Abstract. Natural mineral Dioptase (Cu6Si6O18 · 6H2O) is an S=1/2 antiferromagnet with a unique spin network which consists of helical chains along the c axis connected by inter chain exchange interactions. In order to study the ground state of dioptase, we have performed the magnetization measurement for B//c (easy axis) at 1.5 K and have found the spin-flop transition at 13 T. Moreover, the pressure effect of the magnetic susceptibility is investigated up to 1.78 GPa using the SQUID magnetometer where TN increased while the broad maximum reflecting the low dimensionality of the system decreased by pressure. The results will be discussed in connection with our previous antiferromagnetic resonance (AFMR) measurement results and the phase diagram obtained by Gros et al.
IntroductionNatural mineral Dioptase (Cu 6 Si 6 O 18 · 6H 2 O) is a unique S=1/2 antiferromagnet where Cu 2+ ions form the antiferromagnetic helical chain along the c-axis (the exchange interaction J 2 = J(1 − δ) and the chains are antiferromagnetically connected (the exchange interaction. This unique spin network is called the dioptase lattice and the phase diagram of its ground state is obtained by the Quantum Monte Carlo calculation [1]. As shown in Fig. 1 the calculation suggests that the quantum critical point at δ =0.3 is expected in between the antiferromagnetic order phase (δ <0.3) and the spin gap phase (δ >0.3) [1]. However, the magnetic susceptibility of dioptase shows a broad maximum around 45 K and the antiferromagnetic order is observed at T N =15.5 K. Its analysis by the Quantum Monte Carlo calculation suggest δ =0.1, J=56.6 K, g=2.09 or δ =-0.1, J=53.4 K, g=2.10 for dioptase [1]. Moreover, our previous high field ESR result at 1.8 K clearly shows the existence of antiferromagnetic order below T N by the observation of AFMR, and the existence of the spin-flop