Magnetic and electronic properties of the ferroelectric-photovoltaic ordered double perovskite CaMnTi2O6 investigated by x-ray absorption spectroscopies

The ferroelectric and magnetic phases of the double perovskite $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ with $A$-site order have been investigated by soft x-ray absorption and magnetic circular dichroism. All spectra point to a very ionic state of divalent Mn and tetravalent Ti atoms. The effects of the crystal field produced by O ligands around tetravalent titanium and the dissimilar Mn1 and Mn2 sites were investigated. Both the so-called square-planar and the octahedrally coordinated Mn sites spectroscopically contribute in a rather similar way, with little influence by the oxygen environment. Multiplet calculations suggest a small $O\phantom{\rule{0.16em}{0ex}}2p\text{\ensuremath{-}}\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric $P{4}_{2}mc$ structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the $P{4}_{2}^{\ensuremath{'}}{m}^{\ensuremath{'}}c$ as the most likely magnetic space group keeping invariant the unit cell below ${T}_{\mathrm{N}}$. This symmetry forces the sign of the magnetic coupling along the Mn columns parallel to $c$ to reverse with respect to the coupling between neighboring columns. Below ${T}_{\mathrm{N}}$, the dichroic magnetization loops at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{3}$ edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.