Magnetic interactions in alpha-NaMnO(2): Quantum spin-2 system on a spatially anisotropic two-dimensional triangular lattice

The dc magnetization and the electron spin resonance (ESR) measurements have been performed on alpha-NaMnO(2) polycrystalline sample, a quantum spin system on a frustrated two-dimensional (2D) triangular lattice with spatially anisotropic Heisenberg exchange. The former measurements reveal a realization of the high-spin state (S=2) on magnetic Mn(3+) sites. From the susceptibility curve, we have determined the nearest-neighbor antiferromagnetic exchange coupling constants (namely, in the preferably coupled spin chains, J(1)/k(B)=65 K, and perpendicular to them, J(2)/J(1)=0.44) by employing the finite-temperature Lanczos method. The dominant magnetic anisotropy term of the single-ion type, D/k(B)=-4.1 K, which establishes an easy-axis direction, is evaluated from the ESR linewidth. We find that the temperature dependence of the linewidth reflects the onset of short-range static spin correlations below room temperature.