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

摘要

The ferroelectric and magnetic phases of the double perovskite CaMnTi2O6 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 2p-Ti 3d charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric P4(2)mc structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the P4(2)'m'c as the most likely magnetic space group keeping invariant the unit cell below T-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-N, the dichroic magnetization loops at the Mn L-3 edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.