Magnetic phase transitions in CoCl₂-graphite intercalation compounds

Massachusetts Institute of Technology Cambridge, MA 02139, USA.

Abstract

Magnetic intercalation compounds of stage 1, 2, and 3 CoCl₂-GIC have been successfully prepared and characterized by x-ray diffraction, Raman scattering, and transmission electron microscopy. The temperature dependence of the magnetic susceptibility of these samples show two superposed peaks. The difference in the magnetic field dependence of these two peaks indicates that at least two magnetic states are present. The higher temperature peak is identified with a Kosterlitz-Thouless type phase transition for finite size systems. Above the upper transition temperature (T_cu), the magnetic system consists of unbound vortices and below T_cu the vortices are bound. For low stage samples (stage ≤ 2), the lower temperature peak is identified with a transition showing crossover from two dimensional to three dimensional behavior. Below the lower transition temperature (T_cl), the system is in a phase showing long range magnetic order in which the spins are coupled ferromagnetically in-plane and antiferromagnetically inter-plane. The spin-flop field from the antiferromagnetic to ferromagnetic state can be determined by measuring the magnetic field dependence of the susceptibility. For the higher stage sample (stage 3), the inter-plane coupling is very small and the system can be treated like a 2D-ferroma- gnet.

Résumé

La variation thermique de la susceptibilité magnétique de composés graphite-CoCl₂ de stade 1, 2 et 3 présente deux pics superposés. Le pic de température supérieure est supposé dû à une transition de phase de type Kosterlitz-Thouless pour des systèmes de taille finie. Le second pic, dans le cas des échantillons de stade ≤ 2, semble dû à un cross-over 2 D → 3, avec l'apparition d'un ordre magnétique à grande distance (couplage ferromagnétique dans le plan des couches insérées et antiferromagnétique entre couches successives). Dans le cas de l'échantillon de stade 3, l'interaction entre couches insérées est très petite et le système peut être assimilé à un ferromagnétique bidimensionnel.