  Recent neutron scattering measurements on the spinel CdCr2O4 revealed a rare
example of helical magnetic order in geometrically frustrated pyrochlore
antiferromagnet. The spin spiral characterized by an incommensurate wavevector
Q = 2pi (0, delta, 1) with delta ~0.09 is accompanied by a tetragonal
distortion. Here we conduct a systematic study on the magnetic ground state
resulting from the interplay between the Dzyaloshinskii-Moriya interaction and
further neighbor exchange couplings, two of the most important mechanisms for
stabilizing incommensurate spin orders. We compute the low-energy spin-wave
spectrum based on a microscopic spin Hamiltonian and find a dispersion relation
characteristic of the helimagnons. By numerically integrating the
Landau-Lifshitz-Gilbert equation with realistic model parameters, an overall
agreement between experiment and the numerical spectrum, lending further
support to the view that a softened optical phonon triggers the magnetic
transition and endows the lattice a chirality.

Strongly Correlated Electrons