Strong enhancement of spin ordering by $A$-site magnetic ions in the ferrimagnet $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$

$AB{\mathrm{O}}_3$ perovskite is a kind of very important functional material with versatile physical properties. Although $B$-site chemical substitution with various magnetic ions has been widely investigated, the $A$-site doping with magnetic transition metal is little known. Here we report $A{A}_3^{\prime }{B}_2{B}_2^{\prime }{\mathrm{O}}_{12}$-type $A$- and $B$-site ordered ferrimagnet $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$ with magnetic transition metals occupying three different atomic sites ($A^{\prime }, B$, and $B^{\prime }$ sites). This compound is synthesized by a special high-pressure annealing process. It possesses a much higher Curie temperature $T_{\mathrm{C}}$ of 580 K compared with that of the $B$-site-only ordered $\mathrm{C}{\mathrm{a}}_2\mathrm{FeOs}{\mathrm{O}}_6$ ($T_{\mathrm{C}}=320\mathrm{K}$) without magnetic ion at the $A$ site. First-principles numerical calculations reveal that this enhancement primarily originates from the additional spin interaction between the $A^{\prime }$-site $\mathrm{C}{\mathrm{u}}^{2+}$ and the $B^{\prime }$-site $\mathrm{O}{\mathrm{s}}^{5+}$, generating a strong $\mathrm{C}{\mathrm{u}}^{2+}(\uparrow )\mathrm{F}{\mathrm{e}}^{3+}(\uparrow )\mathrm{O}{\mathrm{s}}^{5+}(\downarrow )$ ferrimagnetic spin coupling. This work opens up an alternative way for enhancing the spin ordering temperature by introducing $A$-site magnetic ions.

Figure 1

(a) Schematic of crystal structure of the $A$- and $B$-site ordered $A{A}_3^{\prime }{B}_2{B}_2^{\prime }{\mathrm{O}}_{12}$-type quadruple perovskite $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$. The corner-sharing $\mathrm{Fe}/\mathrm{Os}{\mathrm{O}}_6$ octahedra and spatially isolated $\mathrm{Cu}{\mathrm{O}}_4$ squares are shown. (b) XRD pattern and structure refinement results obtained at room temperature. The observed (circles), calculated (red line), and difference (bottom line) are shown. The ticks indicate the allowed Bragg reflections with space group $Pn$-3.

Figure 2

(a) XAS of Cu-$L_{2,3}$ edges, (b) Fe-$L_{2,3}$ edges, and (c) Os-$L_3$ edges. The XAS of related references are also shown for comparison.

Figure 3

(a) Field-dependent magnetization measured at different temperatures. (b) Temperature-dependent magnetic susceptibility measured at 0.1 T with ZFC and FC modes. (c,d) XMCD for Fe- and Cu-$L_{2,3}$ edges. The photon spin is aligned parallel (${\mu }^{+}$ black line) and antiparallel (${\mu }^{-}$ red line) to the applied magnetic field, respectively. The difference spectra are shown in blue.

Figure 4

Photocurrent signals excited by light radiation with different wavelengths in $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$ at room temperature. The inset shows the temperature dependence of resistivity.

Figure 5

First-principles numerical results for the band structures and partial densities of states of $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$.

Figure 6

(a) Magnetic interactions in the $B$-site-only ordered $\mathrm{C}{\mathrm{a}}_2\mathrm{FeOs}{\mathrm{O}}_6$ between Fe and Os ions. (b) Magnetic interactions in both $A$-site and $B$-site ordered $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$ among Cu, Fe, and Os ions.