By using theoretical predictions based on first-principle calculations, we explore an interface engineering approach to stabilize polarization states in ferroelectric heterostructures with a thickness of just several nanometers.

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Spin injection is one of the key phenomena exploiting the electron spin degree of freedom in future electronic devices . 1 A critical parameter that determines the efficiency of spin-injection is the degree of spin-polarization carried by the current . Significant interest has been addressed to the spin injection into semiconductors, 2 and recent developments in the field have demonstrated the possibility of efficient spin-injection and spin-detection in various electronic systems . 3,4 All the above results rely however on a "passive" spin-injection where the degree of transport spin polarization is determined by the spin-polarization of the injector and the detector, and the electronic properties of the interface . Adjustable spin-injection with a controllable degree of spin-polarization would be appealing from the scientific point of view and useful for applications . In this work, 5 we demonstrate that ferroelectric (FE) polarization can be used as a control parameter to tune the spin-polarization of injected carries from a ferromagnetic (FM) metal into an electron-doped ferroelectric (n-FE) . Considering a FM/n-FE heterojunction, as shown in Fig . 1, we predict that reversal of FE polarization can significantly alter the magnitude of spin-polarization injected from a FM metal into a FE semiconductor . In addition, we predict that the sign of spin-polarization could be reversed between positive and negative values with a proper doping level of the FE semiconductor . We consider a prototypical all oxide hetero-junction system SrRuO 3 /n-BaTiO 3 (001), where SrRuO 3 is a ferromagnetic metal and n-BaTiO 3 is electron doped FE semiconductor . 6,7 First-principles calculations are performed using the plane-wave pseudopotential code QuantumESPRESSO 8 within the local spin-density approximation . We assume that the electron doping of n-BaTiO 3 is 0 .06 e/formula unit, which is realized by the virtual crystal approximation applied to the oxygen potentials in BaTiO 3 . The spin-dependent interface transmission is calculated using a general scattering formalism implemented in the QuantumESPRESSO . Consistent with our previous work, 9 we find that reversal of FE polarization of n-BaTiO 3 results in a transition between Schottky and Ohmic contact regimes, revealing about three-orders-of-magnitude change in the interface resistance . Fig . 2 shows the transmission for spin-up and spin-down electrons (T É and T Ñ , respectively) calculated over the two-dimensional Brillouin zone (2DBZ) . The transmission is distributed in a ring-shaped area centered around the Γ point (i .e . k || = 0) corresponding to the overlap between the Fermi surface projections of SrRuO 3 and n-BaTiO 3 . For both FE polarization orientations, the spin-down transmission is larger than that of the spin-up transmission, which implies that the spin-polarization of the interface transmission, SP = (T É -T Ñ )/(T É + T Ñ ), is negative . For FE polarization pointing toward the interface (the Ohmic contact), we find that the net spin polarization is -65%, ...

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