Spin transfer in asymmetric Co-Cu-Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to $7.5\mathrm{T}$ applied perpendicular to the junction surface and free-layers thicknesses from $2\text{to}5\mathrm{nm}$. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.

• Received 29 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.144408