Magnetic properties of CocNi1-cCl2-FeCl3 graphite bi-intercalation compounds

CocNi1-cCl2-FeCl3 graphite bi-intercalation compounds (GBIC's) with the stacking sequence -G-CocNi1-cCl2-G-FeCl3-G- along the c axis have been prepared by a method of sequential intercalation. The intraplanar exchange interaction in CocNi1-cCl2 layers is ferromagnetic, while the intraplanar exchange interaction in FeCl3 layers is antiferromagnetic. The magnetic properties of CocNi1-cCl2-FeCl3 GBIC's have been studied by using dc and ac magnetic susceptibility, and low- and high-field SQUID magnetization measurements. The CocNi1-cCl2 layer undergoes a magnetic phase transition at the critical temperature Tc which changes from 19.48 K at c=0 to 9.10 K at c=1. A cluster glass phase appears below Tc where the spin directions of ferromagnetic clusters are frozen because of frustrated interisland interactions. Due to the intervening FeCl3 layer the critical behavior of CocNi1-cCl2-FeCl3 GBIC's at Tc is three dimensional rather than two dimensional. Below TN (4 K) the cluster glass phase may coexist with an antiferromagnetic long-range order occurring in the FeCl3 layers. The effect of antiferromagnetic interplanar interaction between the FeCl3 layer and the CocNi1-cCl2 layer is clearly seen in the magnetization of CocNi1-cCl2-FeCl3 GBIC's. The nature of the coexisting phase is complicated by the spin frustration effect arising from the competition between the interplanar interaction between FeCl3 and CocNi1-cCl2 layers and the antiferromagnetic intraplanar interaction in the FeCl3 layer.