Magnetic droplet solitons generated by pure spin currents

Magnetic droplets are dynamical solitons that can be generated by locally suppressing the dynamical damping in magnetic films with perpendicular anisotropy. To date, droplets have been observed only in nanocontact spin-torque oscillators operated by spin-polarized electrical currents. Here, we experimentally demonstrate that magnetic droplets can be nucleated and sustained by pure spin currents in nanoconstriction-based spin Hall devices. Micromagnetic simulations support our interpretation of the data, and indicate that in addition to the stationary droplets, propagating solitons can be also generated in the studied system, which can be utilized for the information transmission in spintronic applications.

Figure 1

(a) Schematic of the experiment. (b) Calculated distribution of the driving current density in the plane of the device.

Figure 2

(a) and (b) BLS spectra of current-induced magnetization oscillations, measured at the labeled values of the driving current. (c) Peak intensities of the high-frequency (HF) and the low-frequency (LF) modes vs current I. (d) Current dependence of the intensity integrated over the BLS spectrum. The data were obtained at $H_{\left|\right|}=2000\mathrm{Oe}$.

Figure 3

(a) Calculated time dependence of the normalized out-of-plane magnetization component $m_z$, demonstrating the development of the current-induced instability. The current is applied starting at $t=0$. (b) Calculated spatial magnetization map at $t=0.625\mathrm{ns}$, shortly after the droplet is nucleated. Arrows represent the in-plane magnetization component, and the colors represent its out-of-plane component $m_z$. (c) Profile of $m_z$ along the dashed line in panel (b). Simulations were performed at $I=5\mathrm{mA}$ and $H_{\left|\right|}=2000\mathrm{Oe}$.

Figure 4

Temporal evolution of the nucleated droplet soliton, at the labeled instants of time t. The driving current is applied starting at $t=0$. The arrows represent the in-plane magnetization component and the colors represent its out-of-plane component. The simulations were performed at $I=5\mathrm{mA}$ and $H_{\left|\right|}=2000\mathrm{Oe}$.

Figure 5

Fourier spectrum of the time dependence of the spatially averaged out-of-plane magnetization component $m_z$, for the dynamics shown in Fig. 4. Vertical dashed line indicates the central frequency of the spectral peak.

Figure 6

(a) BLS spectra of magnetic oscillations in the droplet mode, measured at $I=4\mathrm{mA}$, at the labeled magnitudes of the magnetic field $H_{\left|\right|}$. (b) Dependence of the central frequency of the spectral peak on $H_{\left|\right|}$, determined from the measurements (open symbols) and from the micromagnetic simulations (solid symbols). Lines are linear fits of the data.

Figure 7

Calculated magnetization maps demonstrating the generation of a propagating droplet soliton, at the labeled instants of time t. The driving current is applied starting at $t=0$. The colors represent the out-of-plane magnetization component. Inset in (a) shows a magnified view of the propagating droplet. The simulations were performed at $I=6\mathrm{mA}$ and $H_{\left|\right|}=1000\mathrm{Oe}$.