Specific heat of stage-2 mncl₂ graphite intercalation compound: Co-Existence of spin glass phase and incommensurate short-Range spin order

Stage-2 MnCl₂ GIC magnetically behaves like a quasi 2D XY antiferromagnet on the triangular lattice with the spin freezing temperature TSG (= 1.1 K). We have undertaken an extensive study on the T dependence of specific heat in the absence of an external field H and in the presence of H along the c-plane. The magnetic specific heat Cmag at H = 0 shows no anomaly at TSG, but exhibits double broad peaks around 5 and 41 K. The anomaly at 41K is the onset of short range spin correlation. The magnetic specific heat C _mag at H = 0 is described by C_mag / α(1/T ²) exp(- ΔT ) with Δ = 1.41 ± 0.03 K, while C _mag at H = 10 kOe is proportional to T. The entropy due to the broad peak around 5K is 1/3 of the total entropy. The residual entropy is 63% of the total entropy because of highly frustrated nature of the system. These results are consistent with the previous DC susceptibility, neutron scattering, and electron spin resonance (ESR) experiments.