The Gilbert damping parameter α plays a central role in magnetization dynamics and is crucial for the performance of spintronic devices. Here, we investigate the composition-dependent damping anisotropy in epitaxial ${\mathrm{Fe}}_{100-x}{\mathrm{Co}}_x$ thin films grown on MgO(001). Angular-dependent broadband ferromagnetic resonance measurements reveal weak damping anisotropy in pure Fe, clear fourfold anisotropy in ${\mathrm{Fe}}_{70}{\mathrm{Co}}_{30}$ and ${\mathrm{Fe}}_{150}{\mathrm{Co}}_{50}$, and the largest angular variation in ${\mathrm{Fe}}_{30}{\mathrm{Co}}_{70}$, where the maximum damping ratio reaches 259%. Anisotropic magnetoresistance measurements show a composition-dependent trend that is broadly consistent with the evolution of damping anisotropy. First-principles calculations further suggest that local tetragonal distortions associated with chemical disorder can modify the spin-orbit-coupled electronic structure and thereby contribute to the anisotropic Gilbert damping. These results show that compositional tuning in epitaxial FeCo alloys provides an effective route to modulate anisotropic magnetic relaxation and offers guidance for the design of spintronic materials with tailored dynamical properties.

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• Anisotropic magnetoresistance
• Magnetic anisotropy
• Magnetism
• Spin dynamics
• Thin films
• Transition metal alloys
• Ferromagnetic resonance
• First-principles calculations