Spiral Antiferromagntic Order in the Topological Semimetal Candidate EuIn₂

The magnetic topological semimetal candidate EuIn₂ is a hexagonal Zintl compound with two layers of triangularly arranged Eu per unit cell. Multiple antiferromagnetic (AFM) transitions (at 14.2, 12.8, and 11 K) have been reported from thermodynamic and transport measurements and Mössbauer spectroscopy. Here, we report results from single-crystal x-ray resonant magnetic scattering and both single-crystal and powder neutron diffraction (PND) experiments which have been used to characterize the AFM structure. We find that temperature-dependent incommensurate magnetic-Bragg peaks emerge upon cooling below 14 K that can be described by an AFM propagation vector (τ,0,0) which shifts upon cooling; τ = 0.3581(1) at 13.5 K and continually reduces to τ = 0.3554(1) at 11.5 K. This shift is followed by a first-order-like jump to a lock in to τ = 0.3443(1) for lower temperatures. Rietveld refinements of PND patterns reveal a fascinating stripe-like spiral structure with the ordered moments aligned in-plane and ferromagnetic coupling between the two Eu layers in the unit cell.