Observation of droplet soliton drift resonances in a spin-transfer-torque nanocontact to a ferromagnetic thin film

Magnetic droplet solitons are nonlinear dynamical modes that can be excited in a thin film with perpendicular magnetic anisotropy with a spin-transfer torque. Although droplet solitons have been proved to be stable with a hysteretic response to applied currents and magnetic fields at low temperature, measurements at room temperature indicate less stability and reduced hysteresis width. Here, we report evidence of droplet soliton drift instabilities, leading to drift resonances, at room temperature that explains their lower stability. Micromagnetic simulations show that the drift instability is produced by an effective-field asymmetry in the nanocontact region that can have different origins.

Figure 1

(a) Schematic of the STNO. An electrical current flows through a nanocontact (150 nm in diameter) to a thin ferromagnetic layer (free layer, FL) and a spin-polarizing layer (PL). (b) Schematic configuration of a droplet soliton centered beneath the nanocontact (left image) and the same soliton moved sideways (right image). (c) Schematic dc resistance and (d) ac response, showing the spin configuration of PL and FL for a nanocontact as a function of the out-of-plane field. In (c) the dashed line shows the expected resistance as a function of magnetic field for a fully reversed FL magnetization in the nanocontact.

Figure 2

(a) Measured normalized resistance $\overline{R}=R/R_{\mathrm{P}}$ as a function of applied current for fields ranging 0.5–1.1 T. Curves are offset for clarity. (b) Stability map of the droplet soliton: in the hysteretic area, triangles show creation of the droplet, and dots annihilation.

Figure 3

High-frequency spectra, color scale in dB above the baseline noise, as a function of applied current for a field of 710 mT. Inset: signal at a fixed current of $I=30\mathrm{mA}$ (white dashed line). The fitted data (red dashed line) show a narrow peak with a full width at half maximum (FWHM) = 10 MHz, and quality factor $Q\approx 2000$.

Figure 4

Low-frequency spectra, color scale in dB above the baseline noise, as a function of applied current for fields of (a) 650 mT, (b) 889 mT, (c) 975 mT, and (d) 1046 mT.

Figure 5

Time evolution of the droplet soliton in an applied field of 1.1 T perpendicular to the film plane first and with an additional in-plane field $(y$ direction) of 0.15 T for times $t\ge 5$ ns. The upper panels show magnetization maps for $m_z$ and $m_x$ at particular times of the simulation. Images correspond to a $400\times 400\text{−}{\mathrm{nm}}^2$ field view. The nanocontact region is outlined in black. The lower panel shows the time evolution of the perpendicular component of the magnetization $m_z$, averaged in the nanocontact area.