Magnetic properties of stage-2 NicMn1-cCl2-graphite intercalation compounds

Stage-2 NicMn1-cCl2-graphite intercalation compounds (with 0c1) are two-dimensional random-spin systems with competing ferromagnetic and antiferromagnetic exchange interactions. The magnetic properties of these compounds have been studied by dc and ac magnetic susceptibilities. The Curie-Weiss temperature increases monotonically with increasing Ni concentration, suggesting that both Ni and Mn ions are randomly distributed on the triangular-lattice sites of the NicMn1-cCl2-intercalate layers. Its sign changes from negative to positive around c0.22. The exchange interaction between Ni-Mn spin pairs is ferromagnetic and is described by J(Ni-Mn)=1.09[-J(Ni-Ni) J(Mn-Mn)-]1/2=1.44 K. The critical temperature Tc decreases rapidly with a dilution of Mn. In spite of ferromagnetic J(Ni-Mn), the ferromagnetic long-range order of Ni2+ disappears below c0.6. The large initial slope of [dlnTc/dc]c=1=2.38 is ascribed to the two-dimensional Heisenberg-like character of stage-2 NiCl2-graphite intercalation compound. The critical behavior of these compounds shows a two-dimensional XY character. The critical exponent appears to be independent of c for c. 0.7. It is determined that =2.270.02 at c=0.9.