Observation of $A$-site antiferromagnetic and $B$-site ferrimagnetic orderings in the quadruple perovskite oxide $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_2{\mathrm{Re}}_2{\mathrm{O}}_{12}$

A quadruple perovskite oxide $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_2{\mathrm{Re}}_2{\mathrm{O}}_{12}$ was synthesized by high-pressure annealing. This compound crystallizes in an $A$- and $B$-site ordered quadruple perovskite structure with space group $Pn\text{−}3$. The charge combination is determined to be $\mathrm{Ca}{{\mathrm{Cu}}^{2+}}_3{{\mathrm{Co}}^{2+}}_2{{\mathrm{Re}}^{6+}}_2{\mathrm{O}}_{12}$ by bond valence sum analysis and x-ray absorption spectroscopy. In contrast to other isostructural $A{\mathrm{Cu}}_3{B}_2{B}_2^{\prime }{O}_{12}$ compounds with a single magnetic transition, a long-range antiferromagnetic phase transition originating from the $A^{\prime }$-site ${\mathrm{Cu}}^{2+}$ sublattice is found to occur at $T_N\approx 28\mathrm{K}$. Subsequently, the spin coupling between the $B$-site ${\mathrm{Co}}^{2+}$ and $B^{\prime }$-site ${\mathrm{Re}}^{6+}$ ions contributes to a ferrimagnetic transition around $T_{\mathrm{C}}\approx 20\mathrm{K}$. Strong electrical insulating behavior is identified by optical measurement with an energy gap of approximately 3.75 eV. The mechanisms of the spin interactions are discussed in detail.

Figure 1

(a) XRD pattern and structure refinement results for CCCRO at room temperature. The observed (black circles) and calculated (red line) results and their difference (bottom blue line) are plotted. The ticks indicate the allowed Bragg reflections for the $Pn\text{−}3$ space group. (b) Crystal structure of $A$- and $B$-site ordered quadruple perovskite CCCRO with $Pn\text{−}3$ symmetry. The corner-sharing $\mathrm{Co}/\mathrm{Re}{\mathrm{O}}_6$ octahedra and spatially isolated $\mathrm{Cu}{\mathrm{O}}_4$ squares are shown.

Figure 2

(a) $\mathrm{Cu}\text{−}{L}_{2,3}$ XAS for CCCRO, and $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ as a ${\mathrm{Cu}}^{2+}$ reference. (b) $\mathrm{Co}\text{−}{L}_{2,3}$ spectra for CCCRO, and $\mathrm{EuCo}{\mathrm{O}}_3$ as a low-spin ${\mathrm{Co}}^{3+}$ reference (from Ref. [27]) and CoO as a high-spin ${\mathrm{Co}}^{2+}$ reference. (c) $\mathrm{Re}\text{−}{L}_3$ XANES for CCCRO, and ${\mathrm{Sr}}_2\mathrm{MgRe}{\mathrm{O}}_6$ as a ${\mathrm{Re}}^{6+}$ reference and ${\mathrm{Sr}}_2\mathrm{FeRe}{\mathrm{O}}_6$ as a ${\mathrm{Re}}^{5+}$ reference.

Figure 3

(a) Temperature dependence of the magnetic susceptibility measured at 0.01 T for CCCRO using ZFC (black) and FC (red) modes. The inset shows the derivative of the susceptibility. (b) Temperature-dependent inverse susceptibility (empty circles) and the Curie-Weiss fitting result from the high-temperature range (blue solid line).

Figure 4

Field-dependent magnetization measured at selected temperatures for CCCRO.

Figure 5

(a) Temperature-dependent ac magnetization measured at different frequencies for CCCRO. The inset shows the enlarged view near $T_{\mathrm{C}}$. (b) Temperature dependence of the specific heat at zero field. The inset shows the specific heat measured at different fields near $T_{\mathrm{N}}$ and $T_{\mathrm{C}}$.

Figure 6

XMCD for (a) Cu- and (b) $\mathrm{Co}\text{−}{L}_{2,3}$ edges measured at 10 K and 6 T for CCCRO. The photon spin is aligned parallel (${\mu }^{+}$ black line) or antiparallel (${\mu }^{-}$ red line) to the applied magnetic field. The difference spectra $({\mu }^{+}-{\mu }^{-})$ are shown as blue lines. (c) XMCD for $\mathrm{Re}\text{−}{L}_{2,3}$ edges measured at 5 K and 5 T. The ${\mu }^{-}$ and ${\mu }^{+}$ lines almost overlap because of the weak XMCD signal.

Figure 7

XMCD magnitude at the (a) Cu- and (b) $\mathrm{Co}\text{−}{L}_{2,3}$ edges measured at 6 T and different temperatures near $T_{\mathrm{N}}$ for CCCRO. (c) Magnetic moments measured at 6 T by different methods. The magenta line was obtained from the susceptibility, the blue dots from the magnetization, and the red triangles from XMCD measurements of Cu- and $\mathrm{Co}\text{−}{L}_{2,3}$.

Figure 8

(a) ${}^{63}\mathrm{Cu}$ NMR spectrum of CCCRO measured at a magnetic field of 9.39 T and 300 K. The FWHM of central line $\left(\mathrm{\Delta }{\nu }_{\mathrm{hf}}\right)$ is shown. (b) Temperature dependence of $\mathrm{\Delta }{\nu }_{\mathrm{hf}}$ and its inverse for ${}^{63}\mathrm{Cu}$ NMR spectra.

Figure 9

UV-vis diffuse reflection spectrum for CCCRO collected at room temperature. The inset shows the Tauc plot of ${\left(a*E\right)}^{1/2}$ vs $E$.