Spin Correlations on a Site Diluted Square Lattice Heisenberg Antiferromagnet Covering the Percolation Threshold

The reduction of the magnetic ordering temperature has been studied in the quasi-two-dimensional square lattice Heisenberg magnet, Mn(HCOO) 2 2(NH 2) 2 CO(T N (1)=3.77 K) with the spin value S =5/2, randomly diluted with non-magnetic Cd 2+ ions. It is found that, as the dilution proceeds, the Neel temperature T N (x) goes down toward 0 K around at x =0.65, higher than x p =0.59 for the square lattice, making the contrast to the cases of the diluted Ising system and La 2 CuO 4, a high Tc superconducting parent compound with S =1/2: The critical index φ in T N (x)∼(x − x p) 1/φ is suggested φ<1 for the Heisenberg and φ>1 for the Ising systems.