Spiral spin structure in the Heisenberg pyrochlore magnetCdCr2O4

Abstract

A frustrated spinel $\mathrm{Cd}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ undergoes a three-dimensional spin-Peierls transition at ${T}_{N}=7.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from a cubic paramagnetic to a tetragonal N\'eel state. The N\'eel state has a spiral magnetic structure with a characteristic wave vector of (0, $\ensuremath{\delta}$, 1) when the $c$ axis is elongated [J.-H. Chung et al., Phys. Rev. Lett. 95, 247204 (2005)]. Here, we report our spherical neutron polarimetry experiments to investigate the spiral spin structure in detail. Our results indicate that the spins lie in the $ac$ plane perpendicular to the direction of the spiral modulation. We also find that the spiral structure in the $ac$ plane is elliptical with the $c$ component larger by $\ensuremath{\sim}24%$ than the $a$ component, suggesting a strong coupling between the magnetic structure and the structural distortion. Unpolarized neutron diffraction under an external magnetic field has also been performed. The magnetic-field dependence suggests the existence of magnetic anisotropy in the N\'eel state, which is consistent with our previous inelastic studies [J.-H. Chung et al., Phys. Rev. Lett. 95, 247204 (2005)].