Domain-Wall Dynamics and Spin-Wave Excitations with Spin-Transfer Torques

A generalization of spin-transfer torques in ferromagnetic structures is proposed. For a spatially nonuniform magnetization, the spin torque has a form nearly identical to that in magnetic multilayers. We show that the domain-wall motion driven by the current has many unique features that do not exist in the conventional domain-wall motion driven by a magnetic field. We also demonstrate that the spin torque can generate bulk and surface spin excitations that have been seen in point-contact experiments.

Figure 1

Domain-wall dynamics of the permalloy wire for different spin currents and for $H_{\mathrm{ext}}=0$. (a) The velocity as a function of time. (b) The displacement of domain wall versus time. (c) The out-of-plane component of the magnetization at the center of the wall, $M_z/M_s$, versus time.

Figure 2

Domain-wall dynamics driven by an external field and/or by a spin current in a permalloy wire. (a) The saturated velocity with and without the spin current. (b) The displacement of domain wall as a function of time. The external field $H_{\mathrm{ext}}=8\mathrm{Oe}$ and the spin current are simultaneously applied along the $x$ direction at $t>0$.