  We investigate the effect of (weak) dipolar interactions on the field
behavior of the temperature at the maximum of the zero-field-cooled
magnetization of a polydisperse assembly of nanoparticles. For this purpose, we
extend the Gittleman-Abeles-Bozowski model for the zero-field-cooled
magnetization by computing the contribution of dipolar interactions to the
longitudinal relaxation time. We show, in good qualitative agreement with many
experimental observations, that the temperature at the maximum of the
zero-field-cooled magnetization as a function of the applied field changes from
a bell-like to a monotonically decreasing curve when the intensity of the
dipolar interactions, or equivalently the sample concentration, increases.
