Altermagnetic XMCD in Hematite Distinct from Weak Ferromagnetic Contributions

Altermagnets are compensated collinear magnets that break time-reversal symmetry without net magnetization, enabling unconventional magneto-optical responses. Here, altermagnetic X-ray magnetic circular dichroism (XMCD) is experimentally demonstrated in hematite $\alpha$-Fe$_2$O$_3$. By employing a symmetry-selective geometry in which the x-ray propagation vector is orthogonal to the Dzyaloshinskii-Moriya-induced weak ferromagnetic moment, we isolate a finite XMCD signal that cannot be attributed to conventional weak ferromagnetism. Moreover, we demonstrate that distinct altermagnetic states characterized by different magnetic symmetries can be reversibly switched through the application of an in-plane external magnetic field. Full-multiplet calculations reveal that the signal originates from an anisotropic magnetic dipole moment realized in the $2p^53d^6$ excited states, despite the isotropic $2p^63d^5$ ground state. Our results establish XMCD as a direct probe of excited-state magnetic multipoles and provide a general route for the optical detection of altermagnetic order in compensated magnets.