Topological and Electronic Properties of Electron-Doped CaMnO3 Thin Films

Lorenzo Vistoli
June 28th, 2019 LORENZO VISTOLI Unité Mixte de Physique CNRS/Thales

Topology is a concept of increasing relevance in physics. In particular, topological spin structures such as magnetic skyrmions are attracting a large amount of interest due to their novel physical properties and promising potential applications. Recently, skyrmions and skyrmion bubbles have been shown to be harbored also in oxides. These discoveries have opened the study of topological spin structures in quantum materials, which provide a platform to investigate how these structures interact with strong correlations and interplaying degrees of freedom.

We take advantage of the strong sensitivity to strain and chemical doping of oxide materials to engineer thin films with topological spin structures. To do so, we use CaMnO3, a charge-transfer insulator whose peculiarity is that the chemical substitution of Ca by Ce induces a transition to a weakly-ferromagnetic and metallic phase at very low doping. In addition, compressive strain strengthens magnetoelastic anisotropy and imposes a perpendicular magnetization axis. The combination of these characteristics makes this compound attractive to probe for the topological Hall effect, a signature of skyrmions and skyrmion bubbles in a metallic system.

First, we use angle-resolved photoemission spectroscopy to determine the electronic structure of lightly doped CaMnO3 thin films. We show that doping first promotes the formation of conduction states with dispersive and non-dispersive bands, showing respectively both signs of weak coupling to phonons and strong polaronic signature. This band-dependent coupling to phonons is able to explain the electronic transport properties of doped CaMnO3, and sheds light on how conduction bands are formed when doping Mott insulators.

We then report the presence of an anomalous Hall effect and a topological Hall signal (THE) in these thin films. Remarkably, both signals are very large and present up to low temperatures. Magnetic force microscopy reveals that magnetic bubbles appear during magnetization reversal. The bubble density directly relates to the THE signal, strongly implying that these are skyrmion bubbles. The magnitude of the THE is much higher than both previous reports, and what one would expect from such a skyrmion bubble density. We discuss mechanisms of enhancement of the THE in doped Mott insulators, and point out how this can be ascribed to the strong correlations close to the metal-to-insulator transition.

Seminar, June 28, 2019, 15:00. ICFO’s Seminar Room

Hosted by Prof. Adrian bachtold