Role of magnetic aftereffect in coercivity enhancement of Co/CoO bilayers

The time dependence of the exchange anisotropy due to the magnetic aftereffect was measured in polycrystalline Co/CoO bilayers as a function of temperature. A technique for explicit separation of the magnetic aftereffect in the antiferromagnetic CoO layer from the magnetic aftereffect in the ferromagnetic Co layer was developed. This technique revealed that after the reversal of the ferromagnetic layer magnetization, the unidirectional exchange anisotropy of the Co/CoO bilayers decreased logarithmically with time due to the magnetic aftereffect in the CoO layer. The maximum relaxation rate of the exchange anisotropy was observed at a temperature close to the blocking temperature of the CoO layer. The data demonstrate that the coercivity enhancement of the bilayers observed at the blocking temperature is related to the magnetic aftereffect in the antiferromagnetic CoO layer. The coercivity increase observed at much lower temperatures is consistent with the spin-flop coupling mechanism between ferromagnetic and antiferromagnetic spins.