The ac susceptibility bridge technique is applied to the measurement of a long spin-lattice relaxation time associated with the magnetic anomaly which has been reported for ${\mathrm{FeCl}}_3$ graphite intercalation compounds at 1.75 K. Investigations of this relaxation mechanism show that the specific heat of the spin system at low temperatures contributes to an anomaly in the relaxation time near the temperature of the magnetic anomaly. Our results indicate that the graphite layers between the magnetic intercalant layers act as nonmagnetic spacing layers allowing the c-axis spin-spin coupling to be varied.

• Received 20 March 1986

DOI:https://doi.org/10.1103/PhysRevB.34.4224