Magnetic phase diagram of the stage-1 CoCl2 graphite intercalation compound: Existence of metamagnetic transition and spin-flop transitions

The H-T phase diagram of a stage-1 CoCl₂ graphite intercalation compound has been determined from measurements of the ac magnetic susceptibility, dc magnetization, and in-plane resistivity in the presence of an external magnetic field along the c plane perpendicular to the c axis. This compound undergoes two magnetic phase transitions at T_cu (=T_N=9.9 K) and T_cl (=7.7 K) at H=0. A metamagnetic transition occurs at a critical point (T₃~8.77 K, H₃~270 Oe). A spin-flop transition occurs at the critical point (T₂~6.8 K, H₂~80 Oe) due to a competition between weak antiferromagnetic interplanar interactions and in-plane anisotropic interactions. Two lines H₊ and H_- above T_N are due to ferromagnetic short-range fluctuations.