This invention relates to spintronic devices. The invention relates particularly to methods and apparatus for injecting strongly spin-polarized electron currents into semiconductors and to electronic devices which operate according to such methods.
A spintronic device is a device in which the behaviour of charge carriers, most typically electrons, is spin-dependent. There are many applications for spintronic devices. Some examples of spintronic devices are the family of devices which exploit giant magnetoresistance (GMR). Spintronic devices may be used, for example, in magnetic field sensors, magnetic memories, spin-based transistors, semiconductor quantum interference devices based on electron spin, classical and quantum computers, heads for reading data from magnetic storage media or the like.
An impediment to the availability of commercially useful spintronic device is that it has so far not been practical to generate strongly spin-polarized currents in semiconductor materials. Spin-polarized currents of electrons can be generated in ferromagnetic materials because the magnetic field of the material (augmented by many-body effects) interacts with the spins of electrons. Thus the spin-up and spin-down states of electrons in a ferromagnetic material have different energy levels. The majority of electrons are in states such that their spins are aligned with the local magnetic field of the material. In such materials electrons occupy the majority and minority spin states asymmetrically.
It is possible, in principle, to take advantage of this asymmetry to create a spin-polarized current in a non-ferromagnetic material by creating a spin-polarized electron current in a ferromagnetic material and causing the current to flow from the ferromagnetic material into an adjacent non-ferromagnetic material by way of an interface. Spin-polarized electron transport has been achieved in this way from ferromagnetic metals into to superconductors (see R. Meservey et al. Phys. Rev. Lett. 25, 1270 (1970)) from ferromagnetic metals into normal metals (see M. Johnson et al. Phys. Rev. Lett. 55, 1790 (1985)), between two ferromagnetic metals separated by a thin insulating film (See M. Julliere Phys. Lett. 54A, 225 (1975))and from magnetic semiconductors into non-magnetic semiconductors (see R. Fiederling et al. Nature 402, 787 (1999) and Y. Ohno et al. Nature 402, 790 (1999)). Although it has long been recognized as an important goal of solid state physics and technology to inject strongly spin-polarized electron currents from ferromagnetic metals into semiconductors this goal has not been achieved.
The best that has been reported to date is the injection of weakly spin-polarized electron currents from ferromagnetic metals into semiconductors (see W. Y. Lee et al. J. Appl. Phys. 85,6682 (1999) and P. R. Hammar et al. Phys. Rev. Lett. 83, 203, (1999). It is not clear that the experimental data presented in these reports establishes conclusively that even weakly spin-polarized currents were successfully injected into the semiconductor (see, for example, F. G. Monzon et al. Phys Rev. Lett. 84, 5022 (2000), B. J. van Wees Phys. Rev. Lett 84, 5023 (2000), and P. R. Hammar et al. Phys. Rev. Lett 84, 5024 (2000)).
It has been suggested that, for devices in the diffusive transport regime, the best that can be achieved, even in principle, is to inject a weak ( less than 0.1%) spin-polarization of electrons from a ferromagnetic metal into a semiconductor (unless the ferromagnetic contact is almost 100% spin polarized, which is not the case for standard ferromagnetic metals such as Fe, Co, Ni and permalloy that are commonly used in devices) (See, for example, G. Schmidt et al. Phys. Rev. B. Vol. 62, pp. R4790-R4793 (2000). The basis for this suggestion is that, in an electric circuit comprising a diffusive semiconductor in series with a metal, the net resistance of the circuit should be dominated by the spin-independent resistance of the semiconductor. Therefore, the spin-up and spin-down currents flowing through the semiconductor should be almost equal.
M. Johnson has suggested that it might be possible to inject spin-polarized electrons from ferromagnetic metals into semiconductors in a manner based on the Rashba effect in quasi-two-dimensional electron gases. It is not clear that this can be practically achieved.
There is a need for methods and systems for injecting strongly spin polarized currents into semiconductors for use in various spintronic devices.
This invention provides methods and apparatus which create a spin filter at an interface between a semiconductor and a ferromagnetic material. The spin filter can be used to provide a current of spin-polarized charge carriers in the semiconductor.
One aspect of the invention provides a spintronic device which comprises a crystalline first semiconductor; and a crystalline ferromagnetic material in atomic registration with the first semiconductor at an interface. The semiconductor and ferromagnetic material are chosen so that transmission of charge carriers in a first spin state from the ferromagnetic material into the first semiconductor is quantum mechanically forbidden while the transmission of charge carriers in a second spin state from the ferromagnetic material into the first semiconductor is quantum mechanically permitted.
Where the first semiconductor is an n-type semiconductor, the charge carriers are electrons in the semiconductor and a projection of a Fermi surface of either majority or minority spin electrons in the ferromagnetic material is not connected to a projection of any wave vector of a lowest conduction band minimum of the first semiconductor by the projection of any sum of reciprocal lattice vectors of the first semiconductor, ferromagnetic material and interface. xe2x80x9cProjectionxe2x80x9d means xe2x80x9cprojection onto the plane of the interfacexe2x80x9d.
Where the first semiconductor is a p-type semiconductor, the charge carriers are holes in the semiconductor and a projection of a Fermi surface of either majority or minority spin electrons in the ferromagnetic material is not connected to a projection of any wave vector of a valence band maximum of the first semiconductor by the projection of any sum of reciprocal lattice vectors of the first semiconductor, ferromagnetic material and interface.
Another aspect of the invention provides a method for injecting a spin-polarized current into a semiconductor. The method comprises providing a semiconductor crystal in atomic registry with a crystal of a ferromagnetic material at an interface wherein transmission of charge carriers in a first spin state from the ferromagnetic material into the first semiconductor is quantum mechanically forbidden while the transmission of charge carriers in a second spin state from the ferromagnetic material into the first semiconductor is quantum mechanically permitted; and, applying a bias voltage across the interface.
Further features and advantages of the invention and specific embodiments of the invention are described below.