Low-nonlinearity spin-torque oscillations driven by ferromagnetic nanocontacts

Spin-torque oscillators are strong candidates as nanoscale microwave generators and detectors. However, because of large amplitude-phase coupling (nonlinearity), phase noise is enhanced over other linear autooscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nanocontacts. Oscillations possessed properties of optical-mode spin waves and at low field (≈200 Oe) had high frequency (15 GHz), a moderate precession amplitude (2-3), and a narrow spectral linewidth (<3 MHz) due to localized excitation at nanocontacts. Micromagnetic simulation showed emission of the resonator's characteristic optical-mode spin waves from disturbances generated by domain-wall oscillations at nanocontacts.