Gilbert damping in FeCo alloy: From weak to strong spin disorder

We study systematically the Gilbert damping of FeCo alloy from first-principles scattering calculations. For collinear magnetization, the nonmonotonic concentration dependence of the reported Gilbert damping is reproduced. At Co-rich concentration, the Gilbert damping in the faced-centred-cubic phase is found to be a half to a third of that in the body-centred-cubic phase for the same alloy concentration. Spin-wave increases the Gilbert damping in the form consistent with the previously discovered spin pumping in noncollinear magnetization. We also consider the Gilbert damping with strong spin disorder that significantly reduces the total magnetization from its saturation magnetization. In such cases, the dimensionless Gilbert damping parameter tends to diverge near the ferromagnetic-paramagnetic phase transition point while the damping frequency saturates and therefore is more appropriate to describe the damping in magnetization dynamics with strong spin disorder. The spin-mixing conductance that gives rise to the interface damping enhancement is almost unaffected by spin waves or strong spin fluctuation even near the transition point.