In this paper, a numerical procedure capable to analyze the nonlinear diffusion of the electromagnetic field into a hysteretic magnetic conductor half-space is presented. The result of such calculation allows for the evaluation of the surface impedance of the material and the formulation of nonlinear impedance boundary conditions. The procedure for calculating the surface impedance is based on the implementation of the modified scalar Preisach model (MSPM) in a finite difference implicit time domain (FDITD) algorithm. Simulations have been performed assuming as input source a periodic magnetic field linearly polarized at the plane interface.