Successive magnetic phase transitions of a ${\mathrm{CoCl}}_2{-\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_3$ graphite bi-intercalation compound

${\mathrm{CoCl}}_2{-\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_3$ graphite bi-intercalation compound (GBIC) has a c-axis stacking sequence of ${-G-I}_1{-G-I}_2{-G-I}_1{-G-I}_2-G-$ $(G=\mathrm{graphite}\mathrm{}\mathrm{layer},$ $I_1={\mathrm{CoCl}}_2\mathrm{}\mathrm{layer},$ and $I_2={\mathrm{FeCl}}_3\mathrm{}\mathrm{layer}).\mathrm{}$ This compound undergoes four magnetic phase transitions at $T_h(=16.4\mathrm{}\mathrm{K}),$ $T_{\mathrm{cu}}(=8.5\mathrm{}–8.7\mathrm{K}),$ $T_{\mathrm{cl}}(=7.3\mathrm{}–7.4\mathrm{K}),$ and $T_{\mathrm{SG}}(=4.7\mathrm{}–5.9\mathrm{K}).\mathrm{}$ The static and dynamic aspects of spin orderings in this compound have been studied employing the temperature, frequency, and field dependence of dispersion ${\chi }_{\mathrm{aa}}^{\prime }$ and ${\chi }_{\mathrm{cc}}^{\prime },$ and the absorption ${\chi }_{\mathrm{aa}}^{″}$ and ${\chi }_{\mathrm{cc}}^{″}.$ A possible helical spin order occurring below $T_h$ is due to the spin frustration effect arising from competing interplanar exchange interactions. Below $T_{\mathrm{cu}}$ two-dimensional (2D) ferromagnetic long-range order appears in each ${\mathrm{CoCl}}_2$ layer. Below $T_{\mathrm{cl}}$ these 2D ferromagnetic ${\mathrm{CoCl}}_2$ layers are antiferromagnetically stacked along the c axis, forming a three-dimensional (3D) antiferromagnetic phase. The spin-glass phase occurring in each ${\mathrm{FeCl}}_3$ layer below $T_{\mathrm{SG}}$ is due to the spin frustration effect arising from competing intraplanar exchange interactions between majority XY ${\mathrm{Fe}}^{3+}$ spins and minority Ising ${\mathrm{Fe}}^{2+}$ spins. The phase transitions near $T_{\mathrm{cu}},$ $T_{\mathrm{cl}},$ and $T_{\mathrm{SG}}$ are partially destroyed by random fields generated in ${\mathrm{FeCl}}_3$ layers and ${\mathrm{CoCl}}_2$ layers, respectively, through competing interplanar exchange interactions.