Altermagnets represent a recently discovered class of collinear magnets, characterized by antiparallel neighboring magnetic moments and an alternating-sign spin polarization in momentum space ($\mathit{k}$ space). However, experimental methods for probing the $\mathit{k}$-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 $\mathit{k}$-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 $\mathit{k}$-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 $\mathit{k}$-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.

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• Altermagnetism
• Anomalous Hall effect
• Dilute magnetic topological insulators
• Magnetic order
• Magnetic texture
• Topological Hall effect
• Altermagnets
• Topological materials