A Mossbauer spectral study of Nd6Fe13X, where X is Cu, Ag, and Au and of the spin reorientation in Nd6Fe13Si

The Mossbauer spectra of Nd(6)Fe(13)X, where X is Si, Cu, Ag, and Au, have been measured between 80 and 500 K. a model corresponding to a basal alignment of the magnetic moments leads to excellent. internally consistent, fits for the Cu, Ag, and Au compounds. The resulting temperature dependences of the spectral hyperfine parameters are uniform and reveal for each Fe site the expected correlations between the isomer shift and the Wigner-Seitz cell volume and the hyperfine field and the number of Fe near neighbors. For Nd(6)Fe(13)Si, a different model must be used because of the presence of a spin reorientation below 155 K. Above 155 K, because of the axial alignment of the moments, and in agreement with the 295 K powder neutron diffraction results, the spectra can be analyzed with four sextets. Below 155 K, five additional sextets are required to fit the spectra because of a progressive transition towards a basal alignment of the Fe moments. At 80 K the mixed magnetic phase is 75% basal and 25% axial, whereas at 110 K the mixture is 50:50. The hyperfine parameters of the basal and axial sextets found for Nd(6)Fe(13)Si are consistent with these found for the basal Cu, Ag, and Au compounds and those observed for basal Nd(6)Fe(13)Sn. The lattice properties of the compounds, obtained from the temperature dependences of the isomer shift and the spectral absorption area, are consistent with the Wigner-Seitz cell volumes and the bonding of each crystallographically distinct Fe site. The magnetic anisotropy of a variety of Nd(6)Fe(13)X compounds is controlled by the s-p hybridization of the Nd-X bonds, a hybridization which is very sensitive to the electronic configuration of X. (C) 1998 American Institute of Physics. [S0021-8979(98)00803-2].