this seminar, after the brief review of our recent works, I will show the topologically nontrivial helimagnetic phases in the simple cubic perovskite SrFeO3, which were discovered by small angle neutron scattering (SANS) measurements. We found that SrFeO3 shows two kinds of multiple-q helimagnetic structures: an anisotropic double-q spin spiral and an isotropic quadruple-q spiral hosting a three-dimensional lattice of topological singularities [3]. As a related topic, our recent discoveries of novel helimagnetic phases in the perovskites Sr1-xBaxCoO3 [4] and Sr1-xCaxCoO3 [5] will be also presented. By magnetization measurements, the magnetic phase diagrams of Sr1-xBaxCoO3 and Sr1‑xCaxCoO3 were established. As for Sr1-xBaxCoO3, the ferromagnetic order tends to be suppressed by the Ba substitution and eventually replaced by a helimagnetic state. Based on the first-principles calculations, the results will be discussed in terms of the competing magnetic order in the Co-O lattice. These results indicate that centrosymmetric perovskite-type transition-metal oxides obtained at high pressures are fruitful show cases for nontrivial helimagnetic phases.
[1] S. Mühlbauer et al., Science 323, 915 (2009).
[2] S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011).
[3] S. Ishiwata et al., arXiv:1806.02309.
[4] H. Sakai, S.I. et al., Phys. Rev. Mater. 2, 104412 (2018).
[5] T. Osaka, S.I. et al., Phys. Rev. B 95, 224440 (2017).