  Shubnikov-de Hass oscillations are directly observed in undoped
antiferromagnetic topological insulator MnBi2Te4. With increasing magnetic
fields, the oscillation period decreases gradually in the magnetic transition
from canted antiferromagnetism to ferromagnetism and then saturates in high
magnetic fields, indicating the field-induced evolution of the band structure.
From the analysis of the high-field oscillations, a nontrivial Berry phase and
a small effective mass are extracted, in agreement with the predicted Weyl
semimetal phase in ferromagnetic MnBi2Te4. Furthermore, rotating the
magnetization of MnBi2Te4 can lead to a splitting of the high-field
oscillations, which suggests the enhanced asymmetry of the Weyl cones in tilted
fields. Therefore, the observation of these magnetically tunable quantum
oscillations clearly demonstrates the indispensable role of field in tuning the
band structure or physical properties of MnBi2Te4.
