Evolution of cation and spin orders in the double–double-double perovskite series ${\mathrm{Ca}}_x{\mathrm{Mn}}_{2-x}{\mathrm{FeReO}}_6$

Variations of perovskite superstructure type and magnetic properties across the high-pressure ${\mathrm{Ca}}_x{\mathrm{Mn}}_{2-x}{\mathrm{FeReO}}_6$ series have been investigated by powder neutron-diffraction, x-ray magnetic circular dichroism, and magnetization measurements. Monoclinic $P{2}_1/n{A}_{2}B{B}^{\prime }{\mathrm{O}}_6$ double perovskite solid solutions with rocksalt-type ordering of $B/B^{\prime }\text{−}\mathrm{Fe}/\mathrm{Re}$ cations but no $A$-cation order are discovered to exist only close to the end members $0<x<0.17$ and $1.73<x<2$. Compositions in the range $0.74<x<\sim 1.1$ adopt a tetragonal $P{4}_2/nA{A}^{\prime }B{B}^{\prime }{\mathrm{O}}_6$ double-double perovskite phase based on ${\mathrm{CaMnFeReO}}_6$, which has both columnar $A/A^{\prime }\text{−}\mathrm{Ca}/\mathrm{Mn}$ and rocksalt $B/B^{\prime }\text{−}\mathrm{Fe}/\mathrm{Re}$ cation orders. Two-phase coexistence of double and double-double perovskites occurs over wide regions between these limits. All samples are ferrimagnetic with Curie temperatures near 500 K. Low-temperature Mn spin order is also observed in the Mn-rich double perovskites and the double-double perovskites. Spin reorientation transitions observed across all of the double perovskites are shown to result from $5d^2{\mathrm{Re}}^{5+}$ orbital ordering, but this does not occur in the double-double perovskites. The Ca-rich double perovskites show significant magnetic coercivity at low temperatures.

Figure 1

Crystal and magnetic structures of the tetragonal $P{4}_2/nA{A}^{\prime }B{B}^{\prime }{\mathrm{O}}_6$ double-double perovskite ${\mathrm{CaMnFeReO}}_6$ showing columnar $A/A^{\prime }\text{−}\mathrm{Ca}/\mathrm{Mn}$ and rocksalt $B/B^{\prime }\text{−}\mathrm{Fe}/\mathrm{Re}$ cation orders. The ${\mathrm{Mn}}^{2+}$ cations occupy alternating tetrahedral (${\mathrm{Mn}}_{\mathrm{TD}}$) and square planar (${\mathrm{Mn}}_{\mathrm{SP}}$) sites within their column. There are thus five different cation sites in total in this structure type. Ferrimagnetic orders of Fe/Re and of ${\mathrm{Mn}}_{\mathrm{TD}}$/${\mathrm{Mn}}_{\mathrm{SP}}$ spins are observed at low temperatures.

Figure 2

Powder neutron-diffraction results for ${\mathrm{Mn}}_2{\mathrm{FeReO}}_6$. (a) Profile fits at 1.5 and 300 K. Labels $m_{\mathrm{A}}$ and $m_{\mathrm{B}}$ refer to magnetic reflections with contributions from $A$ (Mn) and $B$ (Fe/Re) site spin order, respectively. Lower markers show the fit of a trace of ${\mathrm{ReO}}_2$ impurity, and asterisks mark peaks from a trace of ${\mathrm{MnFe}}_3{\mathrm{O}}_5$. Plots show temperature variations of refined quantities for ${\mathrm{Mn}}_2{\mathrm{FeReO}}_6$; (b) monoclinic lattice parameters, (c) Fe-O bond lengths, and (d) Re-O bond lengths.

Figure 3

(a) Refined magnetic structures of ${\mathrm{Mn}}_2{\mathrm{FeReO}}_6$ at 200, 100, and 1.5 K. Fe atoms are shown in red, Re in grey, and Mn in purple. The 1.5-K magnetic structure is shown in the $ac$ plane, as well as the $bc$ plane to emphasize the $b$-axis antiferromagnetic Fe/Re component to the spin order. Plots show (left) the total magnetic moment and cartesian components of spin order, and (right) tilt angles for (b) Fe and (c) Mn spins. ${\theta }_{\mathrm{Fe}\left(\mathrm{Mn}\right)}$ is the angle between the $c$ axis and the Fe(Mn) spin direction in the $ac$ plane. ${\varphi }_{\mathrm{Fe}\left(\mathrm{Mn}\right)}$ is the angle between the $a$ axis and the Fe(Mn) spin direction in the $ab$ plane. Angles ${\theta }_{\mathrm{Tilt}}$ and ${\varphi }_{\mathrm{Tilt}}$ in (b) show the tilts of the ${\mathrm{FeO}}_6$ octahedra defined correspondingly.

