All-Magnonic Spin-Transfer Torque and Domain Wall Propagation

The spin-wave transportation through a transverse magnetic domain wall (DW) in a magnetic nanowire is studied. It is found that the spin wave passes through a DW without reflection. A magnon, the quantum of the spin wave, carries opposite spins on the two sides of the DW. As a result, there is a spin angular momentum transfer from the propagating magnons to the DW. This magnonic spin-transfer torque can efficiently drive a DW to propagate in the opposite direction to that of the spin wave.

Figure 1

Illustration of a transverse DW structure whose $\mathbf{m}$ is denoted by the (blue) arrows. The spin wave (magnon) is a small amplitude precession of $\mathbf{m}$ (represented by the red cones) around the static DW. A linearly polarized microwave $\mathbf{h}(t)$ is applied in a small region on the left side of the DW so that the generated spin wave propagates through the DW from the left. $\Delta$ is the DW width. Inset: The magnons (wavy lines with arrows indicating the propagating directions) pass through the DW (represented by the rectangular block) from the left to the right without any reflection. The magnon spin (solid circle with an arrow) is $-\hslash$ on the left side of the DW and $\hslash$ on the right side. The magnonic STT drives the DW propagating to the opposite direction (green arrow) of the spin wave, with the velocity ${\mathbf{V}}_{\mathrm{DW}}$.

Figure 2

(a) Density plot of $m_z$ in the $z\mathrm{\text{−}}t$ plane at frequency $\Omega =0.75$ for $\alpha ={10}^{-5}$ and $K_{\perp }=2\times {10}^{-3}$ (YIG parameters). The center of the DW is initially at $z=0$. The spin wave is generated in the region of $z\in [-60,-55]$. (b) The frequency dependence of DW velocity. The red circles are for YIG parameters, and the blue squares are the results of the case without damping and transverse magnetic anisotropy.

Figure 3

Field dependence of spin-wave amplitude differences at two sites located in the opposite sides of the DW. Red circles are for the YIG parameters at the optimal frequency ($\Omega =0.75$), and blue squares are the results without damping and the transverse magnetic anisotropy also at its optimal frequency ($\Omega =0.85$).

Figure 4

(a) Field dependence of spin-wave amplitude for YIG parameters (red circles) at frequency $\Omega =0.75$ and the case without damping and transverse magnetic anisotropy (blue squares) at frequency $\Omega =0.85$. (b) Field dependence of the DW velocity for the same cases as those in (a). Inset: DW velocity vs the square of the spin-wave amplitude. Symbols are the simulation data, and solid lines are $\frac{{\rho }^2}{2}{V}_g$ without any fitting parameters.