Direct Current Control of Three Magnon Scattering Processes in Spin-Valve Nanocontacts

We have investigated the generation of spin waves in the free layer of an extended spin-valve structure with a nanoscaled point contact driven by both microwave and direct electric current using Brillouin light scattering microscopy. Simultaneously with the directly excited spin waves, strong nonlinear effects are observed, namely, the generation of eigenmodes with integer multiple frequencies ($2f$, $3f$, $4f$) and modes with noninteger factors ($0.5f$, $1.5f$) with respect to the excitation frequency $f$. The origin of these nonlinear modes is traced back to three-magnon-scattering processes. The direct current influence on the generation of the fundamental mode at frequency $f$ is related to the spin-transfer torque, while the efficiency of three-magnon-scattering processes is controlled by the Oersted field as an additional effect of the direct current.

Figure 1

SEM images of the sample (a) and of the top electrode (b). (c) Schematic representation of the sample with focused light near the point contact in the presence of an external in-plane bias magnetic field $H$.

Figure 2

Spin-wave amplitudes measured with Brillouin light-scattering microscopy as a function of the RF excitation frequency ($x$ axis) and the position in the Brillouin light-scattering spectrum ($y$ axis) for an in-plane magnetic field of (a) $10.297\mathrm{kA}/\mathrm{m}$ (129.4 Oe) and (b) $19.783\mathrm{kA}/\mathrm{m}$ (248 Oe) [12]. (c) Spin-wave dispersion band for the magnetic fields corresponding to panels (a) and (b). The top and bottom branches correspond to wave vectors oriented perpendicular and parallel to the magnetic field. (d) BLS-microscopy image of the two-dimensional distribution of the spin waves radiated from the contact at a pumping frequency of 3.1 GHz.

Figure 3

(a) BLS intensity of the $f$ and $f/2$ modes as a function of microwave excitation power ($x$ axis). The indicated point represents the threshold power for the $f/2$ mode. (b) Relative damping of the directly excited spin waves at 8.9 GHz as a function of the direct current. The straight line represents the best linear fit of the data.

Figure 4

(a) Generation efficiency of the $f/2$ mode ($=4.45\mathrm{GHz}$) as a function of the applied microwave power for different direct currents ($H=22.4\mathrm{kA}/\mathrm{m}$). (b) Extracted threshold power for the $f/2$ mode over the direct current for two values of the applied field. (c) Influence of the scan position on the threshold frequency over the direct current for 20 mW. (d) Spatial distribution of the Oersted field components parallel and perpendicular (inset) to the applied field. The inset figure has a lateral size of $300\mathrm{nm}\times 300\mathrm{nm}$.