The spin-lattice coupling plays an important role in strongly frustrated magnets. In ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$, an excellent realization of the Heisenberg antiferromagnet on the pyrochlore network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at $T_c=12.5\mathrm{K}$. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ tracks the nearest-neighbor spin correlations above $T_c$. The splitting of the phonon triplet below $T_c$ provides a way to measure the spin-Peierls order parameter.

• Received 14 December 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.137202