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

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in CoNi 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 CoNi layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the CoNi layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed.