Magnetic Tunnel Junctions with Ferroelectric Barriers: Prediction of Four Resistance States from First Principles

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588-0111, USA.
Nano Letters (Impact Factor: 13.59). 01/2009; 9(1):427-32. DOI: 10.1021/nl803318d
Source: PubMed


Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization alignment of the electrodes (tunneling magnetoresistance) and the polarization orientation in the ferroelectric barrier (tunneling electroresistance), these multiferroic tunnel junctions (MFTJs) may serve as four-state resistance devices. On the basis of first-principles calculations, we demonstrate four resistance states in SrRuO(3)/BaTiO(3)/SrRuO(3) MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal the exciting prospects of MFTJs for application as multifunctional spintronic devices.

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    • "Based on simple models it was predicted that the TER effect could be as large as several orders of magnitude due to the change in the tunneling potential barrier [4] [5] [7]. These results were elaborated using first-principles calculations of the transport properties of FTJs, which emphasized the importance of the interface bonding as well as evanescent states in TER [8] [9]. Following these theoretical predictions, three experimental groups have independently measured the TER effect associated with the ferroelectric polarization switching in BaTiO 3 (BTO) [10] [11] and Pb 1−x Zr x TiO 3 [12] ferroelectric films, revealing resistance changes by two–three orders of magnitude. "
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    ABSTRACT: We report the effect of compressive strain on the tunneling electroresistance (TER) effect in BaTiO3/SrRuO3 (BTO/SRO) heterostructures. We find that epitaxial strain imposed by the mismatch of NdGaO3 and SrTiO3 lattice parameters with the BTO and SRO layers improves ferroelectric polarization of BTO and concurrently promotes the metallicity of the SRO films. While the enhanced polarization is beneficial for the TER magnitude, the reduced asymmetry in the tunneling barrier due to the shortened screening length of SRO is detrimental for the effect. Thus, a combined effect of strain on the polarization of the ferroelectric barrier and the screening properties of the electrodes needs to be taken into account when considering and predicting the TER effect in ferroelectric tunnel junctions.
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    • "The coupling of different order parameters such as the ferroelectric polarization and ferromagnetic magnetization in an MFTJ structure makes it one of the most complex and yet most interesting multifunctional systems [27] with novel functionalities in the field of spintronics and nanotechnology. The possibility of manipulating the complexity exhibited by such systems via external stimuli makes them even more fascinating for potential applications [22], [28]. Indeed, the giant piezoelectric resistance (GPR) effect predicted for a ferroelectric tunnel junction (FTJ) by Zheng and Woo [29] may serve as a good example of such possibilities. "
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    • "multiferroics or magnetic ferroelectrics will find numerous applications ranging from widely cited ultra-fast magnetoelectric random access memory chips to sensors, actuators, generators, and transducers especially in the THz frequency domain, the latter being of high interest for telecommunication and security (Takahashi et al. 2006; Bibes and Barthélémy 2008; Baek et al. 2010). Multiferroic components incorporated into hybrid nanostructures and nanodevices in form of tunnel junctions with multiferroic barriers will enable unprecedented flexibility and freedom in manipulation with spin and charge in spintronics (Velev et al. 2009; Gajek et al. 2007; Tsymbal and Kohlstedt 2006; Zhuravlev MYe et al. 2005). "
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