抄録
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.
