Dynamic remanent hysteresis loops were measured at several time scales for a L10 ordered Fe45Pt55 nanoparticle array sample. At a fixed percentage of magnetization switched, Sharrock’s formula was applied to obtain both the thermal stability factor and the intrinsic switching field. From the magnetization dependence of the thermal stability factor, the width of the thermal energy barrier distribution was determined to be about 0.30. In comparison with the particle volume distribution width obtained from transmission electron microscopy, the energy barrier width is reduced significantly due to strong interparticle exchange interaction. The magnetization dependence of the intrinsic switching field was used to obtain the intrinsic, i.e., short time, remanent magnetization curves. The intrinsic switching field distribution width was found to be 0.34. © 2003 American Institute of Physics.