Electronic Commensuration of a Spin Moiré Superlattice in a Layered Magnetic Semimetal

Spin moiré superlattices (SMSs) formed by interfacing conventional electronic states with a multi-q magnetic lattice have been proposed as a magnetic analog of crystallographic moiré systems. The electron-minibands created in an SMS are expected to be enriched by the vector-field nature of the magnetic interaction and offer new types of moiré tunability, topological protection, and Berry curvature effects. However, most spin-vortex-hosting systems discovered to date have carrier mean free paths l_mfp significantly shorter than their spin-moiré lattice constant a_spin, inhibiting mini-band-formation. Furthermore, it has proven challenging to realize an SMS in which a significant exchange coupling J is transmitted between conduction electrons and localized spins.

Here we discover that the layered magnetic semimetal EuAg₄Sb₂ overcomes these challenges by forming an interface with a significant J ~ 100 meV transferred between a magnetically frustrated Eu triangular lattice and anionic Ag₂Sb bilayers hosting a high mobility (> 5,000 cm²/Vs) two-dimensional electron band. Neutron scattering experiments demonstrate that the system realizes a SMS with aspin commensurate with the high mobility Fermi momentum observed in ARPES and quantum oscillations, leading to a dramatic quenching of the transport response from mini-band formation. Theoretical modeling shows that the key material design ingredients for this are the realization of the ballistic regime (l_mfp >> a_spin) and a resonant condition of the Fermi and exchange energies. Our findings demonstrate an approach to engineering moiré superlattices based on magnetic degrees of freedom and a potential route to an emergent spin-driven quantum Hall state.