High magnetic field elastic properties of CdCr₂O₄

CdCr2O4 is a geometrically frustrated Heisenberg "pyrochlore" magnet with an incommensurate antiferromagnetic (AF) ordering below TN = 7.8 K. At low temperatures a metamagnetic phase transition at 28 T in this compound is followed by a very wide magnetization plateau with one half of the full moment of S = 3/2 Cr3+ (three spins up and one spin down); the fully polarized state appears above approximately 90 T. Because of the strong spin-lattice coupling some magnetic phase transitions in CdCr2O4 are accompanied by structural distortions. Such spin-lattice coupling can be especially well investigated by ultrasound techniques. We have performed ultrasonic experiments on a high-quality CdCr2O4 single crystal at low temperatures and in pulsed magnetic fields up to 64 T. In our experiments, a longitudinal acoustic mode propagates along the [111] crystallographic direction and the magnetic field is applied in the same direction. A hysteretic minimum followed by a jump in the sound velocity and a peak in the attenuation are observed at 28 T for temperatures below the AF phase transition. An abrupt softening of the acoustic mode is detected at 59 T, where the magnetization plateau terminates. The above-mentioned anomalies get smoother at higher temperatures. The observed behavior of the acoustic characteristics demonstrates a crucial role of the spin-lattice coupling, which leads to lattice instabilities in this geometrically frustrated antiferromagnet in applied magnetic field. Our experimental results will be discussed in the framework of recent theories. Specifically, we construct a simple model, which gives quantitative description of the temperature
variations of the sound velocity in the vicinity of zero-field transition.