Magnetic structures of Mn-rich and of Fe-rich TmMn 6 − x Fe x Sn 6 stannides with (Mn, Fe) kagome networks and related Sn 119 hyperfine magnetic fields

Stannides TmMn6-xFexSn6, with hexagonal HfFe6Ge6-type structures, were studied by x-ray and neutron diffraction, magnetic measurements, and by Sn-119 M-ssbauer spectroscopy. Transition metal atoms form kagome networks in (001) planes with an intraplanar ferromagnetic coupling of their moments. At temperatures >similar to 20 to 50 K, Mn-rich stannides (x=0.4, 0.6, 1.2) have an AFII easy-plane antiferromagnetic (AFM) 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 >1.6 K in Fe-rich stannides (x=4.25, 4.5, 5.0). Fe-rich stannides have an AFI AFM 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 toward the basal plane >4.2 K with maximum rotation angles of similar to 70 degrees and similar to 55 degrees reached at similar to 50 and similar to 80 K for x=4.25 and 4.50, respectively. Transferred hyperfine fields at tin sites of TmMn6-xFexSn6 stannides are the moduli of Sn-119 vectorial hyperfine magnetic fields that are modeled 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 Sn-119 nuclei are measured by Mossbauer spectroscopy. The hyperfine magnetic fields of Sn atoms, whose six transition metal 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 AFM coupled kagome planes are expected proportional to vertical bar p(2)-p(1)vertical bar, where one of the two kagome planes contains p(1) Fe first nearest neighbors and the other p(2). 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 from Mn- to Fe-rich stannides.