We report on the metamagnetic transition observed in oxides ${\mathrm{Ba}}_2{Ln\mathrm{FeO}}_5$ ($\mathit{Ln}$ = Tb, Dy, Ho, Er, Yb), which contain two magnetic sublattices: ${\mathrm{Fe}}^{3+}$ and ${Ln}^{3+}$. The magnetic ordering in these materials results from a competition between several types of interaction (Fe-Fe, Fe-$\mathit{Ln}$, and $\mathit{Ln}-\mathit{Ln}$), combined in most cases with a strong easy-axis anisotropy of the ${Ln}^{3+}$ magnetic moments induced by the crystal electric field (CEF). A specific mechanism is proposed to account for the peculiar metamagnetism in this class of materials, and its variation along the $\mathit{Ln}$ series. This study is based on a set of results including magnetization, x-ray magnetic circular dichroism, neutron powder diffraction, and calculations of the CEF.

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• Magnetic anisotropy
• Magnetic interactions
• Magnetization measurements
• X-ray magnetic circular dichroism