Time-of-flight neutron powder diffraction investigation of Tb2Co17-xGax permanent magnet materials

The crystallographic and magnetic structures of Tb2Co17-xGax (x = 1, 3, 5 and 7) intermetallics have been investigated by time-of-flight neutron powder diffraction on the ROTAX diffractometer at the ISIS spallation neutron source. A Rietveld profile analysis of the diffraction profiles for the two detector banks located in forward and backscattering geometries shows that all four compounds crystallize in the rhombohedral Th2Zn17 structure, with Co atoms fully occupying the 9d site whilst Co and Ga atoms are found to distribute themselves in a non-random manner on the remaining 6c, 18f and 18h sites. These observed site occupations can be correlated with the observed dependence of the saturation magnetization and Curie point with Ga composition. The microscopic magnetic structures of the Tb and Co sublattices have also been refined from the neutron diffraction data and indicate a complex ferrimagnetic behaviour with Ga composition.