Manipulation of ferromagnetic damping via antiferromagnetic Néel order based on spin-pumping theory

Magnetic damping in ferromagnets is a central parameter in spin dynamics and device design. Here, we combine spin-pumping theory with the first-principles scattering-matrix method to investigate how an L1_0-MnPd antiferromagnetic layer modifies the damping of an adjacent Ni_80Fe_20 alloy (Py) layer. In the heterostructure Cu|MnPd|Py|MnPd|Cu, we analyze the angular dependence of the damping enhancement for five representative magnetic configurations. When the pumped spin-current polarization is perpendicular to the Néel vector, the antiferromagnet acts as an efficient spin sink and strongly enhances the damping. When the spin current is polarized parallel to the Néel vector, spin backflow suppresses the net spin-current injection and markedly reduces the damping enhancement. The angular dependence shows a dominant twofold symmetry with a weaker fourfold component. This behavior can be quantitatively explained by the vector structure of the spin-transfer torque. Additional tests involving Cu-spacer insertion, MnPd thickness variation, and several forms of disorder — including spin disorder, lattice disorder, and interface disorder — confirm the robustness of this mechanism. These results identify the Néel order as a new degree of freedom for tuning ferromagnetic damping and provide a theoretical basis for designing antiferromagnet-based spintronic devices with controllable damping.