Developing new electronic devices that incorporate the electron's magnetic properties is at the forefront of device engineering and nanotechnology. Spin-based electronics or "spintronics" is currently having a great impact on electronic devices for information technology. A key discovery was made in 1988 in a new synthetic material, where a small magnetic field could produce a large change in resistance, an effect called Giant Magnetoresistance. This tiny hybrid structure allowed for much smaller magnetic data storage devices (hard disk drives). In 1995 another magnetoresistive device was discovered, that was again a synthetic hybrid structure, but this device relied on quantum mechanical tunneling. Such exotic tunneling can be imagined as a particle colliding with a wall and suddenly reappearing on the other side. This spin-based phenomenon is currently used in memory devices installed in all computers and is also being incorporated in magnetic random access memories (MRAM). One of the outstanding issues today in this field is to build new devices that can produce current-carrying electrons that have a predetermined North-South magnetic orientation. This research award is focused on developing synthetic multilayer semiconductor structures for controlling the electron's magnetic orientation by applying a simple input voltage. Furthering the science and engineering of these spin-based devices is expected to advance future applications that are low-power, high-speed, high-density, and eventually cost effective for information processing. The proposed approach is expected to open doors to new materials and devices having useful properties never before contemplated. Broad impact of this research is assured by educating and training young people in the areas of electronic, magnetic and optical nanostructures that are crucial for developing future applications in information technology.
A novel class of materials has recently been predicted that merge the properties of half-metallic magnets and semiconductors. Theoretical band structure calculations show that these inverse Heusler materials have a Fermi energy lying in a gap for one direction of electron spin, but for the other direction of electron spin the valence and conduction band edges meet at the Fermi energy. One of the great advantages of these spin gapless semiconductors (SGS) for devices relies on the property where a simple gate voltage can tune the spin properties. Furthermore, these inverse Heusler materials encompass half-metallic antiferromagnets (HMAF) that are spin-polarized but nonmagnetic. New functionalities of SGS and HMAF materials would take advantage of several novel and valuable properties, even at room temperature. These valuable assets include: half-metallic high spin polarization (~100 %); generation of spin-polarized holes as well as spin-polarized electrons; voltage-tunable spin polarization; and spin-polarized HMAF without fringing magnetic fields. Thus far, several dozen inverse Heusler materials have been predicted to have SGS properties. These include magnetic Mn2CoAl and antiferromagnetic Mn3Al. Up to now, only a few materials have been synthesized: such as bulk Mn2CoAl and Fe2CoSi; and the first major step to synthesize and investigate thin films (epitaxial Mn2CoAl on GaAs). The research focuses on the synthesis of thin film devices incorporating these X2YZ four-sublattice materials using MBE and sputtering. Multilayer devices will be fabricated with voltage gates in order to vary the Fermi energy with respect to the spin-polarized conduction and valence bands. Tunnel junctions will be fabricated for investigating the spin degrees of freedom. Collaborators include researchers from several universities, and national synchrotron and neutron laboratories. This grant is funded jointly by the Electronics, Photonics, and Magnetic Devices (EPMD) Program in the Division of Electrical, Communications and Cyber Systems (ECCS) and by the Electronic and Photonic Materials (EPM) Program in the Division of Materials Research (DMR).
