Electron Spin Resonance in Second-Stage NiCl2-Graphite Intercalation Compound

ESR measurements have been made at 9 GHz in the temperature range 4.2 K£T£300 K,on the second-stage NiCl2-graphite intercalation compound based on a single crystal of Kish graphite,which approximates to an ideal two-dimensional ferromagnet in the presence of an external magnetic field.The angular and temperature dependences of the ESR resonance field and line-width show that the spin symmetry of this system changes from XY-like to Heisenberg-like at ~Tg(~30 K)as the temperature increases.The experimental results on ESR at sufficiently high temperatures above Tg are qualitatively explained in terms of the magnetic behaviour of the two-dimensional Heisenberg-like ferromagnet.