Spiral spin structure in the Heisenberg pyrochlore magnet Cd Cr2 O4

A frustrated spinel Cd Cr2 O4 undergoes a three-dimensional spin-Peierls transition at TN =7.8 K from a cubic paramagnetic to a tetragonal Néel state. The Néel state has a spiral magnetic structure with a characteristic wave vector of (0, δ, 1) when the c axis is elongated. 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 ∼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 Néel state, which is consistent with our previous inelastic studies.