Abstract
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 Néel ordered state to a disordered state at x c ≃0.07. The coherent exchange integral J p vanishes at x p ≃0.56. For 0≤ 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 Néel temperature of Mg-doped La 2 CuO 4 is larger than that of K 2 CoF 4 and K 2 MnF 4 . The uniform susceptibility as a function of temperature and concentration is obtained.
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