Spectroscopic Visualization of Magneto-Topological Phase Transitions in Topotactically-Converted Zintl Films

Manipulation the band structure and associated phenomenology of electronic matter is highly sought after. Magneto-topological matter provides a promising route as it couples highly tunable magnetic phase diagrams with topological electronic orders. Yet, spectroscopic demonstration of evolution of topology with magnetic order has remained elusive limited by material quality. Here, we report a new topotactic conversion growth of high quality Zintl this films. We map its magnetic phase diagram consisting of two antiferromagnetic transitions and a crossover field to spin polarized state and calculate the corresponding topological phase transitions from semiconducting to axion insulating states. Across the Neel transition we visualize in scanning tunneling spectroscopy an abrupt change between a semiconducting spectrum and that of a surface metal. In magnetic field we visualize a similar transition though it evolves gradually as the spins are progressively polarized. We find corresponding transitions in bulk Shubnikov de Haas oscillations. Longitudinal resistivity exhibits saturation below the Neel transition that further suggests the formation of topological surface modes of a bulk slightly doped axion insulator. A finite anomalous Hall signal below the lower Neel transition may correspond to the altermagnetic nature of that phase. These striking finidings set topotactic conversion growth as well as the Zintl family of materials as promising platforms for manipulationg magneto-topological matter.