The compounds of the $\mathrm{B}{\mathrm{a}}_2\mathrm{LnFe}{\mathrm{O}}_5$ series (Ln being a lanthanide) undergo an antiferromagnetic transition involving both the $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{L}{\mathrm{n}}^{3+}$ spins. The $\mathrm{L}{\mathrm{n}}^{3+}$ cations sit in distorted octahedra and the associated crystalline electric-field (CEF) is expected to have a drastic impact on the magnetism. In this study, the nature of the spin ordering was investigated by Powder Neutron Diffraction (PND), while the $\mathrm{L}{\mathrm{n}}^{3+}$ spin anisotropy was derived from calculations in the point charge approximation. Different propagation vectors were observed for the various Ln, and the magnetic structures were solved in all cases, except for Ln = Er. A couple of easy axis (EA) consistent with the space group symmetry was observed for each Ln (except Er), and the orientation of these EAs exhibits a huge variation along the series. These features are discussed in connection with the CEF results and the literature on related families of materials.

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• Magnetic anisotropy
• Magnetic interactions
• Magnetic order
• Rare-earth magnetic materials
• Crystal-field theory
• Neutron diffraction