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Ferroelectric tunnel junctions for electronics an spintronics

Manuel Bibes, Unité Mixte de Physique CNRS/Thalès, campus de l’école Polytechnique, 1 avenue Fresnel, Palaiseau
Mercredi 2 octobre 2013 à 11 h 00 - INSP - 4 place Jussieu - 75005 PARIS - Barre 22-23 - 3e étage - Salle 317

Because it is spontaneous, stable and electrically switchable the polarization of ferroelectrics is an excellent state variable for non-volatile data storage. In addition, polarization reversal can be as fast as tens of ps [1] and only dissipates the modest power associated with polarization charge switching (with current densities typically lower than 104 A/cm²). When ferroelectrics are made as thin as a few nm, they can be used as tunnel barriers and the tunneling current is influenced by the polarization direction [2] enabling a simple non-destructive readout of the polarization state. In this talk, I will show how the tunnel resistance can vary by more than two orders of magnitude upon polarization switching in highly-strained ultrathin BaTiO3 tunnel barriers. This strong electroresistance effect can be probed using a conductive AFM tip as the top electrode [3], or using solid-state submicron pads. Such ferroelectric tunnel junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching related to ferroelectric polarisation reversal [4]. They emerge as an alternative to other resistive memories, with the additional advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism. Furthermore, a quasi-continuum of resistance states in accessible between the ON and OFF states, which corresponds to a memristive behavior. Coupled transport and domain imaging studies reveal that the memristive response is here determined by ferroelectric domain dynamics [5]. The low-power, electric-field-driven control of information in ferroelectric tunnel barriers can also be interesting for spintronics if ferromagnetic electrodes – here La2/3Sr1/3MnO3 and Fe (or Co) – are used (thereby defining “artificial” multiferroic tunnel junctions). We have found that such devices not only show tunnel magnetoresistance (TMR) and tunnel electroresistance (as expected from their ferromagnetic electrodes, and their ferroelectric barrier, respectively) but also a tunnel electromagnetoresistance effect, i.e. a dependence of the TMR on the ferroelectric polarization direction [5]. This signals a magnetoelectric coupling at the interface between BaTiO3 and Fe (Co), in line with first-principles calculations [6]. A predicted corollary of this coupling is the induction of a magnetic moment by Fe in BaTiO3. I will present soft X-ray resonant magnetic scattering data revealing that, in addition to being ferroelectric, such ultrathin BaTiO3 films possess a spontaneous and hysteretic magnetization at room temperature, qualifying them as novel interface-induced multiferroics [7].

[1] D.S. Rana et al. Adv. Mater. 21, 2881 (2009) [2] E.Y. Tsymbal and H. Kohlstedt, Science 313 (2006) [3] V. Garcia et al, Nature 460, 81 (2009) [4] A. Chanthbouala et al, Nature Nanotech. 7, 101 (2012) [5] A. Chanthbouala et al, Nature Mater. 11, 860 (2012 [6] V. Garcia et al, Science 327, 1106 (2010) [7] C.G. Duan et al, Phys. Rev. Lett. 97, 047201 (2006) ; M. Fechner et al, Phys. Rev. B 78 212406 (2008) ; L. Bocher et al, Nano Lett. 12, 376 (2012) [8] S. Valencia et al, Nature Mater. 10, 753 (2011)