The magnetic structure of polar $G$-type charge and orbital ordered Hg-quadruple manganite perovskites

TL;DR

The paper solves the magnetic ground state of Hg0.7Na0.3Mn3Mn4O12, a Hg-based quadruple manganite with a unique G-type charge and orbital order, using powder neutron diffraction and symmetry analysis. It identifies two antiferromagnetic transitions at approximately 80 K and 60 K, with A'-site Mn showing G-type AFM order in the a-c plane and B-site Mn exhibiting an 'up–up–down–down' AFM order along b accompanied by canting; the magnetic structure is best described by a P2' symmetry with propagation vectors k1=(1,1,1) and k2=(0,0.5,0). The authors discuss symmetry-allowed couplings to secondary lattice distortions (including Zener-polaron-like modes) and propose that while these couplings may fine-tune the ground state, they are unlikely the primary driving force; instead, an improper polarization-related invariant could help stabilize the commensurate B-site order. The results emphasize the distinct magnetic behavior of Hg-based quadruple manganites compared to their Ca/Sr/Cd/Pb counterparts, highlighting the rich interplay of charge, orbital, lattice, and spin degrees of freedom in these materials.

Abstract

The magnetic structure of the novel Hg$_{0.7}$Na$_{0.3}$Mn$_3$Mn$_4$O$_{12}$, a quadruple manganite perovskite that exhibits a unique $G$-type charge and orbital ordered state distinct to other $A^{2+}$Mn$_3$Mn$_4$O$_{12}$ equivalents ($A$ $=$ Ca, Sr, Cd, Pb), has been solved using powder neutron diffraction and symmetry-motivated analysis. A $G$-type-like antiferromagnetic (AFM) ordering of Mn on the $A'$ sites and a `up--up--down--down' AFM moment configuration of Mn spins on the $B$ sites is found to occur. The mechanism for the onset and stabilization of $B$ site `up--up--down--down' AFM order is explored in terms of coupling between structural and magnetic distortions. The results presented here provide evidence of the exotic charge, orbital, electronic and magnetic orderings that quadruple manganite perovskites demonstrate, and further highlighting the distinct chemistry that Hg$^{2+}$ plays in stabilizing novel states compared to other divalent $A$-site cation equivalents.

Figures (6)

• Figure 1: (a) Magnetic susceptibility ($\chi$) measurements under ZFC and FC conditions at 1000 Oe. The inset indicates the presence of hysteresis consistent with the $R\bar{3}$ to $P$2 structural phase transition. (b) Curie-Weiss fit to $\chi^{-1}$ ZFC data within the paramagnetic temperature range 80 K $\leq$$T$$\leq$ 240 K.
• Figure 2: Pawley refinements of models against (a) 100 K PND data and 1.5 K PND data with propagation vectors (b) k$_1$$=$ (1,1,1) and (c) k$_1$$=$ (1,1,1) and k$_2$ (0,0.5,0) with respect to the $Im\bar{3}$ quadruple perovskite structure. Arrows denote magnetic peaks due to Hg$_{0.7}$Na$_{0.3}$Mn$_3$Mn$_4$O$_{12}$. The asterisk in (b) and (c) indicate the presence of a magnetic impurity, attributed to $\alpha$-Mn$_2$O$_3$. Purple ticks are peak positions of the Pawley refined structural model.
• Figure 3: Rietveld refinement of the resulting $P2_{1}^{'}$ magnetic structure model of Hg$_{0.7}$Na$_{0.3}$Mn$_3$Mn$_4$O$_{12}$ against PND data at 1.5 K. Purple ticks are crystal structural and magnetic peak positions of the fitted model. Pink and blue ticks are crystal structural peak positions due to small amounts of the higher temperature $R\bar{3}$ phase ($\sim$3$\%$) and Mn$_2$O$_3$ ($\sim$3$\%$), respectively. Fitting statistics and goodness of fit ($gof$) are given. (b) The resulting magnetic structure model from refinements in (a). Schematics of the crystal structure, magnetic moments of the $A'$ sites and magnetic moments of the $B$ sites are given separately for clarity. Red spheres are oxygen, $A$ sites are omitted for clarity, and magnetic moments are given as blue arrows.
• Figure 4: Temperature dependence of the magnetic moments for $A'$ (upper) and $B$ (lower) sites for the $P2_{1}^{'}$ magnetic structure model. Estimated standard deviations (1$\sigma$) are smaller than the size for each data point, and so they are not given here.
• Figure 5: Proposed exchange mechanism schematic of the $B$ sites for the $P2_{1}^{'}$ magnetic structure of Hg$_{0.7}$Na$_{0.3}$Mn$_3$Mn$_4$O$_{12}$. $d_{z^2}$ orbitals for Mn$^{3+}$ and Mn$^{4+}$, and the 2$p$ orbital for O$^{2-}$ are given. Dark blue arrows are the magnetic moment directions for an arbitrary 'chain' of $B$ cations along the b unit cell direction consistent with Figure \ref{['Rietveld-model']}. Relative spin orientations of each electron for each cation/anion are given for reference, and without canting for clarity.