| The graphite intercalation compounds (GICs) of CoBr 2, FeBr2 and NiBr2 have been prepared by intercalation into highly oriented pyrolytic graphite (HOPG). Stages 1 and 2 CoBr2 -GIC, stage-2 FeBr2-GIC and stages 2, 3 and 5 NiBr2 -GIC have been studied by measuring the dc magnetization, ac susceptibility and heat capacity. A single-crystal sample of stage-3 CoBr2-GIC was studied using neutron diffraction. The CoBr2-GICs do not order at any observed temperature (down to 2 K for stages 1 and 2, 5.5K for stage-3). Stage-2 FeBr2-GIC orders in three stages on cooling, with a transition from a paramagnetic phase to a phase with short-range two-dimensional ordering at 14.5 K, another to a phase with longer-range ordering within individual intercalant domains at 8.5 K and a third to a cluster-glass phase at 3 K. From do magnetization measurements, stages 2, and 3 NiBr2-GIC order two-dimensionally at 6 K and 7 K, respectively. Stage-5 NiBr2-GIC is superparamagnetic below 5 K. The Curie-Weiss law, modified to include a temperature-independent Van Vleck paramagnetism, is fit to the high-temperature dc susceptibility data. The need to include the temperature-independent term is also shown in magnetization curves. Simulations using Green's function calculations show that the absence of three-dimensional ordering down to 2 K in the CoBr2 and NiBr2 compounds is reasonable, but somewhat unexpected. |
