${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$: End point of a spin-polarized metal-insulator transition by $5d$ band filling

In the search for new spintronic materials with high spin polarization at room temperature, we have synthesized an osmium-based double perovskite with a Curie temperature of $725\mathrm{K}$. Our combined experimental results confirm the existence of a sizable induced magnetic moment at the Os site, supported by band-structure calculations, in agreement with a proposed kinetic-energy-driven mechanism of ferrimagnetism in these compounds. The intriguing property of ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$ is that it is at the end point of a metal-insulator transition due to $5d$ band filling and at the same time ferrimagnetism and high-spin polarization are preserved.

Figure 1

(Color online) Rietveld refinements of the neutron powder data of ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$ taken at $553\mathrm{K}$. The observed and calculated diffraction intensities as well as the difference pattern are shown. The upper left inset shows a view along the [111] direction revealing the rhombohedral structure of the double perovskite. The upper right inset shows the temperature dependence of the ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$ (444) diffraction peak (cubic) splitting into a (404) and (0 0 12) peak (hexagonal).

Figure 2

(Color online) Magnetization vs temperature and hysteresis (at $5\mathrm{K}$ and $300\mathrm{K}$) of ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$ (measured in a SQUID). The sample has been cooled and field oriented in diamagnetic $\mathrm{Li}\mathrm{N}{\mathrm{O}}_3$ (melting point $480\mathrm{K}$) because of the high anisotropy field.

Figure 3

(Color online) X-ray appearance near-edge structure (XANES) (left axis) and XMCD signal (right axis) at the Os $L_2$ and $L_3$ edges in ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$. The inset shows the element-resolved magnetic moments of Cr $\left({\mu }_{\mathrm{Cr}}\right)$ and Os $\left({\mu }_{\mathrm{Os}}\right)$, the total magnetic moment $\left({\mu }_{\mathrm{tot}}\right)$, and the lattice distortion as determined from neutron scattering data refinement (see Table ).

Figure 4

(Color online) Left: band-structure sketch for different fillings of the $5d$ spin-down band (as realized in the materials ${\mathrm{Sr}}_2\mathrm{Cr}M{\mathrm{O}}_6$ with $M=\mathrm{Ta},\mathrm{W},\mathrm{Re},\mathrm{Os}$). Right: band-structure calculation for ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$ using the generalized gradient approximation including spin-orbit coupling $(\mathrm{GGA}+\mathrm{SO})$.