Static Spin Correlations in a Diluted Two-Dimensional Ferromagnet K2CuxZn(1-x)F4 Including the Percolation Vicinity

The static spin correlations in a diluted two-dimensional Heisenberg ferromagnet K 2 Cu x Zn (1- x ) F 4 have been studied. At low temperatures T <3.5 K, the magnetic heat capacity C m ( T ) per mole for Cu 2+ ions in the pure system ( x =1.0) has been revealed to obey the spin wave theory for a two-dimensional Heisenberg ferromagnet with the nearest neighbour exchange constant J / k B =11.4 ±0.4 K. Near the percolation limit ( x p =0.59), the absolute value of C m ( T ) in the same temperature range has been found to agree with the exact solution for the one-dimensional Heisenberg ferromagnet with the same interaction term as in the pure system. The quantities of C m ( T ) for x =0.36 and x =0.26 are discussed in comparison with the theoretical results for the finite size of open chains and closed rings, by taking account of the possible shape of the magnetic clusters presented by the computer generated random system in two dimensions.