Anomalous Spin-Density Distribution on Oxygen and Ru in ${\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{S}\mathrm{r}}_{0.5}{\mathrm{R}\mathrm{u}\mathrm{O}}_4$: Polarized Neutron Diffraction Study

By means of polarized neutron diffraction in a magnetic field of 7.0 T at 1.6 K an anomalously large magnetization density is observed on the in-plane oxygen in ${\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{S}\mathrm{r}}_{0.5}{\mathrm{R}\mathrm{u}\mathrm{O}}_4$. Field-induced moments of different ions are determined by refinement on the flipping ratios, yielding ${\mu }_{\mathrm{R}\mathrm{u}}=0.346(11){\mu }_B$, ${\mu }_{\mathrm{O}1}=0.076(6){\mu }_B$, and ${\mu }_{\mathrm{O}2}=0.009(6){\mu }_B$. The moment on the oxygen arises from the strong hybridization between the $\mathrm{R}\mathrm{u}-4d$ and $\mathrm{O}-2p$ orbitals. The maximum entropy magnetization density reconstruction reveals a strongly anisotropic density at the Ru site, consistent with the distribution of the $xy$ ($t_{2g}$ band) $d$ orbitals.

Figure 1

Magnetic susceptibility of ${\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{S}\mathrm{r}}_{0.5}{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ obtained from the polarized neutron intensities of the (004) reflection with the field parallel to the [110] direction.

Figure 2

Magnetic amplitudes of ${\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{S}\mathrm{r}}_{0.5}{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ at 1.6 K and 7 T. Circles represent mixed reflections, normalized by Ru geometrical structure factor. Squares in the inset are the pure oxygen reflections normalized by the geometrical structure factor of O1. The solid line is the magnetic amplitude calculated for the Ru form factor in the dipole approximation. Thin lines show the decomposition of the latter into its components proportional to ${\mu }_L$ and $({\mu }_S+{\mu }_L)$, respectively.

Figure 3

Maximum entropy reconstruction of the magnetization distribution in ${\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{S}\mathrm{r}}_{0.5}{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ at 1.6 K and 7 T. The maps (A) and (B) show the sections passing through the ${\mathrm{R}\mathrm{u}\mathrm{O}}_2$ plane at $z=\frac{1}{8}$ and map (C) shows the section perpendicular to the plane through the Ru position. The edge lengths of the maps correspond to the tetragonal lattice constant of $a=5.34\AA$ for maps (A) and (B) and to $a$ and a quarter of $c$ for map (C), respectively. Note that in space-group $I{4}_1/acd$ a Ru is situated at $(\frac{1}{2},\frac{1}{4},\frac{1}{8})$ and an O1 site lies at $(0.19,0.44,\frac{1}{8})$. The section in (A) relates to the adverse 3D nonuniform prior density used to compute (B) and (C), which biases the MEM reconstruction against any magnetization at the O sites, as well as against an aspherical magnetization at the Ru site. The contour step is $0.03{\mu }_B{\AA }^{-3}$.