Signatures of bi-altermagnetism revealed by sublattice-specific circular dichroism in resonant inelastic X-ray scattering

An altermagnet is a recently identified class of magnets that exhibit a zero net magnetic moment but break symmetry under the combined operations of parity and time reversal. It typically consists of two magnetic sites of opposite spins related by rotation within the unit cell. Here, we use circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) to reveal symmetry signatures of a distinct form of altermagnetism, namely bi-altermagnetism, in the correlated insulator Fe2Mo3O8, which comprises two altermagnetic sublattices: one with alternating quasi-octahedral Fe environments and the other with alternating tetrahedral Fe environments. We experimentally revealed the emergence of CD in an achiral, zero-magnetization system, thereby probing mirror-symmetry breaking associated with altermagnetic order. Notably, the CD appeared at sublattice-specific excitations of the octahedral and tetrahedral sites, indicating symmetry breaking in both altermagnetic sublattices. Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD. Our findings support the realization of bi-altermagnetism in Fe2Mo3O8, and showcase the use of RIXS-CD as a probe of magnetic sublattices in systems with zero net magnetization. The standard definition of altermagnetism is of two magnetic sites of opposite spin related by rotation within the unit of a material. Here, using resonant inelastic X-ray scattering, Channagowdra et al. unveil a ’bi-altermagnetic’ state in Fe2Mo3O8, where the two inequivalent Fe sites each form an opposite-spin sublattice pair that independently satisfies the conventional criteria for altermagnetism.