Ferroelectric and multiferroic tunnel junctions
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Magnetic tunnel junctions Electron tunneling is a quantum-mechanical effect, where electrons can traverse the potential barrier that exceeds their kinetic energy. This phenomenon has been known since the advent of quantum mechanics and reflects the wave nature of electrons.1 Electron tunneling can be realized in tunnel junctions that consist of two metal electrodes separated by a very thin insulating (e.g., Al2O3 or MgO) or vacuum barrier. Numerous useful electronic devices are based on this phenomenon. For example, tunneling between two superconductors separated by a thin insulating layer, called a Josephson junction, has found important practical applications in superconducting quantum interference devices (SQUIDs), integrated circuits, and particle detectors.2 Electron tunneling lies at the heart of scanning tunneling microscopy (STM), which has become a conventional tool for studying the arrangement of individual atoms and molecules on surfaces.3 Field emission in the presence of a high
electric field is another kind of electron tunneling, known also as Fowler-Nordheim tunneling,4 which is used as an electron source in flash memory, electron microscopy, and field emission displays. Significant interest in electron tunneling has been triggered by the advent of spin-electronics (or spintronics), a technology aiming to harness the electron spin in data storage and processing, typically by utilizing heterostructures composed of magnetic and non-magnetic materials.5,6 Electron tunneling from a ferromagnetic metal electrode through a thin insulating barrier is spin-dependent. This is due to a disproportion in the number of electrons parallel and antiparallel to the magnetization of a ferromagnet, usually referred to as majority- and minority-spin electrons. This imbalance leads to the measurable difference in the tunneling current carried by majority- and minority-spin electrons.7 The observation of spin-dependent tunneling led to the idea of a magnetic tunnel junction (MTJ)—a device that consists
E.Y. Tsymbal, University of Nebraska; [email protected] A. Gruverman, University of Nebraska; [email protected] V. Garcia, Unité Mixte de Physique CNRS/Thales, France; [email protected] M. Bibes, Unité Mixte de Physique CNRS/Thales, France; [email protected] A. Barthélémy, Unité Mixte de Physique CNRS/Thales, France; [email protected] DOI: 10.1557/mrs.2011.358
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MRS BULLETIN • VOLUME 37 • FEBRUARY 2012 • www.mrs.org/bulletin
© 2012 Materials Research Society
FERROELECTRIC AND MULTIFERROIC TUNNEL JUNCTIONS
of two ferromagnetic metal layers separated by a thin insulating barrier (Figure 1a).8 In the MTJ, the tunneling current depends on the relative orientation of the magnetizations of the two ferromagnetic layers, which can be changed by an applied magnetic field.9,10 This phenomenon is known as tunneling magnetoresistance (TMR).11 The figure of merit is the relative change in resistance of a MTJ between parallel and antiparallel magnetization orientation, known as the TMR ratio. Since
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