Effective spin-mixing conductance ($g_{\text{eff}}^{\uparrow \downarrow }$) is a key importance parameter for developing future spintronic devices. Here, we report on giant enhancement of $g_{\text{eff}}^{\uparrow \downarrow }$ due to the mutual spin pumping effect in Co/Re/Co heterostructures with antiferromagnetic interlayer exchange coupling (IEC) between asymmetric Co layers of different thicknesses through Re spacer layer. Both static and dynamic time-resolved magneto-optical Kerr effect measurements, performed as a function of magnetic field for different thickness of Re ($d_{\mathrm{Re}}$), reveal occurrence of maximum IEC at $d_{\mathrm{Re}} \sim$ 0.4 nm. Two hybridized magnetization precession modes with frequencies significantly dependent on IEC strength and magnetic anisotropy of the Co sublayers up to second order are well explained within the macrospin precession model applied. Strong antiferromagnetic bilinear exchange coupling, with $J_1$ parameter up to $-1.3\pm 0.1 {\mathrm{mJ}/\mathrm{m}}^2$ at $d_{\mathrm{Re}}=0.4$ nm and noncollinear magnetization ordering due to biquadratic coupling with $J_2$ up to $0.18\pm 0.02 {\mathrm{mJ}/\mathrm{m}}^2$, is determined. The dependence of intrinsic Gilbert damping parameter ${\alpha }_0$ on $d_{\mathrm{Re}}$, inferred from the field dependence of effective damping parameter, exhibits a very strong enhancement at maximum IEC for the system with mixed magnetic anisotropy of the Co layers. A perfect correlation between the ${\alpha }_0$ enhancement and the magnitude of IEC in the structure is found. Large effective spin-mixing conductance up to $g_{\text{eff}}^{\uparrow \downarrow }\sim 1.6\times {10}^3 {\mathrm{nm}}^{-2}$, exceeding the values for structures containing uncoupled or single Co layers by more than an order of magnitude, within the spin pumping model is estimated. It is inferred that the strong enhancement of the interfacial spin-mixing conductance is not only due to the high IEC strength, but is also related to the mixed magnetic anisotropy in the antiferromagnetically coupled Co sublayers.

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• Exchange interaction
• Magnetic anisotropy
• Magnetization dynamics
• Spin pumping
• Heterostructures
• Magnetic multilayers
• Magneto-optical Kerr effect
• Ultrafast femtosecond pump probe