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Topic Name: Researchers race ahead with latest Spintronics achievement

Category: Electronics

Research persons: Ian Appelbaum, BIQIN HUANG, Douwe Monsma

Location: University of Delaware, United States

Details

In a rapid follow-up to their achievement as the first to demonstrate how an electron's spin can be electrically injected, controlled and detected in silicon, electrical engineers from the University of Delaware and Cambridge NanoTech now show that this quantum property can be transported a marathon distance in the world of microelectronics-- through an entire silicon wafer.

The finding confirms that silicon--the workhorse material of present-day electronics--now can be harnessed up for new-age spintronics applications.

The results, published in the Oct. 26 issue of the American Physical Society's prestigious journal Physical Review Letters, mark another major steppingstone in the pioneering field of spintronics, which aims to use the intrinsic “spin” property of electrons versus solely their electrical charge for the cheaper, faster, lower-power processing and storage of data than present-day electronics can offer.

The research team included Ian Appelbaum, UD assistant professor of electrical and computer engineering, and his doctoral student, Biqin Huang, and Douwe Monsma, of Cambridge NanoTech in Cambridge, Mass. Huang was the lead author of the article.

“Our new result is significant because it means that silicon can now be used to perform many spin manipulations both within the space of thousands of devices and within the time of thousands of logic operations, paving the way for silicon-based spintronics circuits,” Appelbaum said.

In Appelbaum's lab at UD, the team fabricated a device that injected high-energy, “hot” electrons from a ferromagnet into the silicon wafer. Another hot-electron structure (made by bonding two silicon wafers together with a thin-film ferromagnet) detected the electrons on the other side.

“Electron spin has a direction, like 'up' or 'down,' ” Appelbaum said. “In silicon, there are normally equal numbers of spin-up and -down electrons. The goal of spintronics is to use currents with most of the electron spins oriented, or polarized, in the same direction.”

In another recent paper published in the Aug. 13 issue of Applied Physics Letters, the team showed how to attain very high spin polarization, achieving more than 37 percent, and then demonstrated operation as the first semiconductor spin field-effect transistor.

“In the future, spintronics may bring a great change to daily life,” Huang added.

A native of China, Huang said he feels fortunate to work in Appelbaum's group. When he completes his doctorate next year, Huang hopes to pursue research in industry or academia.

“An alumnus from my undergraduate school in China was studying here at UD and told me this is a great place. I'm happy I made the right decision to come here,” Huang noted. “I am also lucky to have a chance to work in Dr. Appelbaum's group. I think an excellent adviser is always the reason for students to be here.”

“We're taking the first steps at the beginning of a new road,” Appelbaum said. “Before our initial work on spin transport in silicon, we didn't even know where the road was,” he said with a smile. “There's a lot of fundamental work to be done, which we hope will bring us closer to a new age of electronics.”

In figure, 

Ian Appelbaum (right), UD assistant professor of electrical and computer engineering, and doctoral student Biqin Huang are making pioneering discoveries in spintronics, which seeks to harness an electron's spin in addition to its charge to make cheaper, faster, less power-hungry electronics. The silicon "spin chip" that Huang is holding (closeup at left) contains more than a dozen tiny spin-transport devices.

Note for Spintronics

Spintronics (a neologism for "spin-based electronics"), also known as magnetoelectronics, is an emerging technology which exploits the quantum spin states of electrons as well as making use of their charge state. The electron spin itself is manifested as a two state magnetic energy system.

The discovery of giant magnetoresistance in 1988 by Albert Fert et al. and Peter Grünberg et al. independently is considered as the birth of spintronics.

Theory
Spintronics describes technology with the ability to change or influence the quantum spin state of electrons.

Electrons exhibit the basic properties of spin, charge, and mass. When the intrinsic spin of an electron is measured, it is found in one of two spin states, which we denote as spin up and spin down. Since the Pauli Exclusion Principle dictates that the quantum-mechanical wavefunction of two paired fermions must be antisymmetric, no two electrons can occupy the same quantum state, implying that an entangled pair of electrons cannot have the same spin. There is generally a splitting of the spin-up and spin-down energy levels via the Zeeman effect, so electrons with their spins aligned with an external field are less energetic than electrons with their spins anti-aligned. Electrons absorb or emit photons (quanta of electromagnetic energy) to change valence orbits, and they lose spin coherence by interacting with mutually resonant photon frequencies, causing the electrons to spin flip by energy transfer, through mutual spin-orbit coupling, and through photon emission.

In order to make a spintronic device, the primary requirement is to have a system that can generate a current of spin polarized electrons, and a system that is sensitive to the spin polarization of the electrons. Dmitry Grinevich was one of the original scientists who helped discover the Theory of Polarized Electrons; this theory helped create some of the first spintronics devices ever created (see further reading for details). Most devices also have a unit in between that changes the current of electrons depending on the spin states.

The simplest method of generating a spin-polarised current is to inject the current through a ferromagnetic material. The most common application of this effect is a giant magnetoresistance (GMR) device. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, then an electrical current will flow freely, whereas if the magnetization vectors are antiparallel then the resistance of the system is higher.

Two variants of GMR have been applied in devices, current-in-plane where the electric current flows parallel to the layers and current-perpendicular-to-the-plane where the electric current flows in a direction perpendicular to the layers.

Note for Silicon

Silicon  is the chemical element that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. As the eighth most common element in the universe by mass, silicon occasionally occurs as the pure free element in nature, but is more widely distributed in dusts, planetoids and planets as various forms of silicon dioxide or silicate. On Earth, silicon is the second most abundant element (after oxygen) in the crust, making up 25.7% of the crust by mass.

Silicon has many industrial uses. Elemental silicon is the principal component of most semiconductor devices, most importantly integrated circuits or microchips. Silicon is widely used in semiconductors because it remains a semiconductor at higher temperatures than the semiconductor germanium and because its native oxide is easily grown in a furnace and forms a better semiconductor/dielectric interface than almost all other material combinations.

In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics. It is also a component of silicones, a class-name for various synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often confused with silicon itself.

Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals. It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatoms.

About Researchers:

Ian Appelbaum 
School: University of Delaware 
Location: Newark, DE 
Department: Engineering

Ian Appelbaum obtained his B.S. summa cum laude in Physics and Mathematics at Rensselaer Polytechnic Institute (RPI) in December 1997, and Ph.D. in Physics at the Massachusetts Institute of Technology (MIT) in June 2003. After spending one year as a postdoctoral fellow at Harvard University, he is currently an Assistant Professor of Electrical Engineering at the University of Delaware. 

BIQIN HUANG 
28 Marvin Dr.
Apt B-8 
Newark , DE . 19713 
Cell: 302-397-4823 
Email: bqhuang@udel.edu 

Education: 
 Ph.D. Electrical Engineering, University of Delaware, 2007 (Expected)
 M.S. Electrical Engineering, University of Delaware, 2007, Thesis: "Optical Spin Valve Effects¡±
 M.E. Optical Engineering, Zhejiang University, China, 2004, Outstanding graduate, Thesis: "The omnidirectional reflector and abnormal reflection phenomenon in photonic crystals
 B.S. Optical Engineering, Zhejiang University, China, 2004 , Outstanding graduate