Magnetic structures of the rare-earth quadruple perovskite manganites RMn7O12

We report a neutron powder diffraction study of $R{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ quadruple perovskite manganites with $R=\mathrm{La}$, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the $A$ and $B$ manganese sublattices occurs on cooling below the Néel temperature. The respective magnetic structures are collinear, with one uncompensated ${\mathrm{Mn}}^{3+}$ moment per formula unit as observed in bulk magnetization measurements. We show that both ${\mathrm{LaMn}}_7{\mathrm{O}}_{12}$ and ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$ undergo a second magnetic phase transition at low temperature, which introduces a canting of the $B$-site sublattice moments that is commensurate in ${\mathrm{LaMn}}_7{\mathrm{O}}_{12}$ and incommensurate in ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$. This spin canting is consistent with a magnetic instability originating in the $B$-site orbital order. Furthermore, ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$ displays a third magnetic phase transition at which long-range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition-metal magnetism, orbital order, and the crystal lattice, which may be fine-tuned by cation substitution and rare-earth magnetism.