Ferrimagnetism and spin reorientation in the high-pressure double double perovskites $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_6$ and $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_6$

Two $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_6$ ($B=\mathrm{Cr}$ and Fe) high-pressure double double perovskites are reported. Spins in $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_6$ order below $T_{\mathrm{C}}=49\mathrm{K}$ into a collinear ferrimagnetic arrangement with spins in the $xy$ plane. $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_6$ has the same ordered magnetic structure below $T_{\mathrm{C}1}=55$ K, but also shows a second magnetic transition at $T_{\mathrm{C}2}=21\mathrm{K}$ where the spins reorient towards the $z$ axis. This may reflect a greater degree of magnetic disorder for $B=\mathrm{Fe}$ and A-B intersite charge transfer may also be significant.

Figure 1

(a) Rietveld fits of the double double perovskite $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_6$ structures against 300-K NPD data. Top, $B=\mathrm{Cr}$, secondary phase markers are for $\mathrm{Mn}{\mathrm{Cr}}_2{\mathrm{O}}_4$ and V (sample holder); bottom, $B=\mathrm{Fe}$. (b) $P{4}_2/n \mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_6$ double double perovskite structure, with a view of the ${\mathrm{Mn}}^{\prime }/{\mathrm{Mn}}^{\prime \prime } A$-site column to the right ($A$- cation $\mathrm{Ca}/{\mathrm{Mn}}^{\prime }/{\mathrm{Mn}}^{\prime \prime }=\mathrm{grey}/\mathrm{red}/\mathrm{yellow}$; $B{\mathrm{O}}_6/\mathrm{Sb}{\mathrm{O}}_6 \mathrm{octahedra}=\mathrm{blue}/\mathrm{cyan}$).

Figure 2

Magnetic susceptibility and hysteresis loops (insets) for $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_6$; (a) $B=\mathrm{Cr}$ and (b) $B=\mathrm{Fe}$. Curie-Weiss fits to inverse susceptibilities are also shown.

Figure 3

Magnetic structure determination for $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_6$. (a) Spin structure where red/yellow/blue arrows represent, ${\mathrm{Mn}}^{\prime }/{\mathrm{Mn}}^{\prime \prime }/B=\mathrm{Cr}$ site spins. (b) Rietveld fit to the 1.5–65-K NPD difference pattern. The first three magnetic peaks (observed in the range $2\theta =15–{30}^{\circ }$) are (110)/(101), (111), and (200)/(002).

Figure 4

(a) Rietveld fits to the $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_6$ NPD difference patterns 35–70 K and 3–70 K. The first three prominent magnetic peaks (observed in the range $2\theta =25–{40}^{\circ }$) are (110)/(101), (111), and (200)/(002). The right panels show the refined magnetic structures at 35 (top) and 3 K (bottom). (b) 2D NPD short scan data showing the different thermal evolution of (110) and (111) magnetic peaks (left and right respectively). $T_{\mathrm{C}1}$ and $T_{\mathrm{C}2}$ are labeled as red dashed lines.

Figure 5

Temperature variations of (a) the ${\mu }_x$ and ${\mu }_z$ components, and (b) the resultant moments μ, at Mn and Fe sites from Rietveld fits to the short NPD scans for $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_6$. Critical fits shown to the ${\mu }_x$ and ${\mu }_z$ components are described in the text.