A neutron diffraction study of the magnetic ordering of HoMn6Ge6

Abstract The hexagonal compound HoMn 6 Ge 6 of HfFe 6 Ge 6 -type of structure (P6/mmm) orders antiferromagnetically below T N =466 K and undergoes a second magnetic transition at T t =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 z ) , 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 o S =71.24° 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 and directions. The H ± q 1 satellites (skewed spiral) comprise Ho as well as Mn contributions, while the H + q 2 compr only the Mn antiferromagnetic ordering along c . Above 260 K the ordering consists exclusively of the H + q 2 Mn intensity contributions.