Intermetallic compounds of NdMn6−xFexSn6 () were studied by means of x-ray and neutron diffraction techniques and SQUID magnetic measurements in the temperature range of 30–400 K. The substitution of iron for manganese leads to a phase transition whereby NdMn6Sn6 with the HoFe6Sn6 structure (space group Immm) changes to TbFe6Sn6 (space group Cmcm) for NdMn6−xFexSn6 with . The iron atoms prefer to occupy the 8g sites at iron content x<2.0 due to the longest Mn/Fe–Sn bond distance. The Curie temperature (TC) increases from x = 0 to 1.5 and then decreases for the larger iron content. The magnetic moment of the iron sublattice couples ferromagnetically with the manganese and neodymium moments for the x<2.0 samples. Spin reorientation is observed in samples with iron content up to 1.5, and the spin reorientation temperature (Ts) increases with increasing iron content. Except for NdMn4Fe2Sn6, the easy direction of magnetization for all samples is parallel and perpendicular to the (bc) plane of the unit cell at 300 and 30 K, respectively. The easy direction of magnetization for NdMn4Fe2Sn6 is parallel to the a-axis in the entire temperature range mentioned above, as a result of the anisotropic contraction of the unit cell along the (bc) plane.

The magnetic and crystallographic properties of induction-melted NdMn (6Ϫx) Fe x Ge 6 intermetallics ͑xϭ0, 1.0, and 1.5͒ in the temperature range of 30-475 K have been studied by x-ray and neutron diffraction techniques and SQUID measurements. All of the samples crystallized in the YCo 6 Ge 6-type structure (P6/mmm). A small amount ͑Ͻ5 mol%͒ of Nd͑MnFe͒ 2 Ge 2 is present as an impurity. As expected, iron replaces manganese at the 3g site. The unit cell volume decreases slightly with increasing iron content at an average rate of 1.3% per substituted atom. Lattice parameters a and c contract at a rate of 0.6% and 0.2% per substitution atom, respectively. The net magnetization of these samples decreases rapidly with increasing iron content. According to neutron diffraction data, the magnetic moment of the iron sublattice couples negatively with ferromagnetically coupled manganese and neodymium moments. Addition of iron suppresses the spin reorientation processes observed in NdMn 6 Ge 6. Whereas the net moment in NdMn 5 Fe 1 Ge 6 slowly cants away from the c-axis with increasing temperature, the easy direction of NdMn 4.5 Fe 1.5 Ge 6 is approximately parallel to the c-axis in the temperature range mentioned above.

The magnetic and crystallographic structures of R(Fe/Mn) 6 A 6 ͑RϭNd or Sm and AϭGe or Sn͒ intermetallics have been investigated using x-ray and neutron diffraction techniques and superconducting quantum interference device magnetic measurements. For both stannides ͑AϭSn͒ and germanides ͑AϭGe͒, the lattice contracts with increasing iron content. In the case of the stannides, substitution of manganese by iron enhances the saturation magnetization and Curie temperature at low iron concentrations (xр2) suggesting the presence of an extremely rare occurrence, positive coupling between iron and manganese magnetic moments. In contrast, the magnetic properties of the germanides deteriorate rapidly as manganese is replaced by iron. This difference in the dependence of magnetic properties on the iron content between the germanides and stannides is explained using the Bethe-Slater relationship between near neighbor exchange interactions and interatomic distances. Based on the observations described in this article, it is concluded that the critical near neighbor interatomic distance above which manganese/iron moments couple positively in these intermetallics is ϳ2.614 Å.

The magnetic and crystallographic properties of induction-melted SmMn 6 Fe Ge 6 compounds, where is 0, 0.5, 1.0, and 1.5, have been studied using X-ray diffraction and bulk magnetic measurements between 30 and 490K. All of the samples crystallized in the YCo 6 Ge 6 -type structure with the space group P6/mmm. A small amount, less than 9 mol %, of Sm(MnFe) 2 Ge 2 was present as an impurity in all samples. The unit cell volume decreases with increasing iron content at an average rate of 2.4% per substituted atom at room temperature. The and lattice parameters decrease at an average rate of 1.1 and 0.1% per substituted atom at room temperature, respectively. The magnetic properties deteriorate rapidly as the iron concentration increases beyond = 0 5. Magnetic behavior of SmMn 5 Fe 1 Ge 6 and SmMn 4 5 Fe 1 5 Ge 6 is indicative of antiferromagnetic or ferrimagnetic coupling. According to X-ray diffraction patterns of magnetically aligned powders, the easy direction of magnetization at room temperature for all of the samples lies close to the basal plane of the unit cell.Index Terms-Exchange interaction, magnetic coupling, rareearth and transition metal intermetallics, SQUID bulk magnetic measurements, X-ray.

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.