SKEWED SPIRAL MAGNETIC-STRUCTURE IN ERMN6GE6

ErMn6Ge6 adopts the HfFe6Ge6-type structure and orders antiferromagnetically below T-N = 475 K. At T-t = 80 K it undergoes a first-order spin reorientation transition. Neutron powder diffraction has shown that the low temperature phase is a triple skewed spiral with wave vector q(1) = (0, 0, q(z)), consisting of three ferromagnetic Er and Mn layers Mn(z = -1/4)-Er(z = 0)-Mn(z = 1/4) with the Er moments orientated opposite to the line bisecting the Mn-Mn interplanar angle 2 phi(Mn) = 70 deg. The spiral axis makes a non-zero angle with q(1). The wavevector length is incommensurate with the crystal lattice and weakly temperature dependent. Below 70 K, q(z) locks in to the commensurate value 1/4. Above T-t= 80 K a decoupling of the Mn and Er sublattices sets in, and the skewed spiral structure is destabilized. In the transition region 80-160 K the magnetic ordering is described as a superposition of two Fourier coefficients per atom, associated with the wave vectors q(1) = (0, 0, q(z)) and q(2) = (0, 0, 1/2). The H+/-q(1) satellites (skewed spiral) comprise Er as well as Mn intensity contributions, while only the Mn antiferromagnetic ordering along c contributes to the H+/-q(2) satellites. Above 170 K one observes exclusively H+/-q(2) Mn intensity contributions.