  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 EuAg$_4$As$_2$ via transport,
magnetization, single crystal X-ray and neutron diffraction data.
EuAg$_4$As$_2$ single crystal shows a structural distortion at 120 K, where two
sets of superlattice peaks with the propagation vectors of $q_1=\pm$(0, 0.25,
0.5) and $q_2=\pm$(0.25, 0, 1) emerge. Between 9 K to 15 K, the hexagonal
Eu$^{2+}$ sub-lattice enters an unpinned 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), where the magnetic structure is helical along the $c$ axis
and cycloidal along the $b$ axis with a moment of 6.4 $\mu_B$/Eu$^{2+}$.
Furthermore, rich magnetic phases under magnetic fields, large
magnetoresistance, and strong coupling between charge carriers and magnetism in
EuAg$_4$As$_2$ are revealed.
