Ultrasonic investigation on the distorted diamond chain compound Azurite

Abstract

The natural mineral Azurite [Cu3(CO3) 2(OH)2] has been considered as a model substance for the 1D distorted antiferromagnetic diamond chain, the microscopic couplings of which, however, are still under discussion. Here we present results of the longitudinal elastic constant c22 down to 80 mK and magnetic fields up to 12 T. c22 reveals clear signatures of the magnetic energy scales involved and discloses distinct anomalies at the Néel ordering TN = 1.88 K. Based on measurement as a function of temperature and magnetic field, a detailed B-T phase diagram is mapped out which includes an additional phase boundary of unknown origin at low temperature (T < 0.5 K). Entering the new phase is accompanied by a pronounced softening of the c 22 elastic constant. These observations, together with results obtained by spectroscopic investigations reported in the literature, reflect an unusual long-range magnetically ordered state at very low temperatures. © 2010 IOP Publishing Ltd.