Spin-glass phases in stage-2 ${\mathrm{FeCl}}_3$ graphite intercalation compound

The magnetic phase transitions of stage-2 ${\mathrm{FeCl}}_3$ graphite intercalation compound have been studied using superconducting quantum interference device dc and ac susceptibility measurements in the temperature range between 1.9 and 18 K. The temperature, frequency, and field dependence of ${\chi }_{\mathrm{aa}}^{\prime },$ ${\chi }_{\mathrm{aa}}^{″},$ ${\chi }_{\mathrm{cc}}^{\prime },$ and ${\chi }_{\mathrm{cc}}^{″}$ clearly show that this compound undergoes two kinds of spin-glass phase transition at $T_{\mathrm{SG}}^{\mathrm{}\left(h\right)\mathrm{}}$ $(\approx 4.5–6.1\mathrm{K})$ and $T_{\mathrm{SG}}^{\mathrm{}\left(l\right)\mathrm{}}$ $(\approx 2–2.5\mathrm{K}),$ respectively. Both ${\chi }_{\mathrm{aa}}^{″}$ and ${\chi }_{\mathrm{cc}}^{″}$ have peaks at $T_{\mathrm{SG}}^{\mathrm{}\left(h\right)\mathrm{}}$ that shift to the low-temperature side with decreasing frequency. The spin-glass phase below $T_{\mathrm{SG}}^{\mathrm{}\left(h\right)\mathrm{}}$ may result from a competition between the antiferromagnetic $\mathrm{XY}$-like ${\mathrm{Fe}}^{3+}$ spins as the majority and the ferromagnetic Ising ${\mathrm{Fe}}^{2+}$ spins as the minority. The absorption ${\chi }_{\mathrm{aa}}^{″}$ has a peak at $T_{\mathrm{SG}}^{\mathrm{}\left(l\right)\mathrm{}}$ that shifts to the low-temperature side with decreasing frequency. No anomaly in ${\chi }_{\mathrm{cc}}^{″}$ is observed at $T_{\mathrm{SG}}^{\mathrm{}\left(l\right)\mathrm{}},$ indicating that only the $\mathrm{XY}$ components of spins contribute to this transition. The spin-glass transition below $T_{\mathrm{SG}}^{\mathrm{}\left(l\right)\mathrm{}}$ may result from a competition between the intraplanar nearest-neighbor antiferromagnetic and next-nearest-neighbor ferromagnetic intraplanar exchange interaction, which is responsible for a possible incommensurate in-plane spin structure at low temperatures.