LAUSR

We report a comprehensive investigation of the frustrated magnetism on the delafossite oxides, α-CrOOH and α-CrOOD, which experimentally realize the S = 3/2 nearly-Heisenberg antiferromagnetic (J 1 > 0) model on a triangular lattice with weak single-ion anisotropy (D). The electron spin resonance (ESR), neutron scattering, and specific heat (C m) measurements on both α-CrOOH and α-CrOOD consistently indicate that the long-range 120° Néel order is significantly suppressed and both systems are in the vicinity of a spin-liquid phase with C m ∼ T 2 at low temperatures. The strength of D is quantitatively determined from fitting the high-temperature ESR linewidth and magnetic susceptibility, and its minus sign (D < 0, easy-axis type) is suggested by the low-energy (E ⩽ 2 meV) spin excitations. This easy-axis anisotropy (D/J 1 ∼ −5%) competes with the 120° Néel order and thus enhances the quantum spin fluctuations at low temperatures.