Figure 4

Results for the nominal ${\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{FeReO}}_6$ ($x=0.5$) sample. Fits to (a) 300-K PSXRD and (b) 2- and 300-K PND profiles. The $x=0.5\left(T\right)$ phase is indicated with red labels, while the $x=0.5\left(M\right)$ phase is indicated by black labels. Designations $m_{\mathrm{A}}$/$m_{\mathrm{B}}$ indicate reflections that have magnetic contributions from A- or $B$-site spin order and $m_{\mathrm{B}-\mathrm{AFM}}$ refers to the antiferromagnetic component of Fe/Re spin order along the $b$ axis due to frustration. Asterisks mark a small ${\mathrm{MnFe}}_3{\mathrm{O}}_5$ impurity. Thermal variations of PND lattice parameters for (c) $x=0.5\left(M\right)$ and (d) $x=0.5\left(T\right)$ phases.

Figure 5

Results for the nominal ${\mathrm{Ca}}_{1.5}{\mathrm{Mn}}_{0.5}{\mathrm{FeReO}}_6$ ($x=1.5$) sample. (a) Crystal and magnetic structures of the $x=1.5\left(M\right)$ phase at 200 and 2 K. Fe atoms are shown in red, Re in grey, and Ca/Mn sites in cyan. Magnetic spin structures are shown in the $ac$ plane as well as the $ab$ plane at 2 K regime to show spin orientation. (b) 2- and 300-K PND patterns showing the fit of the $x=1.5\left(M\right)$ phase. Reflections marked as $m_{\mathrm{B}}$ have magnetic contributions from $B$-site magnetic order, no ordered moment from $A$-site Mn is observed from PND. Asterisks mark a small ${\mathrm{MnFe}}_3{\mathrm{O}}_5$ impurity. (c) PSXRD data at 300 K. Rows of markers from top to bottom indicate the $x=1.5\left(M\right)$ double perovskite phase [88.0(2)% by weight, $hkl$ indices in the inset in black], the $x=1.5\left(T\right)$ double-double perovskite phase [11.88(6)% by weight, indices in red], and ${\mathrm{ReO}}_2$ impurity [0.09(1)% by weight]. (d) Thermal variations of PND Re-O bond distances in the $x=1.5\left(M\right)$ structure.

Figure 6

X-ray-absorption near-edge spectra (XANES) and x-ray magnetic circular dichroism (XMCD) data for ${\mathrm{Mn}}_2{\mathrm{FeReO}}_6$ at (a) Re $L_2$ edge, (b) Re $L_3$ edge, and (c) Mn $K$ edge, at several temperatures. XMCD hysteresis of (d) the Re $L_2$-edge and (e) the Mn $K$-edge signals.

Figure 7

Magnetization measurements for ${\mathrm{Ca}}_x{\mathrm{Mn}}_{2-x}{\mathrm{FeReO}}_6$ samples. (a) Field-cooled (filled symbols) and zero-field-cooled (open symbols) for all samples in a 0.5-T field. (b) Field-cooled magnetization for the $x=0.5$ sample, compared against moments at the Mn, Fe, and Re sites and the net moment $m=m_{\mathrm{Fe}}-m_{\mathrm{Re}}$ from neutron refinements of the $x=0.5\left(T\right)$ double-double perovskite phase. Magnetic hysteresis data for (c) the $x=0.5$ and (d) the $x=1.5$ samples.

Figure 8

Overall trends in the $\mathrm{C}{\mathrm{a}}_x\mathrm{M}{\mathrm{n}}_{2-x}\mathrm{FeRe}{\mathrm{O}}_6$ system. (a) Phase diagram showing ferrimagnetic $B$-site order, $T_{\mathrm{C}}$, low temperature $A$-site Mn spin ordering, $T_{\mathrm{Mn}}$, and spin reorientation and Re orbital ordering, $T_{\mathrm{sr}}$, transitions. (b) Evolution of the maximum magnetization, as measured by superconducting quantum interference device magnetometry in a 7-T field and from moments refined by neutron diffraction, and the low-temperature coercive field. (c) Evolution of lattice parameters. Panels (b) and (c) show the estimated single-phase double perovskite ($P{2}_1/n$) and double-double perovskite ($P{4}_2/n$) regions, and the regions of phase coexistence ($P{2}_1/n+P{4}_2/n$).