ORDER-DISORDER TRANSITION OF A 2-DIMENSIONAL SPIN 1 2 HEISENBERG-ANTIFERROMAGNET WITH NONMAGNETIC IMPURITIES/

Considering the reduction effect of the number of magnetic atoms and u sing Kondo and Yamaji's method and the coherent potential approximatio n, we investigate magnetic properties of a spin 1/2 Heisenberg antifer romagnet with nonmagnetic impurities on a square lattice, It is found that the critical impurity concentration x(c) is different from the pe rcolation 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) si milar or equal to 0.56. For 0 less than or equal to x < x(c) the syste m is in the ordered state and for x(c) < x < x(p) in the disordered st ate, This disordered state is characterized by the finite J(p) and ene rgy gap and the exponentially decaying correlation function. This resu lt explains that the reduction rate of the Neel temperature of Mg-dope d La2CuO4 is larger than that of K2CoF4 and K2MnF4. The uniform suscep tibility as a function of temperature and concentration is obtained.