Complex incommensurate helicoidal magnetic ordering of EuNiGe₃

¹⁵¹Eu Mössbauer spectroscopy and neutron powder diffraction are combined to show that the tetragonal (I4mm #107) compound EuNiGe₃ orders magnetically below $T_{\text{N}}\sim 14$ K and adopts a complex incommensurate helicoidal magnetic structure at 3.6 K, with a propagation vector $k=\left[0.255\right(1),0.054(14),0]$ and a Eu moment of 7.1(2) ${\mu }_{\text{B}}$. On warming through 6 K an incommensurate sinusoidal modulation develops and dominates the magnetic order by 12 K. (paper)