Group Leader, NanoElectronics Research Institute, National Institute of Industrial Science and Technology (AIST)
Researcher, Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST)
Coherent spin-polarized electron tunneling in magnetic tunnel junctions with a single-crystal electrode
A new field of electronics called spintronics, in which both the electric charge and the spin of the conduction electrons are utilized, has developed rapidly. A magnetic tunnel junction (MTJ), which consists of two ferromagnetic metal layers (electrodes) separated by a thin insulating layer (tunnel barrier) and exhibits the tunnel magnetoresistance (TMR) effect, is especially important for magnetoresistive random-access memory (MRAM) devices.
The TMR effect originates from spin-polarization of a density of states (DOS) at the Fermi level in the electrodes. However, agreement between the observed TMR effects and theories has not been successful. Additionally, coherent tunneling effects, such as resonant-tunneling, have never been observed in spin-polarized systems. To clarify the physical mechanism of the TMR effect, we developed magnetic tunnel junctions made of well-defined single-crystal electrodes, and have clarified the following points.
- Crystal-orientation-dependence of TMR [1]
We fabricated a MTJ that has a single-crystal Fe electrode in various crystal orientations and observed that TMR significantly depends on the crystal orientation of the electrode. This phenomenon possibly reflects the crystal anisotropy of the spin polarization in the electrode. - A large oscillation of TMR due to spin-polarized resonant tunneling [2, 3]
A non-magnetic Cu(001) ultrathin layer (tCu=0~3nm) was inserted between a ferromagnetic Co(001) electrode and an Al-O tunnel barrier (Fig.1(a)). Spin-polarized quantum well (QW) states were formed in the Cu layer, and spin-polarized resonant tunneling was expected to occur via the QW states (Fig.1(b)). We observed a large TMR oscillation as a function of tCu due to spin-polarized resonant tunneling (Fig.2). This phenomenon is a coherent tunneling effect, which cannot be described simply in terms of the DOS at the electrode/barrier interface. This result is the first observation of coherent tunneling in spin-polarized systems and makes it possible to develop resonant-tunneling-type spintronic devices.
Now we are engaged in the development of all-single-crystal MTJs using MgO(001) as a tunnel barrier in order to create an extremely large TMR effect at room temperature.
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| Fig. 1 Large Image |
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| (a) Cross-sectional transmission electron microscope (TEM) images of a magnetic tunnel junction with a Co(001) /Cu(001) /Al-O /NiFe structure. (b) Schematic diagram of the magnetic tunnel junction, in which electrons with different spin directions tunnel from the upper to the bottom electrodes. Minority-spin electrons are partially confined in the Cu layer and form spin-polarized quantum well states. |
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| Fig. 2 Large Image |
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| Magnetoresistance ratio (MR) at various bias voltages (V) for Co(001)/Cu(001)/Al-O/NiFe junctions as a function of the Cu-layer thickness (tCu). |
Relevant papers
- Yuasa, S., Sato, T., Tamura, E., Suzuki, Y., Yamamori, H., Ando, K. & Katayama, T.
Magnetic tunnel junctions with single-crystal electrodes: A crystal anisotropy of tunnel magneto-resistance
Europhys. Lett. 52, 344-350 (2000). - Yuasa, S., Nagahama, T. & Suzuki, Y.
Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions
Science 297, 234-237 (2002). - Yuasa, S., Nagahama, T., Kawakami, T., Ando, K. & Katayama, T.
A large quantum-well oscillation of the TMR effect
J. Phys. D: Appl. Phys. 35, 2427-2431 (2002).
