Structural distortion and incommensurate noncollinear magnetism in EuAg4As2

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here we report the observation of structural distortion and noncollinear magnetism in layered pnictide ${\mathrm{EuAg}}_{4}{\mathrm{As}}_{2}$ via transport, magnetization, single crystal x-ray, and neutron diffraction data. ${\mathrm{EuAg}}_{4}{\mathrm{As}}_{2}$ single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of ${q}_{1}=\ifmmode\pm\else\textpm\fi{}$(0, 0.25, 0.5) and ${q}_{2}=\ifmmode\pm\else\textpm\fi{}$(0.25, 0, 1) emerge. Between 9--15 K, the hexagonal ${\mathrm{Eu}}^{2+}$ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the $c$ axis and cycloidal along the $b$ axis with a moment of 6.4 ${\ensuremath{\mu}}_{B}/{\mathrm{Eu}}^{2+}$. Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in ${\mathrm{EuAg}}_{4}{\mathrm{As}}_{2}$ are revealed.