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 capacit y 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 n on-magnetic impurities, Mg2+(2p(6)), Zn2+(3d(10)) and Cd2+ (4d(10)), at alm ost the same reduction rate as in the cases of the antiferromagnet K2MnF4 w ith 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 con cavely near T-N(x(p)) = 0 at low temperatures, making a contrast to the con vex 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 dime nsionality, the Curie-Weiss temperature and the Curie constant are discusse d on the base of the absolute values of the magnetic heat capacity and susc eptibility. 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 the oretically reproduced by the contribution from isolated ions and clusters c omputer-generated on the square lattice with the same magnetic concentratio n.