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Atelier spintronique

Ferroelectric tunnel junctions : from electronics to spintronics

par Vincent Garcia, Unité mixte THALES - CNRS

Ferroelectrics possess a polarization that is spontaneous, stable and electrically switchable1 , and submicron-thick ferroelectric films are currently used as non-volatile memory elements with destructive capacitive readout2. Ultrathin ferroelectric films would enable non-destructive resistive readout by tunneling3, but room-temperature polarization switching at very low thicknesses is challenging4,5 . At room temperature, we use piezoresponse force microscopy to show robust ferroelectricity down to 1 nm in highly strained BaTiO3 films, and conductive atomic force microscopy to demonstrate the resistive readout of the polarization state via its influence6 on the tunnel current. The resulting electroresistance effect scales exponentially with the ferroelectric film thickness, reaching 75000 % at 3 nm and demonstrating the influence of the polarization direction on the tunnel transmission coefficient. Our approach exploits the otherwise undesirable leakage current to read the polarization state without destroying it. The giant electroresistance persists for lateral sizes smaller than 70 nm, thereby paving the way towards ferroelectric memories with simplified architectures, higher densities and faster operation. The polarization direction is not only expected to influence the tunnel transmission coefficient but also the interfacial density of states, and thus the interfacial spin-polarization when the metals adjacent to the ferroelectric barrier are ferromagnetic. This provides the unique opportunity to design artificial multiferroic tunnel structures in which the spinpolarization may be controlled electrically and in a non-volatile fashion. We have explored this effect and probed the spin-polarization of electrons tunneling from a Fe layer through a ferroelectric barrier to a half-metallic manganite detector. We find a reproducible 50% variation of the spin-polarization of the Fe interface depending on the polarization direction of the ferroelectric. This provides a novel brick for the purely electrical operation of nextgeneration spintronics devices such as spin field effect transistors. More generally, our results may inspire further exploration of the interplay between quantum tunneling and ferroelectricity at the nanoscale.

1 Dawber, M., Rabe, K.M. & Scott, J.F. Physics of thin-film ferroelectric oxides. Rev. Mod. Phys. 77, 1083-1130 (2005)

2 Scott, J.F. & Paz de Araujo, C.A. Ferroelectric memories. Science 246, 1400-1405 (1999)

3 Tsymbal, E.Y. & Kohlstedt, H. Tunneling across a ferroelectric. Science 313, 181-183 (2006)

4 Junquera, J. & Ghosez, P. Critical thickness for ferroelectricity in perovskite ultrathin films, Nature 422, 506- 509 (2003)

5 Despont, L., Koitzsch, C., Clerc, F., Garnier, M.G., Aebi, P., Lichtensteiger, C., Triscone, J.-M., Garcia de Abajo, F.J., Bousquet & E., Ghosez, P. Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films. Phys. Rev. B 73, 094110 (2006)

6 Zhuravlev, M.Y., Sabirianov, R.F., Jaswal, S.S. & Tysmbal, E.Y. Giant electroresistance in ferroelectric tunnel junctions. Phys. Rev. Lett. 94, 246802 (2005)