Ultrasonic investigation of NiCl2-4SC(NH2) 2

Abstract

We report results of ultrasonic investigations of the quantum S 1 spin-chain magnet NiCl2-4SC(NH2)2, also known as DTN, in magnetic fields up to 18 T and temperatures down to 0.3 K. A field H along the [001] direction induces a transition into an antiferromagnetic phase with TNmax 1.2 K. Accordingly, at T 0 there are two quantum critical points at ∼2.1 T and at ∼12.6 T. The acoustic c 33 mode, propagating along the spin chains, shows a pronounced softening close to the phase transition, accompanied by energy dissipation of the sound wave. The H-T phase diagram obtained from our measurements is compared with results from other experimental investigations and the low-temperature acoustic anomalies are traced up to T > TN. We also report frequency-dependent effects, which open the possibility to investigate the spin fluctuations in the critical regions. Our observations show an important role of the spin-phonon coupling in DTN. PACS numbers: 72.55.+s, 75.45.+j © 2009 IOP Publishing Ltd.