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All-Electric Spintronics Research at UC

Cross-college collaboration puts a new spin on electronics.

Date: 11/30/2009
By: Wendy Beckman
Phone: (513) 556-1826
Photos By: Lisa Ventre, Dottie Stover and Ashley Kempher, photojournalists
What Is Spintronics?
Conventional electronics is based on moving around the electron’s charge and manipulate it using electric fields. The electrical charge of electrons is used to process and store digital information. However, as the device size shrinks and miniaturizes to the nanoscale, the devices operate at higher speed, driving an exponential growth in the amount of power needed to move electrons around. This results in a serious heat dissipation problem.

Scientists have realized that the answer may lie in the spin of the electron. Rather than moving charges around, spin-based devices that would operate by flipping the electron’s spin orientation would use less energy, generate less heat and, would be faster than conventional charge-based devices.

This has resulted in a new field of research — spin electronics or spintronics — that offers a new paradigm for the development of novel devices for use in the post-CMOS era. In spin-based digital information processing, the two orientations of the electron spin — “up” or “down” — can be used, instead of charge accumulation, to represent or encode the bits 1s and 0s, used by all computers. Spin-based computing will be much faster.

The Spintronics Research Team
The multidisciplinary spintronics research team at the University of Cincinnati is at the forefront of research in semiconductor spintronics. The team brings together researchers from the Department of Physics (McMicken College of Arts & Sciences) and the Department of Electrical and Computer Engineering (College of Engineering). It also collaborates with other universities and research institutes.
Dean Richard Newrock.
Richard Newrock (seen here) and Philippe Debray lead the physics portion of the team.

The physics group is led by Philippe Debray and Richard Newrock, while Marc Cahay heads the effort from electrical engineering. In addition to the faculty, the team includes graduate student Saydur Rahman and postdoctoral fellow Krishna Chetry from physics and graduate students Junjun Wan and Partha Pratim Das from electrical engineering.

Junjun Wan.
Junjun Wan is a graduate student in electrical engineering.

The team also collaborates with Steven Herbert of Xavier University (a close collaborator, who assists in sample processing and characterization, and also provides access to the facilities for them available in his laboratory) and Sergio Ulloa of Ohio University, who often offers his theoretical insight and support.

“Frequent consultation with him has been a source of encouragement for the spintronics team,” says Philippe Debray. Mark Johnson, a well-known spintronics researcher at the Naval Research Laboratory, has provided semiconductor materials and helped with useful suggestions.

The experimental work of all-electric spintronics is carried out in the laboratory for research on charge and spin transport in one-dimensional electron systems located in the Department of Physics. Newrock heads this laboratory.
Saydur Rahman.
Saydur Rahman is a graduate student in physics.

“Before I came to UC in 2004, the laboratory was focused on experimental investigations of statistical mechanics, using large arrays of classical Josephson junctions to study low-dimensional phase transitions,” says Debray. “When Professor Newrock, a pioneer in this research area, became Dean of the College of Applied Science, he transferred the responsibility for the operations of his laboratory to me, but maintained his interest in the work being done.”

Debray, who had published many papers on transport in nanoscale devices (such as quantum dots and quantum point contacts) in international journals, re-oriented the laboratory’s research efforts to spin and charge transport in nanoscale one-dimensional semiconductor devices. Debray, Newrock and coworkers’ experimental work on Coulomb drag between parallel quantum wires to obtain experimental evidence of the Luttinger liquid state remains unmatched by any other research laboratory. The work on Coulomb drag was supported by a National Science Foundation grant.

Philippe Debray, Marc Cahay, Partha Pratim Das and Krishna Chetry.
Philippe Debray, Marc Cahay, grad student Partha Pratim Das and post-doc Krishna Chetry.

In early 2006, Philippe Debray met Marc Cahay of the Electrical Engineering Department in the corridors of the Physics Department, which he calls “an unplanned, fortuitous meeting” that led to the creation of this multidisciplinary research team. Cahay is a well-known spintronics theorist specializing in theoretical modeling of spin devices using non-equilibrium Greens function techniques. He collaborated with S. Bandyopadhyay of Virginia Commonwealth University to measure extremely long spin relaxation time in organic semiconductor nanowires. Cahay and his graduate student, Junjun Wan, together constitute the theoretical pillar of the spintronics research team.

About two decades ago, in a pioneering work, Supriyo Datta and Biswajit Das of Purdue University proposed a one-dimensional spin field-effect transistor or FET, based on electron spin polarization created by ferromagnetic contacts. The UC team decided, in 2007, to implement a Datta-Das spin FET without using ferromagnetic contacts or external magnetic field. beginning the search for all-electric spintronics or spintronics without ferromagnetism. A proposal for developing an All-Electric Datta-Das Spin FET was submitted to NSF in early 2007. It was acclaimed by the review panel as creative, interesting, and — if successful — expected to be ground-breaking. The proposal was funded.

Spintronics Breakthrough
Since an electron’s spin is associated with a magnetic moment, attempts so far used ferromagnetic contacts or electrodes to filter electrons by spin state, “up” or “down” and an external magnetic field to flip the spin. However, the use of embedded ferromagnetic materials and an external magnetic field to flip the spin make the devices bulky and add to design complexities.

Philippe Debray and Maureen Murage, WISE student.
Philippe Debray works here with undergraduate mechanical engineering student Maureen Murage as part of the WISE program.

In early 2008, the UC spintronics team used a novel method to create for the first time completely spin-polarized current by purely electrical means. The team used a quantum point contact — a short quantum wire — fabricated in indium arsenide to generate strongly spin-polarized current by tuning the electrons’ confinement in the wire by bias voltages of the gates that create it. The spin of an electron entering the quantum point contact could be set up or down by tuning the voltage potential of the gates.The holy grail of spintronics is to take any electron as an input and change its spin state to up or down under purely electrical control.

The creation of spin polarization and control of spin orientation by purely electrical means is a major milestone in the brief history of spintronics and paves the way for a breakthrough in spintronics leading to faster computing.

Kalyan Garre, PhD student, and Marc Cahay.
PhD student Kalyan Garre with Marc Cahay.

Saydur Rahman contributed significantly to the experimental work and Junjun Wan did the same for theoretical analysis. Rahman received his PhD in 2008 and moved to Queen’s University in Canada, where he is doing research in nanomechanics and nanoelectronics.

Though a major advancement, the work mentioned above was done on indium arsenide with relatively short spin coherence length and at very low temperatures. For practical devices, a relatively large spin coherence length and operation at higher temperatures are required. The team is now planning to use quantum point contacts made from gallium arsenide, a mainstream semiconductor with large spin coherence length, to achieve the same results as with indium arsenide quantum point contacts.