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

Abstract

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 Tcl ( = 6.8-7.2 K) and Tcu ( = 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 Tcl and Tcu, where Θ1'/h = χ1' + 3χ3'h2/4 + and Θ1''/h = χ1'' + 3χ3''h2/4 + . We find (i) a shift of Tcl 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 Tcl, 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 Tcu. The H-T phase diagram is also determined for H along the c plane.