Magnetism of a graphite bi-intercalation compound with two types of ferromagnetic layers: double hysteretic transition in CrCl3–NiCl2–C

An atomic-scale multilayer with two types of ferromagnetic layers is achieved by graphite intercalation. It is a bi-intercalation compound with stacking sequence CrCl3NiCl2NiCl2/… where/represents a graphite layer. The two types of chloride layers are much as they occur in the pristine chlorides and have the same lattice parameters and intralayer intertransition–metal ferromagnetic exchange interactions. The transition temperatures for singly intercalated CrCl3 and NiCl2 graphite compounds are, respectively, 11.3, and 20.2 K. The bi-intercalation compound shows a "double transition" with Tc1 and Tc2 being equal to the relevant intercalate-specific temperatures. It is proposed that, as the temperature is lowered, the NiCl2 layers order first at Tc2 into a 3-D antiferromagnetic stacking of ferromagnetic planes, followed by onset of intra-CrCl3-layer ferromagnetic order at Tc1. Below Tc1 the two types of ferromagnetic planes probably assume a 3-D antiferromagnetic stacking that involve both types of layers, requiring that the NiCl2 stacking be different than at Tc1 < T < Tc2. The main interlayer interactions are believed to be dipole–dipole forces and these are seen to give large low-field temperature hysteresis effects. This is in contrast to many layered materials with antiferromagnetic in-plane interactions in which: (i) dipole–dipole forces do not play a significant role, (ii) 3-D order is not intercalate specific but occurs at temperatures that are much lower than the in-plane interaction strengths and that are highly dependent on the interlayer interaction strengths, and (iii) hysteresis effects are not observed.