GERMANYStrong electronic correlations can produce remarkable phenomena such as metal-insulator transitions 1 and greatly enhance superconductivity 2 , thermoelectricity 3 , or optical non-linearity 4 . In correlated systems, spatially varying charge textures also amplify magnetoelectric effects 5 or electroresistance in mesostructures 6 . However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) 7,8 in thin films of a lightly electron-doped charge-transfer insulator, (Ca, Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems 9 . The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.* manuel.bibes@cnrs-thales.fr  these authors contributed equally to the manuscript.

High quality thin films of topological insulators (TI) such as Bi2Se3 have been successfully synthesized by molecular beam epitaxy (MBE). Although the surface of MBE films can be protected by capping with inert materials such as amorphous Se, restoring an atomically clean pristine surface after decapping has never been demonstrated, which prevents in-depth investigations of the intrinsic properties of TI thin films with ex-situ tools.Using high resolution scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrate a simple and highly reproducible Se decapping method that allows recovery of the pristine surface of extremely high quality Bi2Se3 thin films grown and capped with Se in a separate MBE system then exposed to atmosphere during transfer into the STM system. The crucial step of our decapping process is the removal of the surface contaminants on top of amorphous Se before thermal desorption of Se at a mild temperature (~210 °C). This effective Se decapping process opens up the possibility of ex-situ characterizations of pristine surfaces of interesting selenide materials and beyond using cutting-edge techniques.

We report a systematic study of ferromagnetic domains in both single-crystal and thin-film specimens of magnetic topological insulators Cr doped (Bi0.1Sb0.9)2Te3 using magnetic force microscopy (MFM). The temperature and field dependences of MFM and in situ resistance data are consistent with previous bulk transport and magnetic characterization. Bubble-like ferromagnetic domains were observed in both single crystals and thin films. Significantly, smaller domain size (∼500 nm) with narrower domain wall (∼150 − 300 nm) was observed in thin films of magnetic topological insulators, likely due to vertical confinement effect. These results suggest that thin films are more promising for visualization of chiral edge states.

A systematic magnetic force microscopy (MFM) study of domain behavior in thin films of the magnetic topological insulator Sb 1.89 V 0.11 Te 3 reveals that in the virgin domain state, after zero-field cooling, an equal population of up and down domains occurs. Interestingly, the cooling field dependence of MFM images demonstrates that a small cooling magnetic field (approximately 5-10 Oe) is sufficient to significantly polarize the film despite the coercive field (H C) for these films being on the order of a tesla at low temperature. By visualizing the magnetization reversal process around H C of V-doped Sb 2 Te 3 , we observed a typical domain behavior of a ferromagnet, i.e., domain nucleation and domain wall propagation. Our results provide direct evidence of ferromagnetic behavior of the magnetic topological insulator, a necessary condition for a robust quantum anomalous Hall effect.

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