Ultrasound at high magnetic fields: first scientific results at the HLD

Ultrasound investigations are a powerful experimental tool to study various phase transitions and critical phenomena. This technique is now available at the Dresden High Magnetic Field Laboratory (HLD) as part of the pulsed field user program. The possible application of the infrared radiation produced by next-door free electron lasers for photoacoustic spectroscopy will be discussed. Here I will present two results recently obtained at the HLD by use of ultrasound technique. First, I report results of magnetoacoustic studies of the quantum spin-chain magnet NiCl₂-4SC(NH₂)₂ (DTN) having a field-induced ordered antiferromagnetic (AF) phase. The longitudinal acoustic c33 mode, which propagates along the spin chains, modulates the in-chain exchange interaction and shows a softening, accompanied by energy dissipation in the acoustic wave in the vicinity of the quantum critical points. Another example is CdCr₂O₄, a geometrically frustrated magnet with a metamagnetic phase transition at 28 T followed by a very wide magnetization plateau with one half of the full moment of S = 3/2 Cr³⁺. We have performed an ultrasonic investigation of a high quality CdCr₂O₄ single crystal in pulsed magnetic fields up to 64 T. A minimum in the sound velocity and a peak in the attenuation have been observed at 28 T for temperatures below the temperature of the AF phase transition T_N = 7.8 K. Another anomaly in the sound velocity has been observed at 60 T, where the magnetization plateau is terminated. The observed anomalies become smoother at higher temperatures. The experimental data are analyzed by use of a model where the main contribution to the spin-lattice interaction arises from the exchangestriction coupling.