Electronic commensuration of a spin moiré superlattice in a layered magnetic semimetal

Spin moiré superlattices (SMSs) have been proposed as a magnetic analog of crystallographic moiré systems and a source of electron minibands offering vector-field moiré tunability and Berry curvature effects. However, it has proven challenging to realize an SMS in which a large exchange coupling J is transmitted between conduction electrons and localized spins. Furthermore, most systems have carrier mean free paths l_mfp shorter than their spin moiré lattice constant a_spin, inhibiting miniband formation. Here, we discover that the layered magnetic semimetal EuAg₄Sb₂ overcomes these challenges by forming an interface with J ~ 100 milli-electron volts transferred between a Eu triangular lattice and anionic Ag₂Sb bilayers hosting a two-dimensional electron band in the ballistic regime (l_mfp >> a_spin). The system realizes an SMS with a_spin commensurate with the Fermi momentum, leading to a marked quenching of the transport response from miniband formation. Our findings demonstrate an approach to magnetically engineering moiré superlattices and a potential route to an emergent spin-driven quantum Hall state.