We performed Monte Carlo simulations considering two different models for antiferromagnetic small particles with Ising spins. The spins of the particle are disposed at the sites of the two dimensional arrays with coordination numbers z = 4 and z = 6, around a central spin. The core spins interact antiferromagnetically and the spins at the surface of the particle are disordered. In the first model, we consider an antiferromagnetic core surrounded by a disordered surface of the spin-glass type. In the second model, the core is still antiferromagnetic, but some bonds at the surface are broken. We determined the hysteresis curves, the zero-field-cooling (ZFC) and field-cooling (FC) curves. We have shown that the model with a disordered surface of the spin-glass type fits better the experimental measurements determined for the antiferromagnetic nanoparticles.
