Magnetic properties of transition metal dichloride - graphite intercalation compounds

The magnetic properties of four transition metal dichloridegraphite intercalation compounds (GIC's) have been studied by neutron scattering and static magnetization measurements. Because of the large spatial anisotropy introduced by intercalation, the compounds studied represent good approximations to magnetic systems in two dimensions. Magnetization measurements as a function of applied field and temperature have been used to determine the dominant coupling terms and anisotropies in the spin Hamiltonian, and magnetic neutron scattering is used to measure dynamic spin correlations above and below the critical temperatures. In addition, the atomic positions and structural dynamics of these compounds have been probed through x-ray and neutron scattering. Stage-2 CoC12 -GIC approximates a two-dimensional easy-plane ferromagnet on a triangular lattice. It is found to order in two iv steps, the higher-temperature phase showing no three-dimensional static correlations, and the lower phase exhibiting a weak antiferromagnetic correlation between neighboring intercalate layers . We have probed the wave vector and temperature dependence of the static and dynamic spin correlations in detail. Some of the predictions for a Kosterlitz-Thouless type transition are met in this compound, at least qualitatively , including an apparent jump in the spin stiffness at the critical point and a diffusive central peak in the scattering function originating from vortex autocorrelations. However, there are substantive inconsistencies that prevent unambiguous assignment of the upper critical temperature as a vortex-binding transition . Intercalation compounds of NiC12 , MnC12 , and CuC12 have also been studied to a lesser extent. The magnetism in NiC12 -GIC is found to be nearly identical to that in CoC12 -GIC . . MnC12 - and CuC12 -GIC's approximate two-dimensional antiferromagnets. MnC12 -GIC shows an easy-plane anisotropy and orders at Tc - 1.1 K to a 2/3 X 2/3 superstructure, indicating strong exchange coupling with far neighbors. In CuC12 -GIC the triangular lattice is deformed, making the spin correlations stronger in one direction than the other. We find no evidence by neutron scattering of magnetic ordering above 9 K.U of I OnlyThesi