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(NH 2 ) 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 N\({\rm \acute{e}}\)el temperature T N ( x ) has been found to decrease as d/ d x { T N ( x )/ T N (1)}= R =2.7 just below x =1 for the three kinds of non-magnetic impurities, Mg 2+ (2p 6 ), Zn 2+ (3d 10 ) and Cd 2+ (4d 10 ), at almost the same reduction rate as in the cases of the antiferromagnet K 2 MnF 4 with S =5/2 and the ferromagnet K 2 CuF 4 with S =1/2. The present value R =2.7, however, makes a contrast to R =3.1 for the diluted La 2 CuO 4 , 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 concavel...