Propagating spin waves excited by spin-transfer torque: A combined electrical and optical study

Nanocontact spin-torque oscillators are devices in which the generation of propagating spin waves can be sustained by spin transfer torque. In the present paper, we perform combined electrical and optical measurements in a single experimental setup to systematically investigate the excitation of spin waves by a nanocontact spin-torque oscillator and their propagation in a $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$ extended layer. By using microfocused Brillouin light scattering we observe an anisotropic emission of spin waves, due to the broken symmetry imposed by the inhomogeneous Oersted field generated by the injected current. In particular, spin waves propagate on the side of the nanocontact where the Oersted field and the in-plane component of the applied magnetic field are antiparallel, while propagation is inhibited on the opposite side. Moreover, propagating spin waves are efficiently excited only in a limited frequency range corresponding to wavevectors inversely proportional to the size of the nanocontact. This frequency range obeys the dispersion relation for exchange-dominated spin waves in the far field, as confirmed by micromagnetic simulations of similar devices. The present results have direct consequences for spin wave based applications, such as synchronization, computation, and magnonics.

Figure 1

(a, b) SW intensity maps measured by $\mu \text{−}\mathrm{BLS}$ in the green area of panel (c) for two different directions of the in-plane component $\left(H_{\mathrm{IP}}\right)$ of the external field, $H=700\mathrm{mT}$ and $I=30\mathrm{mA}$. (c) Scanning electron microscope (SEM) image of the device with the GSG pads, NC position and the direction of the Oersted (Oe) field generated by the current (I) flowing into the NC.

Figure 2

SW intensity (circles) measured by $\mu \text{−}\mathrm{BLS}$ as a function of the distance $(X-X_0)$ from the NC position. Best-fit curve (line) obtained using Eq. (1).

Figure 3

(a) $\mu \text{−}\mathrm{BLS}$ and (b) SA spectra measured as a function of the intensity of the external field $H$ for a fixed value of the injected dc $I=30\mathrm{mA}$. (c, b) Color plots of the sequence of spectra in panels (a) and (b). (e) Results of micromagnetic simulations obtained under the same conditions.

Figure 4

(a) $\mu \text{−}\mathrm{BLS}$ and (b) SA spectra measured as a function of the intensity of the injected dc $\left(I\right)$ for a fixed value of the external field $H=680\mathrm{mT}$. (c, b) Color plots of the sequence of spectra in panels (a) and (b).

Figure 5

(a) $\mu \text{−}\mathrm{BLS}$ spectra measured as a function of the intensity of the injected dc $\left(I\right)$ for a fixed value of the external field $H=680\mathrm{mT}$. (b) Simulated SW dispersion curve of the NiFe free layer.

Figure 6

SPWV transform of simulated SWs along $X$ for (a) $I=34\mathrm{mA}$ and (b) $I=25\mathrm{mA}$ and an external field of $H=800\mathrm{mT}$. The vertical black lines indicate the boundaries of the NC while the white dashed lines indicate wavelength obtained from the SW dispersion relation at each frequency, namely (a) $19.2\mathrm{GHz}$ and (b) $17.1\mathrm{GHz}$. The color contrast indicates the normalized magnitude in logarithmic scale of the SW associated with each wavelength ($y$ axis) as a function of $X$.