Cascade of Spin Moiré Superlattices with In-Plane Field in Triangular Lattice Semimetal EuAg ₄ Sb ₂

EuAg₄Sb₂ is a rhombohedral europium triangular lattice material that exhibits a rich phase diagram of spin moiré superlattices (SMS) and single-q magnetic phases. In this paper, we characterize the incommensurate phases accessible with a field applied in the plane with small-angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with a magnetic field applied along the a and a* directions. Many of these phases can be understood to be multi-q phases. One phase in particular, ICM2b (ICM = incommensurate magnetic phase), is rather unconventional in that it is an anisotropic multi-q phase that can rotate freely within the ab-plane, dependent on the magnetic field direction and history. The stabilization of tunable multi-q incommensurate spin textures via an in-plane field sets this class of materials apart from conventional skyrmion materials. We further identify that the propagation vectors of the in-plane phases have a significant commensuration with the diameter of the smallest pocket of the Fermi surface (2kF). The multi/single-q nature is also correlated with the enhancement of resistivity, suggesting that a gap opens in the electron bands at q = 2kF. We also compare with a phenomenological model of the phase diagram, which predicts several of these in-plane-field multi-q phases to host finite scalar spin chirality. The richness of phases revealed in this study hints at the frustrated nature of the incommensurate magnetism present in EuAg₄Sb₂ and motivates further probes of these phases and the origin of the stability of spin moiré superlattices. Finally, the coupling of the multi-q nature and q = 2kF commensuration conditions reveals the key requirements for a strong SMS transport response.