Recent concepts for novel spintronic devices have emerged, and theses are generating new enthusiasm for spin transistor research. More specifically, motivation for realizing robust electrical spin injection now arises from recent developments in spin transistor. The former refers the use of spin-polarized currents to achive magnetic actuation, and the latter refers to prospects for electrical control of the polarization of localized nuclear or electric moments. Both phenomena may ultimately play important roles in future spintronic systems and in the longer-term quest for realizing solid-state quantum logic.

Spin-valve Transistor (SVT)
    Consider an F₁-N-F₂ system, simple band models can be used to illustrate device operation [M. Johnson, J. Magn. Magn. Mater. 156, 321(1996)]. In the figure, a bias current Ie is applied to F₁ and N, and D represents a detector characterized by a variable load resistance. The nonequilibrium magnetization M in N generates an electric field at the N-F₂ interface such that the voltage across, or current through. Therefore, D is positive when the magnetizations M₁ and M₂ of F₁ and F₂ are parallel and is negative when M₁ and M₂ are antiparallel.

    The SVT is based on spin-dependent transport of hot electrons rather than electrons near the Fermi energy. The energy of the hot electrons is determined by the Schottky barrier height at the emitter side of the device. The injected electrons can only be collected if they have retained enough energy to surmount the collector Schottky barrier, which is slightly lower than the emitter barrier. Although large magnetocurrent effects (>300%) and high field sensitivities have been demonstrated at room temperature, a small output current (~20 nA) limits the application potential of the SVT. The reason for the small output current is mostly the limited collection efficiency due to the small difference between the two Schottky barrier heights.
    The IBM group [S. Van Dijken, X. Jiang and S. S. P. Parkin, Phys. Rev. B 66, 094417 (2002)] presents the room temperature operation of a magnetic tunneling transistor(MTT). The MTT consists of a ferromagnetic emitter, a tunnel barrier, a single ultrathin FM base layer, and a GaAs(111) collector. The spin-polarized electron current that is injected from the emitter into the base is further spin filtered by spindependent scattering in the FM base layer. As a consequence, the collector current depends critically on the orientation of the magnetic moment of the base with respect to that of the emitter. Output currents larger than 1 mA and magnetocurrent changes of 64% are obtained at a bias voltage of 1.4 V across the tunnel barrier. The output current of this single base layer MTT is 2¡V3 orders of magnitude larger than the output current previously reported for SVT.

    There are some main research subjects for spin valve transistor: (a) How to make the transfer ratio of the device higher? (b) How to improve the output current and characterize the noise behavior? Many researches have been focused on the output current level and noise sources. Further improvements are still required.
    The basic concept of the SVT has also led to the development of a number of related devices, such as the MTT. The variable of the hot-electron energy gives possibilities of spectroscopic studies, while enhanced base transmission is anticipated. Moreover, it has been realized that hot-electron spin filtering may have some attractive features for spin-injection into a semiconductor, in particular the ability to reach a spin polarization near 100% with conventional ferromagnets.
Another interesting research is the study of hot-electron spin-transport in half-metallic ferromagnets oxides. Such materials may also offer new types of spintronic devices combining ferromagnets and semiconductors.

Spin-polarized Field-effect Transistor
    In 1990, Datta and Das proposed a design for a spin-polarized field-effect transistor, or spin FET [S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)]. This idea is at first glance much like a conventional FET. In a conventional FET, a narrow semiconductor channel runs between two electrodes named the source and drain. When voltage is applied to the gate electrode, which is above the channel, the resulting electric field drives electrons out of the channel (for instance), turning the channel into an insulator.

    The spin FET has ferromagnetic source and drain so that the current flowing into the channel is spin-polarized. The current¡Vcarrying medium would be a channel layer formed at the heterojunction between InAlAs and InGaAs. The two-dimensional electron gas (2DEG) in that layer would provide a very high mobility channel, free of spin-flip scattering events. When a voltage is applied to the gate, the current modulation in the proposed structure arises from spin precession due to the strong spin-orbital coupling in narrow-gap semiconductors (InGaAs, for this system).
The modulation of current can be expected by controlling the alignment of a carrier¡¦s spin with respect to the magnetization vector in the collector electrode. The gate electrode on the top of the device can be used for controlling the spin-orbit interaction if the interaction is dependent on the interface electric field. Such that one of the key ideas of the above device is that how the spin-orbital interaction in a narrow gap semiconductor and the electric field causes the precession of the spin carriers.
    There are some main research subjects for realizing a spin FET: (a) How to modify the strength of spin-orbit coupling by the gate voltage? (b) What is an efficient way to inject spin-polarized currents, or spin currents, into a semiconductor? (c) What is the transport behavior of spin polarized electrons in 2DEG? How long can a spin current retain its polarization in a semiconductor? (d) What happens to spin currents at boundaries between different semiconductors? (e) Can we make semiconductors that are ferromagnetic at room temperature? The devices should be able to operate at room temperature.
    The (a) has been verified [Junsaku Nitta et al., Phys. Rev. Lett. 78, 1335(1997)]. The second problem remains unclear, although there are some papers describing the success of spin injection to semiconductors. With respect to the (c), it has been observed that the spin diffusion length in the 2DEG is in the order of £gm, and an electric field could push the spin polarized carriers over distance in the order of 100£gm. [D. Hagelle et al., Appl. Phys. Lett. 73, 1580(1998)20]
    A spin FET would have several advantages over a conventional FET. To flip an electron's spin takes much less energy and can be done much faster than pushing an electron out of the channel. However, no one has succeeded in making a working prototype of the Datta-Das spin FET because of difficulties in efficiently injecting spin currents from a ferromagnetic metal into a semiconductor. Such that, the spin-injection is one of the most important subjects that scientist interest in. Since Fe₃O₄ can be epitaxially grown on GaAs, we will combine Fe₃O₄ and III-V-based DMS to form spin MOSFET.

    Schmidt et al. ( Phys. Rev. B62, R4790 (2000) )revealed that the basic obstacle for spin injection from a FM metal emitter into a semiconductor originates from the conductivity mismatch between these materials. E. I. Rashba ( Phys. Rev. B62, R16267 (2000) ) indicated that the spin injection coefficient controlled by the element of a FM-T-N-junction having the largest effective resistance. We plan to fabricate Fe₃O₄ based TMR junctions and investigate how the interfacial chemical state and the spin polarization affect the spin transport. Finally, we will use Fe₃O₄ and DMS to form spin transister.