An iron-57 and tin-119 Moessbauer spectral study of NdMn6-xFexSn6

The iron-57 Moessbauer spectra of the NdMn6-xFexSn6 compounds with x = 0.5, 1.0, 1.5 and 2.0 have been obtained at 4.2, 78 and 295 K, and the tin-119 Moessbauer spectra of the NdMn6-xFexSn6 compounds with x = 0.0, 0.5, 1.0, 1.5 and 2.0 have been obtained between 85 and 370 K. A successful and rational analysis of the spectra is based upon a Wigner-Seitz cell analysis of the HoFe6Sn6-structure with the Immm space group for NdMn6Sn6 and of the TbFe6Sn6-structure with the Cmcm space group for the NdMn6-xFexSn6 compounds with x = 0.5, 1.0, 1.5 and 2.0. Both the iron-57 and the tin-119 spectra reveal that the spin reorientation exhibited by these compounds at low temperature is extremely sensitive to the cooling rate of the samples. Specifically, samples that are slowly cooled from 295 to 78 K retain their 295 K magnetic structure and do not exhibit a spin reorientation. In contrast, samples that are quenched from 295 to 78 K and then further cooled to 4.2 K exhibit a spin reorientation. The ca 15 T iron-57 hyperfine fields observed at 4.2 K are unusually small, whereas the ca 25 T tin-119 transferred hyperfine fields observed at 85 K are unusually large. These latter large fields, as well as the improvement in Curie temperature and magnetization with increasing iron content in the NdMn6-xFexSn6 compounds, are discussed in terms of earlier electronic structure calculations