Cascade of Spin Moir\'e Superlattices with In-Plane Field in Triangle Lattice Semimetal EuAg4Sb2
Abstract
EuAg4Sb2 is a rhombohedral europium triangle lattice material that exhibits a rich phase diagram of spin moir\'e superlattices (SMS) and single-q magnetic phases. In this paper, we characterize the incommensurate phases accessible with field applied in the plane with small angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with 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 magnetic field direction and history. The stabilization of tunable multi-q incommensurate spin textures via 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. The richness of phases revealed in this study hint at the frustrated nature of the incommensurate magnetism present in EuAg4Sb2 and motivate further probes of these phases and the origin of the stability of spin moir\'e superlattices. Finally, the coupling of the multi-q nature and q=2kF commensuration condition reveals the key requirements for a strong SMS transport response.
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