The rich phase diagrams of complex oxides include phase boundaries where large changes in properties, such as conductivity and magnetism, can be achieved by switching the material across the boundaries. The combination of the ferroelectric field effect and novel mechanisms of conduction in complex oxides provides a route to a new family of logic and memory devices with attributes such as non-volatility, re-configurability, radiation hardness, and low power consumption. Non-volatile switching is accomplished using a ferroelectric oxide gate (e.g. PbZr0.2Ti0.8O3 (PZT), BaTiO3, etc.) that controls the reversible metal-insulator transition in the channel. Furthermore, in a complex oxide field-effect device, the atomic displacements in the ferroelectric and associated changes in local atomic and magnetic arrangements at the gate-channel interface can be engineered to enhance the device performance (Figure 1).
Figure 1: Ferroelectric modulation of structural and electronic properties at interfaces between complex oxides is a key element in the realization of new non-volatile switching devices.
We realize this type of coupling in PZT-LaNiO3 non-volatile switches, where the ferroelectric polarization is used to modulate the conductivity of the LaNiO3 channel (Figure 2). Polarization-dependent metallic states appear in the ferroelectric interfacial layer, resulting in a 100% conductivity change at room-temperature .
Other functionalities can emerge from the coupling of magnetic and electric order, such as modulating orbital occupancies. Through scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) measurements, collaborators R. F. Klie and coworkers at the University of Illinois-Chicago are able to observe polarization-dependent displacements in the Mn and O planes at the BaTiO3 - La0.8Sr0.2MnO3 interface. Ab initio calculations show that the induced polar states effect a change in the Mn orbital configuration and hence the interface conductance, suggesting other ways to tailor the physical properties of the conducting layer . Due to the magnetoresistive nature of manganites, both the transport and magnetic properties can be modulated electrostatically via the ferroelectric field effect. We study magnetoelectric field effect devices in which the accumulation and depletion of charge in the La0.8Sr0.2MnO3 conducting channel is responsible for a change in the magnetic phase of the interface, switching from ferromagnetic to antiferromagnetic. This approach provides a means for electronic control of spin in complex oxides .
Figure 2: a) Switching conductivity in the nickelates. a) Calculations of the metallic states that appear in the ferroelectric PbO layer at the interface of PbTiO3-LaNiO3 heterostructures. b) Polarization-switched resistivity in a PZT/3-u.c.LaNiO3 device. c) Polarization-dependent carrier mobility determined from Hall measurements for a PZT-4-u.c. LaNiO3 device .
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 H. Chen, Q. Quiao, M. S. J. Marshall, A. B. Georgescu, A. Gulec, P. J. Phillips, R. F. Klie, F. J. Walker, C. H. Ahn, S. Ismail-Beigi “Reversible Modulation of Orbital Occupations via an Interface-Induced Polar State in Metallic Manganites” Nano Letters 14, 4965 (2014)
 C. A. F. Vaz, J. Hoffman, Y. Segal, J. W. Reiner, R. D. Grober, Z. Zhang, C. H. Ahn, F. J. Walker “Origin of the magnetoelectric effect in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures” Physical Review Letters 104, 127202 (2010)