Specific heat of stage-2 MnCl₂ graphite intercalation compound: co-existence of spin glass phase and incommensurate short-range spin order

Stage-2 MnCl$_{2}$ GIC magnetically behaves like a quasi 2D $XY$ antiferromagnet on the triangular lattice. It shows a typical spin glass behavior below the spin freezing temperature $T_{SG}$ (= 1.1 K). The AC magnetic susceptibility shows a peak at $T$ = $T_{SG}$ at $H$ = 0. This peak shifts to the low temperature side with increasing $H$, according to the de Almeida-Thouless critical line. 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 $C_{mag}$ at $H$ = 0 shows no anomaly at $T_{SG}$, but exhibits double broad peaks around 5 K and 41 K. The magnetic specific heat $C_{mag}$ at $H$ = 0 is described by $C_{mag} \propto (1/T^{2})\exp(-\Delta/T)$ with $\Delta = 1.41 \pm 0.03$ K, while $C_{mag}$ at $H$ = 10 kOe is proportional to $T$. The entropy due to the broad peak around 5 K is 1/3 of the total entropy. The residual entropy is 63 % of the total entropy because of highly frustrated nature of the system. The magnetic neutron scattering indicates that a short range spin order associated with the incommensurate wave vector $| {\bf Q}_{incomm}|$ ($= 0.522 \AA^{-1}$ at 0.45 K) appears below 5 K and remains unchanged down to 63 mK. The in-plane spin correlation length is only 18 $\AA$ at 0.45 K. The low temperature phase below $T_{SG}$ is a kind of reentrant spin glass phase where the spin glass phase coexists with a short range spin order associated with $| {\bf Q}_{incomm}|$.