A magnetic, neutron diffraction, and Mössbauer spectral study of Nd2Fe15Ga2 and the Tb2Fe17−xGax solid solutions

Abstract

An x‐ray diffraction study of the substitution of gallium in Tb₂Fe₁₇ to form the Tb₂Fe_17−xGa_x solid solutions indicates that the compounds adopt the rhombohedral Th₂Zn₁₇ structure. The unit cell volume and the a‐axis lattice parameter increase linearly with increasing gallium content. The c‐axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb₂Fe₁₇ to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd₂Fe₁₅Ga₂ and Tb₂Fe_17−xGa_x, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples antiferromagnetically with the iron sublattice and that there is a change in easy magnetization direction from planar to axial with increasing gallium concentration. This change in easy magnetization direction is explained in terms of a sign reversal of the second‐order crystal field parameter, A⁰₂, the most important parameter responsible for determining the terbium sublattice anisotropy. The Mössbauer effect spectra indicate a larger room‐temperature average hyperfine field at the iron site in the Tb₂Fe_17−xGa_x solid solutions than in several related R₂Fe₁₇ compounds. The large observed increase in the isomer shift with increasing gallium content results from interatomic charge transfer and intraatomic s‐d charge redistribution in the presence of gallium.