Giant Anisotropy of Gilbert Damping in Epitaxial CoFe Films

Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics and spin-torque related applications. By investigating broadband spin-torque ferromagnetic resonance of Fe50Co50 and Fe50Co50/Pt, we observe as a function of magnetic field orientation a giant anisotropy of the Gilbert damping in epitaxial FeCo thin film with a maximum-minimum damping ratio up to 400%. The symmetry of damping follows the cubic magnetocrystalline anisotropy. In addition, we identify the origin of this damping anisotropy as the variation of spin-orbit coupling with magnetization orientations, which is reflected in the intensity of the rectification signals in FeCo as opposed to the density of state anisotropy. The damping anisotropy is compared with the angular dependence of the crystalline-induced anisotropic magnetoresistance, which was simultaneously obtained in the spin-torque resonance measurements. Our results provide new insights for the damping mechanism in metallic ferromagnets, which are important for optimizing dynamic properties of future magnetic devices. We will also discuss a few following up works that explores such an interesting anisotropy in CoFe films.