Structure of the modulated magnetic phase of Mn3Sn (abstract)

The intermetallic compound Mn3Sn has the hexagonal DO19 crystal structure with six Mn and two Sn atoms per unit cell. The triangular antiferromagnetic order that develops below TN=420 K becomes modulated below 270 K in what is generally described as a helical modulation. However, magnetic excitation spectra show the presence of an energy gap of about 4 meV for both the triangular and the modulated phases, and this can occur for a helical modulation only if the period is commensurate with the lattice. This prompted us to re-examine the structure of the modulated phase by neutron diffraction. We find two distinct moment modulations, both propagating along the c axis and both of which are commensurate below 120 K with periods of 11c0 and 14c0. Above 120 K, both modulation wave vectors increase with temperature and become incommensurate. The satellite reflections exhibit quite different intensities and temperature dependencies, and this suggests coexistent modulated phases. For the predominant 11c0 (22 layer) phase, we observe a third harmonic with about 20% of the amplitude of the first harmonic and with no fifth harmonic to 1% in amplitude. This shows that the modulation in Mn3Sn is definitely not that of a uniform helix. The observed intensity distributions are compared with configurational models in which multilayer blocks of the triangular order align along the easy sixfold axes in the basal plane and rotate by 60° per block in proceeding along the c axis.