Strongly enhanced Gilbert damping anisotropy at low temperature in high quality single-crystalline Fe/MgO (001) thin film

The anisotropic behavior of Gilbert damping provides a new dimension for controlling and tuning magnetization relaxation within the same magnetic device. The single-crystalline ferromagnetic metals with a high symmetry have been predicted to possess significantly anisotropic damping for one decade. By decreasing temperature down to T = 4.5 K, we found that the anisotropic damping factor Q, defined as the fractional difference of damping between easy and hard axis, can be approached to 60%-90% in Fe (15 nm)/MgO (0 0 1) without applied fields dependent behavior. It is much larger than Q = 10%-20% obtained at room temperature. Furthermore, it is followed by explanations on the basis of the breathing Fermi surface model giving around Q = 30%-40% when electron-phonon scattering rate is small enough. This discovery will enrich the understanding of damping mechanism, and can provide clues for searching strongly anisotropic damping within 3d transition metals and alloys.