Field-tunable Weyl points and large anomalous Hall effect in the degenerate magnetic semiconductor EuMg2Bi2

Magnets, with topologically nontrivial Dirac/Weyl points, have recently attracted significant attention owing to their unconventional physical properties, such as a large anomalous Hall effect. However, they typically have a high carrier density and a complicated band structure near the Fermi energy. In this Letter, we report a degenerate magnetic semiconductor ${\mathrm{EuMg}}₂{\mathrm{Bi}}₂$, which exhibits a single valley at the $\mathrm{\ensuremath{\Gamma}}$ point, where field-tunable Weyl points form via a magnetic exchange interaction with the local Eu spins. By the high-field measurements on high-quality single crystals, we observed quantum oscillations in the resistivity, elastic constant, and surface impedance, which enabled us to determine the position of the Fermi energy ${E}_F$. In combination with a first-principles calculation, we revealed that the Weyl points are located in the vicinity of ${E}_F$ when the Eu spins are fully polarized, leading to a peak of energy-dependent anomalous Hall conductivity due to the Berry curvature. Accordingly, in the forced ferromagnetic phase, we observed a large anomalous Hall effect (Hall angle ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{AH}}\ensuremath{\sim}0.07$) qualitatively consistent with the calculation, which demonstrates a marked impact of the Weyl points in the simple band structure.