Electron-spin-resonance study of two-dimensional antiferromagnet stage-2 ${\mathrm{MnCl}}_2$-graphite intercalation compound

Electron-spin-resonance (ESR) measurements have been made at 9.42 GHz in the temperature range 1.38 K≤T≤293 K on the stage-2 ${\mathrm{MnCl}}_2$-graphite intercalation compound (GIC) which approximates an ideal two-dimensional antiferromagnet with a Néel temperature $T_N$=1.1 K. The temperature and angular dependences of the ESR linewidth ΔH have been examined. As temperature is lowered toward $T_N$, the linewidth, ΔH, initially decreases, passes through a minimum, and then increases rapidly. The angular dependence of ΔH changes with temperature: a form (3 ${\cos }^2$θ${\mathrm{-}1)}^2$ at 65 K, and a form ${\sin }^2$θ(1+9 ${\cos }^2$θ) at 1.38 K. These phenomena may be qualitatively explained by a theory of ΔH developed for the two-dimensional antiferromagnet on the triangular lattice sites.