Soft x-ray spectroscopic study of the ferromagnetic insulator ${\text{V}}_{0.82}{\text{Cr}}_{0.18}{\text{O}}_2$

The chromium-vanadium oxide system ${\text{V}}_{1-x}{\text{Cr}}_x{\text{O}}_2$ $\left(0.1<x<0.2\right)$ displays both insulating character in the rutile phase and room-temperature ferromagnetism. A combination of x-ray photoemission spectroscopy, resonant inelastic x-ray scattering, and resonant x-ray emission spectroscopy of the $\text{V}{L}_{3,2}$, $\text{O}K$, and $\text{Cr}{L}_{3,2}$ edges was used to study the electronic structure near the Fermi level of ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$. Our results show that the chromium enters as ${\text{Cr}}^{3+}$ with the $d^3$ configuration, resulting in the formation of ${\text{Cr}}^{3+}{\text{-V}}^{5+}$ ion pairs, in contrast to the simple ${\text{Cr}}^{4+}$ substitution expected from the end members ${\text{VO}}_2$ and ${\text{CrO}}_2$. The occupied $\text{Cr}3d$ orbital is located $\sim 2\text{eV}$ below the Fermi level. Comparison with the parent material ${\text{VO}}_2$ reveals significant changes in the $\text{O}2p$ bandwidth and increased $\text{O}2p\text{-V}3d$ hybridization in ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$, which are attributed to the reduced atomic spacing upon doping. Two energy loss features due to $d\text{−}{d}^{\ast }$ transitions are observed at 0.95 and 1.75 eV in the $\text{V}{L}_3$-edge x-ray scattering spectra and are explained in terms of the splitting of the $t_{2\text{g}}$-derived $\pi$ band.

Figure 1

(Color online) The $\text{O}1s$ and $\text{V}2p$ XPS of ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$ (blue, upper) and ${\text{VO}}_2$ (red, lower). The peak energies of the $\text{V}2p_{3/2}$ associated with different charge states are also displayed.

Figure 2

(Color online) The VB-XPS of ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$ (blue) and ${\text{VO}}_2$ (red). The vertical lines reflect features of insulating ${\text{VO}}_2$ reported by Koethe et al. (Ref. 5).

Figure 3

(Color online) (a) The $\text{V}{L}_{3,2}$-edge and $\text{O}K$-edge XAS of ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$. The arrows (a)–(f) represent incident energies used to excite the $\text{V}{L}_3$-edge RIXS/RXES process. (b) The corresponding $\text{V}{L}_3$-edge RIXS/RXES spectra vertically offset. (c) The enlarged region of RIXS features near the elastic peak (zero energy) plotted on a common energy-loss plot.

Figure 4

(Color online) (Top) The $\text{O}K$-edge XAS ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$. The arrows (g)–(i) represent incident energies used to excite RXES process. (Bottom) The corresponding $\text{O}K$-edge RXES spectra, vertically offset.

Figure 5

(Color online) (Top) The $\text{Cr}{L}_{3,2}$-edge XAS ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$. The arrows (j)–(l) represent incident energies used to excite RXES process. (Bottom) The corresponding $\text{Cr}{L}_{3,2}$-edge RXES spectra, vertically offset. Reference $\text{Cr}{L}_{3,2}$-edge RIXS spectra (with similar excitation energies) for (dark gray empty circles) ${\text{CuCrO}}_2$ (Ref. 30) and (light gray lines) ${\text{CrO}}_2$ (Ref. 31) are also displayed for direct comparison.

Figure 6

(Color online) The (a) $\text{V}{L}_3$- and (b) $\text{O}K$-edge peak RXES spectra of ${\text{V}}_{0.82}{\text{Cr}}_{0.12}{\text{O}}_2$ (blue) and ${\text{VO}}_2$ (red).

Figure 7

(Color online) (a) A schematic representation of the rutile ${\text{VO}}_2$ structure with the oxygen octahedron (small, blue spheres) around the edge and center vanadium ions (large, red spheres) shown, along with the (110) plane (shaded plane).

Figure 8

(Color online) A schematic representation of the molecular orbitals near the Fermi level for rutile $R$ $\left(P{4}_2/mnm\right)$ and monoclinic $M_1$ $\left(P{2}_1/c\right)$, $M_2$ $\left(C2/m\right)$, and $M_4$ $\left(P2/\text{m}\right)$ phases of ${\text{VO}}_2$ and Cr-doped ${\text{VO}}_2$ proposed by Goodenough and Hong (Ref. 7).