Probing k-Space Alternating Spin Polarization via the Anomalous Hall Effect

Altermagnets represent a recently discovered class of collinear magnets, characterized by antiparallel neighboring magnetic moments and an alternating-sign spin polarization in momentum space (k space). However, experimental methods for probing the k-space spin polarization in altermagnets remain limited. In this Letter, we propose an approach to address this challenge by interfacing an altermagnet with the surface of a topological insulator. We show that the altermagnet's unique k-space spin polarization imprints a momentum-dependent, sign-alternating Dirac mass onto the otherwise massless surface states of the topological insulator, a direct consequence of breaking time-reversal symmetry. This engineered Dirac mass results in a unique, alternating half-quantized anomalous Hall effect. By measuring the Hall conductance, we can extract the local k-space magnetic moment. Moreover, we can map the global magnetic moment distribution by tuning the Dirac point position using an in-plane magnetic field, thereby revealing the k-space spin density of the altermagnet. This Letter establishes the Dirac fermion on the topological insulator surface as a sensitive probe for unveiling spin characters of altermagnets and those of other unconventional antiferromagnets.