Ginzburg-Landau theory of spin pumping through an antiferromagnetic layer near the Néel temperature

Spin pumping is a microwave-driven means for injecting spins from a ferromagnet into the adjacent target material. The insertion of a thin antiferromagnetic layer between the ferromagnet and the target material is known to enhance the spin pumping signal. Here, in view of describing dynamic fluctuations of the N\'eel order parameter, we develop Ginzburg-Landau theory of the spin pumping in a ferromagnet/antiferromagnet/heavy metal trilayer in the vicinity of the antiferromagnetic N\'eel temperature ${T}_{\mathrm{N}}$. When there exists an interfacial exchange interaction between the ferromagnetic spins and the antiferromagnetic N\'eel order parameter at the ferromagnet/antiferromagnet interface, we find a strongly frequency-dependent enhancement of the pumped spin current that is peaked at ${T}_{\mathrm{N}}$. The present finding offers an explanation for the enhanced spin pumping with strong frequency dependence observed in a ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}/\mathrm{CoO}/\mathrm{Pt}$ system.