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.

Topological Hall effect (THE) of electrons coupled to a noncoplanar spin texture has been studied so far for the strongand weak-coupling regimes separately; the former in terms of the Berry phase and the latter by perturbation theory. In this letter, we present a unified treatment in terms of spin gauge field by considering not only the adiabatic (Berry phase) component of the gauge field but also the nonadiabatic component. While only the adiabatic contribution is important in the strong-coupling regime, it is completely canceled by a part of the nonadiabatic contribution in the weak-coupling regime, where the THE is governed by the rest of the nonadiabatic terms. We found a new weak-coupling region that cannot be accessed by a simple perturbation theory, where the Hall conductivity is proportional to M, with 2M being the exchange splitting of the electron spectrum. arXiv:1711.08366v1 [cond-mat.mes-hall]

Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g-factor g e and effective damping constant e of the GdFeCo films were estimated by using a numerical calculation of Landau-Lifshitz-Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition M of the GdFeCo, while the estimated g e and e increased around the angular momentum compensation composition A . The compositional dependences of g e and e were roughly described by a mean-field model. The g e and e were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near M . The FMR method was unable to excite the magnetization near M because of the small net magnetization.