Structural and magnetic properties of random mixture graphite intercalation compounds

Abstract

The structural and magnetic properties of the stage 2 Co_cNi_1−cCl₂- and CO_cFe_1−cCl₂-graphite intercalation compounds (GICs) for 0 ≤ c ≤ 1 have been studied by x-ray scattering and dc magnetic susceptibility. The stage 2 Co_cNi_1−cCl₂-GICs approximate two-dimensional randomly-mixed ferromagnets with XY spin symmetry. The average effective magnetic moment P_eff, the Curie-Weiss temperature θ, and the paramagnetic-to-ferromagnetic phase transition temperature T_c have been determined as continuously varying functions of Co concentration c. They indicate that the Co²⁺ and Ni²⁺ spins are randomly distributed on the triangular lattice sites of each intercalate layer. They also show that the intraplanar exchange interaction J(Co–Ni) between the Co²⁺ and Ni²⁺ spins is enhanced and is larger than the interaction J(Co–Co) between two Co²⁺ spins and J(Ni–Ni) between two Ni²⁺ spins. This enhanced interaction, J(Co–Ni), can be expressed as J(Co–Ni) = 1.28 [J(Co–Co) · J(Ni–Ni)]^1/2. The stage 2 Co_cFc_1−cCl₂-GICs approximate two-dimensional randomly mixed ferromagnets with competing spin anisotropy. The dc magnetic susceptibility results suggest that Co²⁺, Fe³⁺ rather than Fe²⁺ are distributed in the intercalate layer. The repeat distance along the c-axis (d-spacing) versus Co concentration deviates from Vegard’s law which states that the d-spacing is proportional to Co concentration. The broad peak of d-spacing observed at c = 0.75 is discussed in terms of the double layer model developed by Jin and Mahanti.