Specific heat of stage-2 MnCl${}_2$ graphite intercalation compound: co-existence of spin glass phase and incommensurate short-range spin order

Abstract

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 $|{\mathbf{Q}}_{incomm}|$ ($=0.522{\mathring{A}}^{-1}$ at 0.45 K) appears below 5 K and remains unchanged down to 63 mK. The in-plane spin correlation length is only 18 $\mathring{A}$ 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 $|{\mathbf{Q}}_{incomm}|$.

14 pages, 20 figures