Imaging, controlling and harnessing non-collinear magnetism in perovskite oxides*

In magnetic perovskites, first-neighbour antiferromagnetic super-exchange interactions usually dominate, but may coexist with other terms such as ferromagnetic double-exchange or Dzyaloshinskii-Moriya interactions. This often produces non-collinear spin configurations leading to weak ferromagnetism or to spatially modulated spin structures. A prototypical non-collinear magnetic oxide is multiferroic BiFeO₃ that shows a cycloidal order with a 64 nm period in the bulk. In this talk, I will present real-space images of the cycloidal structure and its manipulation by electric field [1]. In a second part, I will report the observation of a very large topological Hall effect (THE) in thin films of a lightly electron-doped manganite. Magnetic force microscopy reveals the presence of small magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems. The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition, suggesting its amplification by strong correlations, in the vicinity of the Mott transition [2].
[1] I. Gross et al, Nature 549, 252 (2017).
[2] L. Vistoli et al, Nature Phys. (2018) http://dx.doi.org/10.1038/s41567-018-0307-5