A NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC-ORDERING OF HOMN6GE6

The hexagonal compound HoMn6Ge6 of HfFe6Ge6-type of structure (P6/mmm) orders antiferromagnetically below T-N = 466 K and undergoes a second magnetic transition at T-1 = 200 K. Neutron powder diffraction has shown that the low-temperature phase is a triple skewed spiral with wave vector q(1) = (0, 0, q(2)), consisting of ferromagnetic Ho and Mn layers coupled almost antiparallel in a three-layer sequence: Mn(+)-Ho(-)-Mn(+). The plane of the moments is rotated about an axis in the hexagonal plane so that its normal makes a nonzero angle with q(1). This structure may be regarded as a combination of an elliptic helix with a longitudinal wave with the same wave vector. The wave vector length is incommensurate with the crystal lattice and is temperature dependent, at 9 K q(z) = 0.1979(2) r.l.u., which corresponds to an interplanar turn angle of Phi(S) = 71.24 degrees in the direction of q(1). Above T-t = 200 K a spin reorientation transition, associated with a decoupling of the Mn and Ho sublattices sets in, and the skewed spiral structure gets destabilised. In the transition region 200-260 K the magnetic ordering is described as a superposition of two Fourier coefficients/atom, associated with the wave vectors q(1) = (0, 0, q(z)) and q(2) = (0, 0, 1/2) corresponding to a distorted spiral with fluctuating Mn moment values and directions. The H + q(1) satellites (skewed spiral) comprise Ho as well as Mn contributions, while the H + q(2) comprise only the Mn antiferromagnetic ordering along c. Above 260 K the ordering consists exclusively of the H + q(2) Mn intensity contributions.