Site dilution study of a square lattice Heisenberg antiferromagnet with S=5/2 covering the percolation threshold

The site dilution problems on a square lattice antiferromagnet with spin S = 5/2, Mn(HCOO)(2)2(NH2)(2)CO (T-N = 3.77 K), have been studied for a wide range of magnetic concentration x, covering the percolation threshold x(p) = 0.59 for the square lattice, by the measurements of magnetic heat capacity and susceptibility The Neel temperature T-N(x) has been found to decrease as d/dx{T-N(x)/T-N(1)} = R = 2.7 just below x = 1 for the three kinds of non-magnetic impurities, Mg2+(2p(6)), Zn2+(3d(10)) and Cd2+ (4d(10)), at almost the same reduction rate as in the cases of the antiferromagnet K2MnF4 with S = 5/2 and the ferromagnet K2CuF4 with S = 1/2. The present value R = 2.7. however, makes a contrast to R = 3.1 for the diluted La2CuO4, an S = 1/2 Heisenberg antiferromagnet. The extrapolation of T-N(x), with R = 2.7(3.1) down to T-N(x) = 0 gives a pseudo-critical concentration x = x(c) = 0.63(0.68) deviating from x(p) = 0.59. This indicates that T-N(x) must draw concavely near T-N(x(p)) = 0 at low temperatures, making a contrast to the convex curve of decreasing T-N(x) down to zero at x = x(p) for Ising systems. As x decreases, dilution effects reflected on the reduction of lattice dimensionality, the Curie-Weiss temperature and the Curie constant are discussed on the base of the absolute values of the magnetic heat capacity and susceptibility. An anomalous enhancement of the heat capacities has been found in the external field of 50 kOe for the system with x = 0.12, which was theoretically reproduced by the contribution from isolated ions and clusters computer-generated on the square lattice with the same magnetic concentration.