Magnetic and electronic properties of the ferroelectric-photovoltaic ordered double perovskite $\mathrm{CaMnT}{\mathrm{i}}_2{\mathrm{O}}_6$ 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 $O2p\text{−}\mathrm{Ti}3d$ charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric $P{4}_{2}mc$ structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the $P{4}_2^{\prime }{m}^{\prime }c$ 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 $\mathrm{Mn}{L}_3$ edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.

Figure 1

(a) Crystallographic projection of the polar phase of $\mathrm{CaMnT}{\mathrm{i}}_2{\mathrm{O}}_6$ (space group $P{4}_{2}mc$). The two positions of Ca atoms are represented in grey; $\mathrm{Ti}{\mathrm{O}}_6$ octahedra appear in green color; the two independent Mn sites are easily identifiable: Mn1 in tetrahedral coordination (purple) and Mn2 in square-planar coordination (orange). (b) X-ray diffraction frame of one of the single crystals $(30\times 20\times 10\mu \mathrm{m})$ that formed the sample used in our experiments.

Figure 2

Experimental x-ray absorption (solid, black line) and x-ray magnetic circular dichroism (solid, blue) spectra of $\mathrm{C}\mathrm{a}{\mathrm{MnTi}}_2{\mathrm{O}}_6$ across the $\mathrm{Mn}{L}_{2,3}$ edges at 5 K and 6 T applied field. The dashed, green line corresponds to the XMCD integration curve used for spin and orbital sum rules. (Inset) Comparison of the XMCD spectra at 5 K (blue) and 300 K (orange). The latter has been normalized to the low-$T$ spectrum applying the multiplicative factor 11.7.

Figure 3

Multiplet spectra calculated for Mn atoms in $\mathrm{C}\mathrm{a}{\mathrm{MnTi}}_2{\mathrm{O}}_6$ across $L_{2,3}$ absorption edges. Panels from top to bottom respectively show (i) a scheme of atomic $3d$ orbital relative energies (eV) for each local symmetry around Mn atoms, (ii) XAS, and (iii) XMCD spectra for these local structures, as commented in the main text, and (iv) the best XAS (red) and XMCD (magenta) simulated spectra for ${\mathrm{CaMnTi}}_2{\mathrm{O}}_6$ after considering the relative weight of tetrahedral and pseudo square-planar $\mathrm{M}{\mathrm{n}}^{2+}$ sites. The green dashed curve corresponds to the corresponding XMCD simulated integral curve. The experimental data (open circles) shown in Fig. 2 are also plotted for comparison. Note that nonresonant absorption has not been subtracted from the experimental data, and that it is not comprehended in calculated spectra.

Figure 4

(a) Experimental x-ray absorption (solid, black line) and x-ray magnetic circular dichroism (solid, blue) spectra of $\mathrm{C}\mathrm{a}{\mathrm{MnTi}}_2{\mathrm{O}}_6$ across the $\mathrm{Ti}{L}_{2,3}$ edges at 5 K and 6 T applied field. The latter signal has been multiplied by a factor 30 for convenience. The dashed, green line corresponds to the XMCD integral used for spin and orbital sum rule calculations. The correspondence of the different spectroscopic features to $t_{2g}$- and $e_g$-symmetry empty $3d$ states is indicated; (b) $\mathrm{Ti}{L}_{2,3}$ calculated XAS (solid, grey line), XMCD (solid, light blue; multiplied by 30), and XMCD integral (dashed, green) curves for CMTO.

Figure 5

Magnetization curves for $\mathrm{C}\mathrm{a}{\mathrm{MnTi}}_2{\mathrm{O}}_6$ as measured at the main feature (at 640 eV) of the $\mathrm{Mn}{L}_{2,3}$ XMCD spectrum at 5 K (blue) and 25 K (red).