Incommensurate spin spiral of the geometrically-frustrated antiferromagnet CdCr₂O₄

CdCr₂O₄ is a cubic spinel with geometrically-frustrated antiferromagnetic interactions between Cr³⁺ ions. We investigated the structural and magnetic phase transitions in powder samples by using magnetization and neutron diffraction measurements. An antiferromagnetic phase transition was observed at T _N = 8.1 K, below which the crystallographic unit cell was tetragonal (c < a = b). Its incommensurate antiferromagnetic ordering below T _N resembles dimerized spin chains in the ab planes with the propagation vector k = (0, 0.088, 1). We argue that the Dzyaloshinskii-Moriya interaction and the magnetic frustration play important roles in establishing the observed magnetic ordering.