Magnetic properties of ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-${\mathrm{FeCl}}_3$ graphite bi-intercalation compounds

${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_{2\mathrm{-}}$${\mathrm{FeCl}}_3$ graphite bi-intercalation compounds (GBIC’s) with the stacking sequence -G-${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-G-${\mathrm{FeCl}}_3$-G- along the c axis have been prepared by a method of sequential intercalation. The intraplanar exchange interaction in ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ layers is ferromagnetic, while the intraplanar exchange interaction in ${\mathrm{FeCl}}_3$ layers is antiferromagnetic. The magnetic properties of ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_{2\mathrm{-}}$${\mathrm{FeCl}}_3$ GBIC’s have been studied by using dc and ac magnetic susceptibility, and low- and high-field SQUID magnetization measurements. The ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ layer undergoes a magnetic phase transition at the critical temperature ${\mathit{T}}_{\mathit{c}}$ which changes from 19.48 K at c=0 to 9.10 K at c=1. A cluster glass phase appears below ${\mathit{T}}_{\mathit{c}}$ where the spin directions of ferromagnetic clusters are frozen because of frustrated interisland interactions. Due to the intervening ${\mathrm{FeCl}}_3$ layer the critical behavior of ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-${\mathrm{FeCl}}_3$ GBIC’s at ${\mathit{T}}_{\mathit{c}}$ is three dimensional rather than two dimensional.

Below ${\mathit{T}}_{\mathit{N}}$ (≊4 K) the cluster glass phase may coexist with an antiferromagnetic long-range order occurring in the ${\mathrm{FeCl}}_3$ layers. The effect of antiferromagnetic interplanar interaction between the ${\mathrm{FeCl}}_3$ layer and the ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ layer is clearly seen in the magnetization of ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-${\mathrm{FeCl}}_3$ GBIC’s. The nature of the coexisting phase is complicated by the spin frustration effect arising from the competition between the interplanar interaction between ${\mathrm{FeCl}}_3$ and ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ layers and the antiferromagnetic intraplanar interaction in the ${\mathrm{FeCl}}_3$ layer.