Lattice Instabilities in the Frustrated Magnet CdCr₂O₄: An Ultrasonic Study

We report results on ultrasound studies of the frustrated magnet CdCr₂O₄. This compound demonstrates an antiferromagnetic ordering at T_N = 7.8 K and a metamagnetic phase transition at 28 T followed by a wide magnetization plateau with one half of the full moment of S = 3/2 of Cr³⁺ ions. A longitudinal acoustic mode in the [111] crystallographic direction exhibits pronounced effects both in the temperature and magnetic-field dependencies. Pulsed-field measurements show a drastic change in the sound velocity just below and above the 1/2 magnetization plateau. Our results suggest a large spin-strain coupling and give evidence for a pronounced interplay between spin and lattice degrees of freedom in CdCr₂O₄.