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 EuAg4As2 via transport, magnetization, single crystal x-ray, and neutron diffraction data. EuAg4As2 single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of q(1) = +/-(0, 0.25, 0.5) and q(2) = +/-(0.25, 0, 1) emerge. Between 9-15 K, the hexagonal Eu2+ 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 mu(B)/Eu2+. Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in EuAg4As2 are revealed.