A magnetic and Mössbauer spectral study of TbFe₁₁Ti and TbFe₁₁TiH

Magnetic and iron-57 Mössbauer spectral measurements between 4.2 and 640 K have been carried out on TbFe₁₁Ti and TbFe₁₁TiH. The insertion of hydrogen into TbFe₁₁Ti to form TbFe₁₁TiH increases its ordering temperature, magnetization, magnetic hyperfine fields, and isomer shifts as a result of lattice expansion. Further, the insertion of hydrogen reinforces the basal magnetic anisotropy of the terbium sublattice and, as is shown by ac susceptibility measurements and thermomagnetic analysis, the spin reorientation observed in TbFe₁₁Ti at 338 K disappears in TbFe₁₁TiH. The Mössbauer spectra have been analysed with a model that considers both the easy magnetization direction and the distribution of titanium atoms in the near-neighbour environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with a Wigner-Seitz cell analysis of the three iron sites in RFe₁₁Ti and RFe₁₁TiH, where R is a rare-earth element. A complete analysis of the quadrupole interactions in both magnetic phases and in the paramagnetic phase of TbFe₁₁Ti supports the Mössbauer spectral analysis, and indicates that in the basal magnetic phase the iron magnetic moments are oriented along the equivalent [100] and [010] directions of the unit cell.