Nonlinear AC magnetic susceptibility of the stage-2 CoCl₂ graphite intercalation compound

The stage-2 CoCl₂ graphite intercalation compound behaves magnetically like a quasi-two-dimensional XY-like ferromagnet with a very weak antiferromagnetic interplanar exchange interaction. This compound undergoes two magnetic phase transitions at T_cl ( = 6.8-7.2 K) and T_cu ( = 8.9 K). The SQUID AC magnetic susceptibility at a fixed AC frequency (f = 1 Hz) has been measured as a function of AC magnetic field strength h (1 mOe⩽h⩽4.2 Oe), temperature T (1.9⩽T⩽12 K) and external magnetic field (0⩽H⩽1 kOe). The dispersion (Θ₁'/h) and absorption (Θ₁^''/h) are strongly dependent on h for h>50 mOe in the vicinity of T_cl and T_cu, where Θ₁'/h = χ₁' + 3χ₃'h²/4 + ··· and Θ₁^''/h = χ₁^'' + 3χ₃^''h²/4 + ···. We find (i) a shift of T_cl in χ₁^'' to the low-temperature side with decreasing frequency and (ii) a positive local maximum in χ₃' at 7.3 K. These results are indicative of a re-entrant spin-glass-like phase below T_cl, where the AF phase and spin-glass phase coexist. A positive broad peak of χ₃' around 8.4-8.9 K shows the occurrence of two-dimensional ferromagnetic order below T_cu. The H-T phase diagram is also determined for H along the c plane.