# Regensburg 2022 – wissenschaftliches Programm

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# KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

## KFM 28: Topological Insulators (joint session MA/KFM)

### KFM 28.2: Vortrag

### Donnerstag, 8. September 2022, 15:30–15:45, H37

Tuning the magnetic gap of a topological insulator — •Marcus Liebmann^{1}, Philipp Küppers^{1}, Jannik Zenner^{1}, Stefan Wimmer^{2}, Gunther Springholz^{2}, Oliver Rader^{3}, and Markus Morgenstern^{1} — ^{1}II. Phys. Inst. B, RWTH Aachen Univ., Germany — ^{2}Inst. Halbleiter- u. Festkörperphysik, Johannes Kepler Univ., Linz, Austria — ^{3}Helmholtz-Zentrum Berlin f. Mater. u. Energie, Germany

Mn-rich MnSb_{2}Te_{4} is a ferromagnetic topological insulator with yet the highest Curie temperature T_{C} = 45−50 K. It exhibits a magnetic gap at the Dirac point of the topological surface state that disappears above T_{C}. We probe the gap size by scanning tunneling spectroscopy, varying in-plane magnetic field B_{||} and temperature. We demonstrate shrinkage of the average gap size with B_{||} revealing that the gap opening originates from out-of-plane magnetization. In line, the gap does not close completely up to B_{||} = 3 T as the magnetization is only partially rotated in-plane. In addition, we demonstrate significant spatiotemporal fluctuations of the gap size at temperatures as low as T_{C}/2, above which the remanent magnetization indeed decays. Thus, the gap is tightly bound to the out-of-plane magnetization, as expected theoretically but not demonstrated experimentally yet. The partial in-plane rotation at B_{||} = 3 T and the low temperature onset of fluctuations stress the important role of competing magnetic orders in the formation of the favorable ferromagnetic topological insulator in Mn-rich MnSb_{2}Te_{4}, providing insight into the complex magnetic gap opening that is decisive for quantum anomalous Hall devices.