Spin-polarized currents driven by spin-dependent surface screening

We demonstrate the mechanism of spin current generation in ultrathin ferromagnetic film by voltage-induced interface magnetoelectric effect and provide a rigorous theoretical and numerical description of the phenomenon. Surprisingly, for MgO-Cu-Co-MgO systems the spin-dependent screening in thin (less than 20 nm) Co film produces spin accumulation 7 times higher than the accumulation induced by the bulk effect of spin-dependent conductivity in thick Co films. An experimental approach to validate our numerical predictions is proposed. The demonstrated effect opens routes to design highly miniaturized, voltage-controlled spintronic or magnonic devices, while the developed model is a useful tool to study spin currents driven by surface screening.

Figure 1

The schematic of considered composites. In system A there is no D-FM interface, while in system B the single D-FM interface is present. With an ac voltage applied to system B, the spin accumulation appears at the Co/MgO interface as a consequence of the spin-dependent surface screening. The spin accumulation diffuses to the bulk of the metallic multilayer.

Figure 2

Schematic diagrams of $s$ and $d$ bands in Co (a) at electric field $E=0$ and in equilibrium; (b) at $E\ne 0$ with equal band shift $-V_c$; (c) at $E\ne 0$ with screening exchange splitting $V_s$. (d) Dynamic nonequilibrium charges (with respect to equilibrium spin level $s_0$) $\mathrm{\Delta }{n}^{\uparrow }$ and $\mathrm{\Delta }{n}^{\downarrow }$ developing as a consequence of the dynamic exchange splitting. In (e) the equilibrium state with the exchange splitting of the screening charges is shown.

Figure 3

Spatial distribution of (a) spin accumulation $s$ and (b) spin current $J_s$ through the thickness of the Cu-Co bilayer in the system B with separated contributions from spin-dependent conductivity (SC) and spin-dependent surface screening (SS). The vertical dash-dotted line indicates the Cu/Co interface.

Figure 4

(a) Spin accumulation and (b) spin current in the Cu layer in dependence on the thickness of the Co layer for system A and system B. The insets show the same dependences but with the extended scale.