MS-12-O-2474 Electron doping and structure of interfaces and defects in multiferroic heterostructures with ferroelectric barrier

The ferroelectric (FE) control of ground states in multiferroic nanostructures based on strongly correlated oxides is an important task in solid state physics and spintronics. Recently a ferroelectrically induced modulation of resistivity [1] and carrier densities [2] has been shown for nanostructures between the multiferroic BiFeO₃ (BFO) and the Mott insulator CaMnO₃ (CMO). The strong doping sensitivity of CMO makes it susceptible to electronic state transitions induced interfacially by the large polarization of BFO. Understanding the specific properties of this and other similar oxide-based heterostructures, requires an accurate estimation of small charge modulations, strain, presence of impurities, cationic inter-diffusion, interface structure and defects, ferroelectricity, appearance of dead layers.
In this study, we address at the atomic level the electronic and structural properties of Ca_1-xCe_xMnO₃ (CCMO)/BFO (x=0, 2, 4 at% Ce) nanostructures by STEM-EELS. A HAADF image and a Ce concentration profile are given in Fig. 1(a) and (c). Upon doping with tetravalent Ce⁴⁺ partial mixed valence appears in the manganite. The changes in the oxidation state are revealed by a fine chemical shift on the Mn-L_3,2 edges, Fig. 2(a). Analyzing the atomically-resolved Mn-L₃ edges, we therefore propose a method to evaluate small changes in the electron density at the scale of a single unit cell for the different Ce doping levels, Fig. 2(b). This permits to estimate charge densities at the interfaces between the BFO and the CCMO layers and between the substrate and the CCMO layers, Fig. 1(b). Using theoretical calculations, the estimated two-dimensional electron gas at the BFO/CCMO interface is interpreted in terms of electrostatic doping and polar discontinuities. The as-grown interfacial termination planes are important for the stabilization of the direction of the FE polarization and for the accumulation of carriers. In addition, the STEM-EELS data reveal the presence of a dead layer at the substrate/CCMO interface related to different structural defects [3].
Further such effects as interface structure, polar discontinuity and appearance of defects will be discussed in the context of other important multiferroic heterostructures such as LaNiO₃/BiFeO₃ and (La,Sr)MnO₃/BaTiO₃/FeRh.
References
[1] H.Yamada, V.Garcia, S.Fusil, S.Boyn, M.Marinova, A.Gloter, S.Xavier, J.Grollier, E.Jacquet, C.Carrétéro, C.Deranlot, M.Bibes, A.Barthélémy, ACS Nano 7, 5385 (2013).
[2] H.Yamada, M.Marinova, P.Altuntas, A.Crassous, L.Bégon-Lours, S.Fusil, E.Jacquet, V.Garcia, K.Bouzehouane, A.Gloter, J.E.Villegas, A.Barthélémy, M.Bibes, Sci. Rep. 3, 2834 (2013).
[3] M.Marinova, A.Gloter, C.Carrétéro, H.Yamada, V.Garcia, K.March, O.Stéphan, A.Barthélémy, M.Bibes and C.Colliex, submitted.