Ferromagnetic resonance study of reduced NiCl2 graphite intercalation compound

A commercial reduction compound of graphite intercalated with NiCl₂ has been investigated by FMR. The X-band measurements for 573 ? T ? 3.5 K and Q-band for 293 ? T ? 150 K show the presence of metallic Ni and some unreduced NiCl₂. The Ni resonance signals have very large linewidths, δH_pp ～ 1800 ? 2000 Oe for 300 ? T ? 150 K. Analysis of the lineshape indicates that Ni remains intercalated in the graphite. The intercalated Ni exhibits a Curie temperature which is approximately 50 K lower than that of bulk Ni. The NiCl₂ X-band signals observed for 150 ? T 3.7 K show a maximum in intensity and linewidth at about 15 K, which is attributed to thermally induced magnetic excitations in the remaining finite NiCl₂ clusters. An in-plane exchange constant, J/k = 15 K, is estimated for NiCl₂ from the temperature dependence of the signal intensity.