Altermagnetic spin splitting and symmetry-enforced partial spin degeneracy in hexagonal MnTe

Abstract: Besides hosting several intriguing physical properties, the recently discovered time-reversal-asymmetric antiferromagnets, known as altermagnets, hold immense promise for technologies based on spintronics. Understanding the symmetry conditions leading to the spin-splitting becomes the key to further progress in the field. Hexagonal MnTe emerges as an even-parity magnet within the altermagnet family. In this work, using ab initio density functional theory (DFT) within a combination of an appropriate exchange-correlation functional and the relevant corrections, we uncover the spin-splitting features of MnTe. Our calculations reveal the spin degeneracy to be preserved in the $k_z = 0$ and $k_y = 0$ planes, while spin-splitting is observed everywhere else in the Brillouin zone, except the nodal lines identified here. To explain these findings, we provide a comprehensive symmetry analysis based on magnetic space group theory and introduce an insightful symmetry-adapted model Hamiltonian that qualitatively describes the spin-splitting behavior in different parts of the Brillouin zone. Our calculations considering spin-orbit interaction reveal no weak ferromagnetism in MnTe. Nevertheless, we discuss plausible explanations for weak ferromagnetism and anomalous Hall effect reported from experiments. Our comprehensive analysis of the magnetic space group symmetry and the DFT results leads to a thorough understanding of altermagnetism in MnTe, paving the way for possible future technology.