Large-amplitude coherent spin waves excited by spin-polarized current in nanoscale spin valves

Abstract

We present spectral measurements of spin-wave excitations driven by direct spin-polarized current in a free layer of nanoscale ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Cu}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ spin valves. The measurements reveal that large-amplitude coherent spin-wave modes are excited over a wide range of bias current. The frequency of these excitations exhibits a series of jumps as a function of current due to transitions between different localized nonlinear spin-wave modes of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanomagnet. We find that micromagnetic simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented by the Slonczewski spin-torque term (LLGS) accurately describe the frequency of the current-driven excitations including the mode transition behavior. However, LLGS simulations give qualitatively incorrect predictions for the amplitude of excited spin waves as a function of current.