Nanoscale zerovalent iron (NZVI), promising for in situ degradation of many environmental contaminants, has been shown to aggregate reducing reactivity and mobility. Aggregation is attributed to the magnetic attractive forces between particles. In this study an alternating gradient force magnetometer (AGFM) is used to measure magnetic properties of NZVI. The magnetization curve is modeled according to the Langevin theory, evidencing a non-superparamagnetic nature of the particles. The measured ratio of remanent and saturation magnetization, 0.16, excludes the possibility of the dispersion being composed of Stoner-Wohlfarth (SW) type structures, probably due to the aggregated state of the particles at the measurement time. An extension of the Stoner-Wohlfarth model is therefore adopted to keep into account particle interaction and domain nucleation and found to be suitable for the system. Extended Derjaguin-Landau-Verwey-Overbeek (DLVO) model is applied to experimental data, under the hypothesis that primary particles behave according to the SW model, proving magnetic attraction to prevail on electrostatic repulsion.