Magnetic and structural properties of ferromagnetic Fe₅PB₂ and Fe₅SiB₂ and effects of Co and Mn substitutions

Crystallographic and magnetic properties of Fe₅PB₂, Fe₄CoPB₂, Fe₄MnPB₂, Fe₅SiB₂, Fe₄CoSiB₂, and Fe₄MnSiB₂ are reported. All adopt the tetragonal Cr₅B₃ structure-type and are ferromagnetic at room temperature with easy axis of magnetization along the c-axis. The spin reorientation in Fe₅SiB₂ is observed as an anomaly in the magnetization near 170 K and is suppressed by substitution of Co or Mn for Fe. The silicides are found to generally have larger magnetic moments than the phosphides, but the data suggest smaller magnetic anisotropy in the silicides. Cobalt substitution reduces the Curie temperatures by more than 100 K and ordered magnetic moments by 16%-20%, while manganese substitution has a much smaller effect. This suggests Mn moments align ferromagnetically with the Fe and that Co does not have an ordered moment in these structures. Anisotropic thermal expansion is observed in Fe₅PB₂ and Fe₅SiB₂, with negative thermal expansion seen along the c-axis of Fe₅SiB₂. First principles calculations of the magnetic properties of Fe₅SiB₂ and Fe₄MnSiB₂ are reported. The results, including the magnetic moment and anisotropy, are in good agreement with experiment.