An iron-57 and tin-119 Mössbauer spectral study of NdMn_6−xFe_xSn₆

Abstract

The iron-57 Mössbauer 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 Mössbauer 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.

A Mössbauer spectral study of the NdMn6−xFexGe6 solid solutions

“…Even though the presence of ferromagnetism in the RMn 6−x Fe x Ge(Sn) 6 compounds requires a minimum distance [14] between transition metal near neighbours of 2.614Å, there is no direct relationship between the Curie temperature and the distance. As was observed [7] for the NdMn 6−x Fe x Sn 6 solid solutions, the introduction of iron in NdMn 6 Ge 6 adds some electrons to the 3d majority band and, as a consequence, both the Curie temperature and the magnetic moment increase, an increase that is in agreement with observation [4]. However, with increasing iron content, the near-neighbour distance decreases below 2.614Å and favours antiferromagnetic over ferromagnetic interactions.…”

“…6. At each temperature, the average hyperfine field is larger in NdMn 6−x Fe x Ge 6 than in NdMn 6−x Fe x Sn 6 , in agreement with the larger iron magnetic moments measured by neutron diffraction [4,7]. The difference between the fields in NdMn 6−x Fe x Ge 6 and in NdMn 6−x Fe x Sn 6 at 295 K is amplified by the difference in Curie temperature between the two series (see Fig.…”

“…In order to clarify the role of the unit-cell parameters and of germanium or tin on the magnetic properties of these compounds, band structure calculations are required. The increases in the compositional dependence of the average hyperfine field in the NdMn 6−x Fe x Sn 6 solid solutions are related to changes in the magnetic structure [7]. In a similar fashion, the larger fields in NdMn 5.5 Fe 0.5 Ge 6 indicate a different magnetic structure for this compound at 78 K.…”

“…Herein, we present the iron-57 Mössbauer spectra of the NdMn 6−x Fe x Ge 6 solid solutions with x = 0.5, 1.0 and 1.5, at 4.2, 78 and 295 K, with the goal both of investigating the compositional dependence of the hyperfine parameters and of comparing them with those observed [7] for the NdMn 6−x Fe x Sn 6 solid solutions with 0.5 ≤ x ≤ 2.…”