Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathbf{O}}_7$

${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in–all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by the Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions.

Figure 1

Raman spectra at 300 and 4 K on the (111) surface of a ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ single crystal in parallel polarization. The inset shows a single crystal of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$.

Figure 2

Spectra of the intensity of the 006 reflection in the $\pi -{\sigma }^{\prime }$ channel ($\psi =-47°$) at 10 K and x-ray absorption near the Os $L3$ edge at room temperature. $M$ represents the multiple scattering. The right inset shows the experimental setting in the $\pi -{\sigma }^{\prime }$ channel, where the azimuthal angle $\psi$ (angle in the in-plane rotation of the crystal) is defined to be zero when the $x$-axis is parallel to the $a$-axis of the crystal. The left inset shows the polarization dependence of the 006 reflection at 10.874 keV.

Figure 3

(a) Temperature dependence of the integrated intensity of the 006 magnetic reflection measured in the $\pi -{\sigma }^{\prime }$ channel at $\psi =-47°$. Upon cooling, it increases gradually below $T_{\mathrm{MI}}=227\mathrm{K}$ following a power law of temperature, indicating a second-order transition. (b) Resistivity in a zero magnetic field and magnetic susceptibility in $H=20\mathrm{kOe}$ measured after zero-field cooling, both of which show clear anomalies at $T_{\mathrm{MI}}$.

Figure 4

Spin arrangements on a regular tetrahedron for selected basis functions ${\Psi }_i$ with $i=1$, 2, 3, 4, and 8 and the corresponding irreducible representations ${\Gamma }_s$. All of them are noncollinear antiferromagnetic spin arrangements. Only the all-in–all-out ${\Psi }_1$ indicates cubic magnetic symmetry; the other antiferromagnetic basis functions indicate tetragonal magnetic symmetry.