An applied field is used to perform Ga + ion irradiation on a CoFe/PtMn bilayer. Effects of the applied field and energy transfer between Ga + ions and antiferromagnetic (AFM) atoms on the exchange bias field 𝐻ex are investigated. A partially reversed 𝐻ex is found in CoFe/PtMn specimens irradiated at a dose of 1 × 10 14 ions/cm 2 with an applied field anti-parallel to the original exchange bias direction. We believe that the rapid energy transfer and local temperature increase originating from the interaction between Ga + ions and AFM atoms result in spin reversal and the formation of reversed AFM domains when specimens are irradiated with anti-parallel fields. The decrease in 𝐻ex when annealing the film in a negative saturation field indicates a thermal decay process. The AFM moments are reversed by thermal activation over an energy barrier distribution, which may change in some way as the temperature increases.