Spin-torque driven ferromagnetic resonance of Co∕Ni synthetic layers in spin valves

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co∕Ni synthetic layers with perpendicular anisotropy confined in spin valve based nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude rf excitation, from 1to20GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co∕Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth, and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements.