Magnetic Phase Transition for Dilute Heisenberg Spin System in Two-Dimensional Lattice

The Magnetic co-operative phenomena in two-dimensional Heisenberg spin system diluted with non-magnetic ions, Mn x Mg (1- x ) (HCOO) 2 ·2H 2 O, were studied by measurements of the specific heat and magnetic susceptibility. The Néel temperature T N ( x ), associated with the antiferromagnetic long range ordering, ran down linearly to the lower temperature side for 0.76< x <1.00. The critical concentration was estimated to be about 0.60 from the disappearance of the sharp peak on the specific heat curve which indicates the phase transition. By diluting the system with non-magnetic ions Mg 2+ , we could observe in the specific heat curve the long range ordering peak and the broad maximum characteristic of short range ordering at separated temperatures. The appearance of the sharp peak and the broad maximum in the cource of dilution can be explained by that in the Ising spin system in two dimensions. Form these results, the behaviour similar to the Ising spin system and the two-dimensionality of this Mn 2+ ion salt are discussed in comparison with other experiments and theoretical results.