Intermediate magnetic phase of the magnetoelectric compound (Ca,Sr)BaCo4O7 described with the superspace formalism

Abstract

In this work we report the temperature and doping-level evolution of the crystallographic and magnetic properties of the Sr-doped cobaltates ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{BaCo}}_{4}{\mathrm{O}}_{7}$. The noncollinear ferrimagnetic phase known for the magnetoelectric parent compound is found to persist only for a small amount of Sr doping ($x=0.02$) and is accompanied by a strong unit cell distortion. In turn, further Sr doping blurs this distortion of lattice parameters and favors other magnetic arrangements. In particular, this work focuses on an intermediate temperature region $62\phantom{\rule{0.28em}{0ex}}\mathrm{K}lTl82\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ that shows a plateau in the magnetization. Using neutron powder diffraction at selected temperatures and compositions, we have solved the magnetic structure of this intermediate phase. By means of the superspace group theory and its implementation in the Rietveld refinement, we found that the phase belongs to the superspace group $Pna{2}_{1}{1}^{\ensuremath{'}}(1/2,1/2,g)qq0s$ with a basic orthorhombic structure and a modulation of the magnetic moments along the $c$ axis. The propagation vector is $\mathbf{k}=(\frac{1}{2},\frac{1}{2},g)$ with $g\ensuremath{\sim}0.02$ and possibly dependent on temperature. A modulated spin structure with distinct behaviors of the triangular and kagome cobalt sites is reported in ``114'' cobaltates.