A MOSSBAUER SPECTRAL STUDY OF ND6FE13X, WHERE X IS CU, AG, AND AU ANDOF THE SPIN REORIENTATION IN ND6FE13SI

The Mossbauer spectra of Nd6Fe13X, 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 consi stent, fits for the Cu, Ag, and Au compounds. The resulting temperatur e dependences of the spectral hyperfine parameters are uniform and rev eal for each Fe site the expected correlations between the isomer shif t and the Wigner-Seitz cell volume and the hyperfine field and the num ber of Fe near neighbors. For Nd6Fe13Si, a different model must be use d because of the presence of a spin reorientation below 155 K. Above 1 55 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 r equired 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. Th e hyperfine parameters of the basal and axial sextets found for Nd6Fe1 3Si are consistent with these found for the basal Cu, Ag, and Au compo unds and those observed for basal Nd6Fe13Sn. The lattice properties of the compounds, obtained from the temperature dependences of the isome r shift and the spectral absorption area, are consistent with the Wign er-Seitz cell volumes and the bonding of each crystallographically dis tinct Fe site. The magnetic anisotropy of a variety of Nd6Fe13X compou nds is controlled by the s-p hybridization of the Nd-X bonds, a hybrid ization which is very sensitive to the electronic configuration of X. (C) 1998 American Institute of Physics. [S0021-8979(98)00803-2].