Finite-size effects and magnetic order in the spin-1/2 honeycomb-lattice compound InCu2/3V1/3O3

High-field electron spin resonance, nuclear magnetic resonance, and magnetization studies addressing the ground state of the quasi-two-dimensional spin-1/2 honeycomb lattice compound InCu2/3V1/3O3 are reported. Uncorrelated finite-size structural domains occurring in the honeycomb planes are expected to inhibit long-range magnetic order. Surprisingly, ESR data reveal the development of two collinear antiferromagnetic (AFM) sublattices below similar to 20 K, whereas NMR results show the presence of the staggered internal field. Magnetization data evidence a spin reorientation transition at similar to 5.7 T. Quantum Monte Carlo calculations show that switching on the coupling between the honeycomb spin planes in a finite-size cluster yields a Neel-like AFM spin structure with a substantial staggered magnetization at finite temperatures. This may explain the occurrence of a robust AFM state in InCu2/3V1/3O3 despite an unfavorable effect of structural disorder.