Magnetic interactions inα−NaMnO2: Quantum spin-2 system on a spatially anisotropic two-dimensional triangular lattice

Abstract

The dc magnetization and the electron spin resonance (ESR) measurements have been performed on $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Na}\mathrm{Mn}{\mathrm{O}}_{2}$ 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 ${\mathrm{Mn}}^{3+}$ sites. From the susceptibility curve, we have determined the nearest-neighbor antiferromagnetic exchange coupling constants (namely, in the preferably coupled spin chains, ${J}_{1}∕{k}_{B}=65\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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, $D∕{k}_{B}=\ensuremath{-}4.1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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.