Gilbert Damping Modulated by the Exchange Spring Effect in IrMn/CoFe Bilayers

The spin dynamic process of the IrMn/CoFe system has been investigated by the time-resolved magneto-optical Kerr effect. It is found that both spin pumping and the interfacial exchange spring effect between IrMn and CoFe layers contribute to the total effective Gilbert damping. With the increase in IrMn layer thickness (t_IrMn), the saturated damping factor initially increases and subsequently starts to decline beyond 2.4 nm. The results demonstrate that the damping factor drops due to the exchange spring effect in the IrMn/CoFe system, where the IrMn layer features additionally as a spin source when the antiferromagnetic order of the IrMn layer is built at t_IrMn> 2.4 nm. With an inserted Cu layer in between, the damping factor behaves differently by remaining nearly constant in a small range with increasing t_IrMnafter its initial increase. This results from the exchange-decoupling of IrMn and CoFe layers in the presence of a 3 nm thick Cu interlayer and the disappearance of the exchange spring effect.