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.