Order-disorder transition of a two-dimensional spin 1/2 Heisenberg antiferromagnet with nonmagnetic impurities

Considering the reduction effect of the number of magnetic atoms and using Kondo and Yamaji's method and the coherent potential approximation, we investigate magnetic properties of a spin 1/2 Heisenberg antiferromagnet with nonmagnetic impurities on a square lattice, It is found that the critical impurity concentration x(c) is different from the percolation threshold x(p) in the ground state. The ground state changes from the Neel ordered state to a disordered state at x(c) similar or equal to 0.07. The coherent exchange integral J(p) vanishes at x(p) similar or equal to 0.56. For 0 less than or equal to x < x(c) the system is in the ordered state and for x(c) < x < x(p) in the disordered state, This disordered state is characterized by the finite J(p) and energy gap and the exponentially decaying correlation function. This result explains that the reduction rate of the Neel temperature of Mg-doped La2CuO4 is larger than that of K2CoF4 and K2MnF4. The uniform susceptibility as a function of temperature and concentration is obtained.