Thermal Decay and Reversal of Exchange Bias Field of CoFe/PtMn Bilayer after Ga⁺ Irradiation

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 H _ex are investigated. A partially reversed H_ex is found in CoFe/PtMn specimens irradiated at a dose of 1×10¹⁴ ions/cm² 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 H _ex 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.