Finite size effects on spin-torque driven ferromagnetic resonance in spin valves with a Co∕Ni synthetic free layer

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in Co∕Ni synthetic layers confined in 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 excitation in a linear response regime. The resulting resonance fields were compared with those obtained using conventional rf field driven FMR on extended films with the same Co∕Ni layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the Co∕Ni layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed.