Structures magnétiques de stannures TmMn 6 − x Fe x Sn 6, soit riches en manganèse, soit riches en fer où les atomes de Mn et de Fe forment des réseaux de kagome et champs magnétiques hyperfins de 119Sn associés

International audienceStannides TmMn6-xFexSn6, with hexagonal HfFe6Ge6-type structures, were studied by X-ray and neutron diffraction, magnetic measurements and by 119Sn Mössbauer spectroscopy. Transition metal atoms form kagome networks in (001) planes with an intraplanar ferromagnetic coupling of their moments. At temperatures higher than ~ 20 to 50 K, Mn-rich stannides, (x = 0.4, 0.6, 1.2), have an AFII easy-plane antiferromagnetic structure, with interplanar couplings ++-- along the c-axis. The thulium sublattice orders magnetically at low temperature. The complexity of the resultant neutron diffraction patterns arises from a mixture of several magnetic phases, some being incommensurate. The Tm sublattice does not order above 1.6 K in Fe-rich stannides (x = 4.25, 4.5, 5.0). Fe-rich stannides have an AFI antiferromagnetic structure, this time with interplanar couplings +-+-. The M = Mn/Fe magnetic moments are directed along the c-axis for x=5 at any temperature. Further, neutron diffraction shows that moments rotate from the c-axis towards the basal plane above 4.2 K with maximum rotation angles of ~70° and of ~55° reached at ~50 K and ~80 K for x = 4.25 and 4.50 respectively. Transferred hyperfine fields at tin sites of TmMn6-xFexSn6 stannides are the moduli of 119Sn vectorial hyperfine magnetic fields that are modelled for the AFI and AFII magnetic structures assuming a random substitution of Mn with Fe. Models involve sums of dipolar-type and of isotropic vector components, with simple assumptions about the model parameters. The transferred hyperfine magnetic fields at 119Sn nuclei are measured by Mössbauer spectroscopy. The hyperfine magnetic fields of Sn atoms, whose six transitionmetal nearest neighbors are ferromagnetically coupled, are predicted and observed to vary linearly with their number of Fe first nearest neighbors. The hyperfine magnetic fields on Sn atoms sandwiched between two antiferromagnetically-coupled kagome planes are expected proportional to abs(p2-p1) , where one of the two kagome planes contains p1 Fe first nearest neighbor and the other p2 . Tin atoms, with a ferromagnetic first nearest neighbor shell, experience negative hyperfine fields in Mn-rich stannides whereas they experience positive hyperfine fields in the Fe-rich stannides. This change is explained by a change of sign of the isotropic hyperfine magnetic fields which occurs when going fom Mn-rich to Fe-rich stannides