Magnetic susceptibility measurement under high pressure and magnetization measurement of S = 1/2 dioptase lattice antiferromagnet

Abstract

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

High-field spin-flop state in green dioptase

“…The magnetic properties of green dioptase have been reported by a number of authors [14,[16][17][18]20]. We have performed bulk characterization of our sample, and the results agree with those reported in Refs.…”

“…Early nuclear magnetoresonance (NMR) and specific-heat measurement indicated that it orders antiferromagnetically (AFM) below 21 K [12,13]. This is close to the results from the most recent susceptibility, specific heat, and neutron diffraction measurements reporting Néel temperatures of about 15 K [9, [14][15][16] and much lower than T N obtained from susceptibility measurements [17,18]. The ordered magnetic moment stemming from the Cu 2+ ions is reduced and equals to ∼0.4-0.5 μ B [9,15,18,19].…”

“…The absolute value of the magnetization was checked against the low-field measurements on a SQUID magnetometer. For B c there is a metamagneticlike transition at about B * = 12.2 T. This is slightly less than the transition field of 13 T reported by Ohta et al [14]. The inset in Fig.…”

“…Recently, Ohta et al reported a spin-flop transition in green dioptase in a magnetic field of about 13 T applied along the c axis at 1.5 K [14]. Although a spin-flop transition would be expected in the case of the AFM chains in dioptase, the authors noted a difference in the slopes of the magnetization curves for B ⊥ c and for B c beyond 13 T. As single-ion anisotropy is not expected in the case of S = 1 2 antiferromagnet, further investigations are required.…”

Ground State of the Spin-1/2 Chain of Green Dioptase at High Fields

“…The field region of NMR is approximately 13.2 -13.7 T, which well falls within the field region of broad magnetization jump at H C = 13 (±1) T [9]. So, we consider that the result shown in Fig.…”