A neutron diffraction and M ssbauer spectral study of the magnetic spin reorientation in Nd₆Fe₁₃Si

Powder neutron diffraction and Mössbauer spectral studies have been carried out on Nd 6 Fe 13 Si between 2–295 K. The nuclear neutron scattering shows that Nd 6 Fe 13 Si has the tetragonal I 4/ mcm crystal structure at all temperatures. A collinear antiferromagnetic structure with the wavevector q = (0, 0, 1) is observed below the Néel temperature of 421 K. A spin reorientation is observed at ∼100 K in both the neutron diffraction patterns and the Mössbauer spectra. Above and below 100 K, the magnetic moments of the four iron and the two neodymium crystallographic sites are ferromagnetically coupled within one block along the c axis and the resulting magnetic moment of this block is antiferromagnetically coupled with that of the adjacent block along the c axis through a layer of silicon atoms. Above and below 100 K, the magnetic moments are found to be parallel or very close to the c axis and within or close to the ( a , b ) basal plane of the tetragonal unit cell, respectively. This first-order spontaneous zero-field spin reorientation occurs cooperatively and involves both the neodymium and iron magnetic moments. The powder neutron diffraction between 10 and 200 K indicates anisotropic lattice changes associated with a change in the magnetoelastic coupling in this compound. The Mössbauer spectral hyperfine parameters are those expected for axial and basal magnetization directions in Nd 6 Fe 13 Si and further indicate the existence of a 75% basal and 25% axial mixed magnetic phase even at 4.2 K, a mixed magnetic phase which results from the influence of the secondary ferromagnetic Nd 2 Fe 17 phase on the primary Nd 6 Fe 13 Si phase.