Magnetization Measurements of the Second Stage NiCl2-Graphite Intercalation Compound

We have investigated the induced (M) and residual (Mr) magnetizations in the 2nd stage NiCl2-graphite intercalation compound at 4.2 to 300 K in the fields of 0.1 to 4.5 x 104 Oe with using the well-staged single crystal samples. Two successive magnetic phase transitions have been observed at 71-1 = 17.5 K and 7CU = 22.0 K. Above Tcu the compound is paramagnetic with 0=7O�1 K and leff= 3.29 juB. In the intermediate phase (Tci<T<Tcu) the index S in M~Hjl6t has a remarkable temperature dependence from 1.0(7>7,cu) to 2.0(7T<18.5 K) in the intermediate fields (1 <Tint <80 Oe) whereas it takes unity in the low fields (i7int;S 1 Oe). Mt measured in a weak cooling field Hc shows two stepwise decreases at temperatures immediately below Tcl and rcu, respectively, and Mt being finite in the intermediate phase. This two-step decrease of Mr has not been observed for Hc 10 Oe. These results suggest the existence of an ordered state at 1 Oe in the intermediate phase. The critical behavior of %(T) and S(T) and the magnetic structure of the ordered state are also discussed.