Magnetic Behavior of Spin-1/2 Frustrated System Hg₂Cu₂F₆S

Abstract

Experimental studies have been carried out on geometrically frustrated quantum spins of Cu²⁺ ions of Hg₂Cu₂F₆S with pyrochlore structure. No magnetic and structural phase transitions have been observed in the temperature (T) range down to 2 K. The magnetic correlation length ξ deduced from the T dependence of the NMR longitudinal relaxation rate is found to be as small as the atomic distance even at T much lower than the Cu²⁺–Cu²⁺ exchange coupling energy J∼100 K. With decreasing T from ∼200 K, the NMR Knight shift first increases, has a maximum at ∼90 K and decreases rather rapidly. This low-T behavior suggests that the system approaches the spin-liquid state. The upturn of the uniform magnetic susceptibility observed in the very low T region with decreasing T is due to lattice imperfections, and effects of these imperfections can be considered to be an origin of the large broadening of the NMR spectra observed at low T.