Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2 Te4 is, however, antiferromagnetic with 25 K Néel temperature and is strongly n-doped. In this work, p-type MnSb2 Te4 , previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45-50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn-Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2 Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

2 Institute of Physics B and JARA FIT RWTH Aachen Unversity 52074 Aachen Germany

Central European Institute of Technology Brno University of Technology Purkyňova 123 Brno 612 00 Czech Republic

Centro de Física de Materiales Centro Mixto CSIC UPV EHU San Sebastián Donostia 20018 Spain

Departamento de Polímeros y Materiales Avanzados Física Química y Tecnología Facultad de Ciencias Químicas Universidad del País Vasco UPV EHU San Sebastián Donostia 20080 Spain

Department of Condensed Matter Physics Masaryk University Kotlářská 267 2 Brno 61137 Czech Republic

Department of Physics and Astronomy Universitet Uppsala Lägerhyddsvägen 1 Uppsala 75120 Sweden

Donostia International Physics Center San Sebastián Donostia 20018 Spain

Helmholtz Zentrum Berlin für Materialien und Energie Albert Einstein Straße 15 12489 Berlin Germany

IKERBASQUE Basque Foundation for Science Bilbao 48011 Spain

Institut für Halbleiter und Festkörperphysik Johannes Kepler Universität Altenberger Straße 69 Linz 4040 Austria

Institut für Physik und Astronomie Universität Potsdam Karl Liebknecht Straße 24 25 14476 Potsdam Germany

Institute for Theoretical Physics Johannes Kepler Universität Altenberger Straße 69 Linz 4040 Austria

Max Planck Institute of Microstructure Physics Weinberg 2 06120 Halle Germany

Peter Grünberg Institut and Institute for Advanced Simulation Forschungszentrum Jülich and JARA 52425 Jülich Germany

Saint Petersburg State University Saint Petersburg 198504 Russia

Tomsk State University Tomsk 634050 Russia

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