text: A number of metal halides intercalate into graphite. The chloride derivatives have been most extensively studied. Examples include  (M = Zn, Ni, Cu, Mn),  (M = Al, Fe, Ga),  (M = Zr, Pt), etc. The materials consists of layers of close-packed metal halide layers between sheets of carbon. The derivative  exhibits spin glass behavior. It proved to be a particularly fertile system on which to study phase transitions. A stage n magnetic graphite intercalation compounds has n graphite layers separating successive magnetic layers. As the stage number increases the interaction between spins in successive magnetic layers becomes weaker and 2D magnetic behaviour may arise.