The value of exchange field of two FeMn-NiFeCo-based spin valves with varying thickness of the pinned ferromagnetic layer has been determined as a function of temperature. The complexities caused by thermal activation of the antiferromagnet during measurement have been overcome by the development of a measurement protocol. The values of the exchange field obtained provide a measure of the degree of order in the antiferromagnet. Thus it is possible to determine the distribution of energy barriers to reversal for the system. We find that for a 110-Å-thick pinned NiFeCo layer a broad distribution exists, whereas for an 80-Å layer, the distribution is bimodal and has a component subject to thermal activation at temperatures down to 260 K.