In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately similar to 50% of the exchange bias, while the remaining 50% is due to the order in the bulk. For both samples, the median blocking temperature was found to be greater for the interfacial spins by similar to 25 K. The energy barriers to reversal for both bulk and interfacial spins have been calculated from the magnetic data and we find that the distributions are of similar form but with that for the interfacial spins occurring at higher values. (c) 2008 American Institute of Physics.