Electric Control of Spin Injection into a Ferroelectric Semiconductor

Electric-field control of spin-dependent properties has become one of the most attractive phenomena in modern materials research due to the promise of new device functionalities. One of the paradigms in this approach is to electrically toggle the spin polarization of carriers injected into a semiconductor using ferroelectric polarization as a control parameter. Using first-principles density-functional calculations, we explore the effect of ferroelectric polarization of electron-doped ${\mathrm{BaTiO}}_3$ ($n{\text{−}\mathrm{BaTiO}}_3$) on the spin-polarized transmission across the ${\mathrm{SrRuO}}_3/n{\text{−}\mathrm{BaTiO}}_3(001)$ interface. Our study reveals that, in this system, the interface transmission is negatively spin polarized and that ferroelectric polarization reversal leads to a change in the transport spin polarization from $-65\%$ to $-98\%$. Analytical model calculations demonstrate that this is a general effect for ferromagnetic-metal–ferroelectric-semiconductor systems and, furthermore, that ferroelectric modulation can even reverse the sign of spin polarization. The predicted effect provides a nonvolatile mechanism to electrically control spin injection in semiconductor-based spintronics devices.

Figure 1

Polarization controlled band alignment and spin polarization at the interface between a ferromagnetic metal, e.g., ${\mathrm{SrRuO}}_3$, and electron-doped ferroelectric ($n$-FE), e.g., $n{\text{−}\mathrm{BaTiO}}_3$. Horizontal arrows indicate the ferroelectric polarization direction. Light shaded areas correspond to occupied states, and dark shaded areas correspond to unoccupied states. (a) Schottky and (b) Ohmic contacts are created for polarization pointing away from and into the interface, respectively. Waves depict incident and transmitted Bloch states for spin-up and spin-down electrons.

Figure 2

${\mathbf{k}}_{\mathrm{\parallel }}$-resolved transmission through the Schottky interface for (a) spin-up and (b) spin-down electrons. (c) ${\mathbf{k}}_{\mathrm{\parallel }}$-resolved spin polarization for the Schottky interface. Note that transmission is only plotted in a small region around ${\mathbf{k}}_{\mathrm{\parallel }}=0$; all other points in the 2DBZ have zero transmission. (d)–(f) Same as in (a)–(c) for the Ohmic interface.

Figure 3

Fermi surfaces of ${\mathrm{SrRuO}}_3$ for (a) spin-up and (c) spin-down electrons and (b),(d) their views along the $z$ direction, respectively. Colors are used to aid the eye in delineating different sheets, and different sides of the same sheet, of the Fermi surface. The concentric rings in (b) and (d) approximately demark the minimum and maximum radii of the Fermi surface of $n{\text{−}\mathrm{BaTiO}}_3$.

Figure 4

(a),(c) Spin-up and (b),(d) spin-down ${\mathbf{k}}_{\mathrm{\parallel }}$-resolved local density of states on the interfacial Ti atom for (a),(b) Schottky and (c),(d) Ohmic contacts.

Figure 5

Spin polarization as a function of the Schottky barrier height $U$ for $k_z^{\uparrow }\approx 0.079\AA$, $k_z^{\downarrow }\approx 0.634\AA$, and $\gamma =5.55$. The inset shows schematically the potential profiles for spin-up (solid line) and spin-down (dashed line) electrons.