Phase-resolved x-ray ferromagnetic resonance measurements of spin pumping in spin valve structures

Element-specific phase-resolved x-ray ferromagnetic resonance (FMR) was used to study spin pumping within Co${}_{50}$Fe${}_{50}$(3)/Cu(6)/Ni${}_{80}$Fe${}_{20}$(5) (thicknesses in nanometers) spin valve structures with large areas, so that edge effects typical of nanopillars used in standard magnetotransport experiments could be neglected. The phase of precession of the Co${}_{50}$Fe${}_{50}$ fixed layer was recorded as FMR was induced in the Ni${}_{80}$Fe${}_{20}$ free layer. The field dependence of the fixed layer phase contains a clear signature of spin transfer torque (STT) coupling due to spin pumping. Fitting the phase delay yields the spin-mixing conductance, the quantity that controls all spin transfer phenomena. The STT coupling is destroyed by insertion of Ta into the middle of the Cu layer.