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)₂2(NH₂)₂CO (T_N = 3.77K), 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 Néel 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⁺(2p⁶), Zn²⁺(3d¹⁰) and Cd²⁺(4d¹⁰), 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 TN(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.