Revisiting the magnetic structure and charge ordering in La 1 /3 Sr 2 /3 FeO 3 by neutron powder diffraction and Mössbauer spectroscopy

The magnetic ordering of ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3}$ perovskite has been studied by neutron powder diffraction and $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector $\mathbf{k}=(0,0,1)$ in $R\overline{3}c$ space group. In the former model, the magnetic moments of Fe adopt the magnetic space group $P{3}_{2}21$ and have helical and antiferromagnetic ordering propagating along the $c$ axis. The model allows only a single Fe site, with a magnetic moment of 3.46(2)${\ensuremath{\mu}}_{\mathrm{B}}$ at 2 K. In the latter model, the magnetic moments of iron ions adopt the magnetic space group $C2/c$ or $C{2}^{\ensuremath{'}}/{c}^{\ensuremath{'}}$ and are aligned collinearly. The model allows the presence of two inequivalent Fe sites with magnetic moments of amplitude 3.26(3)${\ensuremath{\mu}}_{\mathrm{B}}$ and 3.67(2)${\ensuremath{\mu}}_{\mathrm{B}}$, respectively. The neutron-diffraction pattern is equally well fitted by either model. The M\"ossbauer spectroscopy study suggests a single charge state ${\mathrm{Fe}}^{3.66+}$ above the magnetic transition and a charge disproportionation into ${\mathrm{Fe}}^{(3.66\ensuremath{-}\ensuremath{\zeta})+}$ and ${\mathrm{Fe}}^{(3.66+2\ensuremath{\zeta})+}$ below the magnetic transition. The compatibility of the magnetic structure models with the M\"ossbauer spectroscopy results is discussed.