Hexagonal TmMn6Sn4Ge2 investigated by neutron diffraction and 119Sn Mössbauer spectroscopy

The hexagonal compound TmMn6Sn4Ge2 has been investigated by neutron diffraction and 119Sn Mössbauer spectroscopy. The location of Ge atoms on the 2(c) site has been confirmed. Above 230 K, an easy plane helimagnetic structure is stable and below 70 K an easy axis ferrimagnetic structure prevails. The magnetic moments at 2 K are μTm = 6.93 μB and μMn = 2.25 μB. In the intermediate temperature range, we observed a tilting of the spiral plane. The intermediate magnetic structure is clearly observed by the 119Sn local probe through a field distribution for the Sn(2e) site and a quadrupolar distribution for the Sn(2d) site. The change of the magnetocrystalline anisotropy of the Tm sublattice with respect to the easy plane observed in the parent stannide TmMn6Sn6 might be related either to a shortening of the Tm-Mn distances or to a deformation of coordination shell around the Tm atom, both being caused by the location of Ge atoms on the 2(c) site. The stabilisation of the ferrimagnetic structure at low temperature might be driven by stronger Tm-Mn interactions that result from the reduced Tm-Mn bond lengths.