The magnetic structures of rare-earth quadruple perovskite manganites $R$Mn$_7$O$_{12}$

TL;DR

This study uses neutron diffraction to explore magnetic structures in rare-earth quadruple perovskite manganites, revealing complex magnetic phase transitions and the influence of orbital order and rare earth elements on magnetism.

Contribution

It provides the first detailed analysis of magnetic ordering and phase transitions in $R$Mn$_7$O$_{12}$ compounds, highlighting the interplay between orbital order, lattice, and rare-earth magnetism.

Findings

Concomitant magnetic ordering of A and B manganese sublattices occurs below the Néel temperature.

Low-temperature phase transitions induce spin canting, with different periodicities in La and Nd compounds.

NdMn$_7$O$_{12}$ exhibits a third magnetic phase transition involving Nd sublattice ordering.

Abstract

We report a neutron powder diffraction study of $R$Mn$_7$O$_{12}$ quadruple perovskite manganites with $R$ = 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 N$\mathrm{\acute{e}}$el temperature. The respective magnetic structures are collinear, with one uncompensated Mn$^{3+}$ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn$_7$O$_{12}$ and NdMn$_7$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 LaMn$_7$O$_{12}$ and incommensurate in NdMn$_7$O$_{12}$. This spin canting is consistent with a magnetic instability originating in the $B$ site orbital order. Furthermore, NdMn$_7$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.