Accidental Discovery Could Enable Development Of Faster Computers

Tag : mgo oxide

The researchers were experimenting with ferromagnet/semiconductor(FM/SC) structures, which are key building blocks for semiconductorspintronic devices (microelectronic devices that perform logicoperations using the spin of electrons). The FM/SC structure issandwich-like in appearance, with the ferromagnet and semiconductorserving as microscopically thin slices between which lies a thinnerstill insulator made of a few atomic layers of magnesium oxide(MgO).
The researchers found that by simply altering the thickness of theMgO interface they were able to control which kinds of electrons,identified by spin, traveled from the semiconductor, through theinterface, to the ferromagnet.
Study results appear in the June 13 issue of Physical ReviewLetters.
Experimental results
The spin of an electron is represented by a vector, pointing up foran Earth-like west-to-east spin; and down for an east-to-west spin.
In the researchers' experiment with the FM/SC structures, both spinup and spin down electrons were allowed to travel from thesemiconductor to the ferromagnet.
The researchers found that when the structure's MgO interface isvery thin (less than two atomic layers), spin down electrons passthrough to the ferromagnet, while spin up electrons are reflectedback, leaving only spin up electrons in the semiconductor.
They also found that when the interface is thicker than six atomiclayers, both spin up and spin down electrons are reflected back,leaving electrons with zero net spin in the semiconductor.
But the surprising result for the researchers was that at anintermediate thickness, ranging from two to six atomic layers, theselectivity of the interface completely changes.
"We see a dramatic and complete reversal in the spin ofelectrons that pass through the interface," said RolandKawakami, an assistant professor of physics who led the researchteam. "This time, spin up electrons pass through while spindown electrons are reflected back to the semiconductor. In otherwords, the thickness of the MgO interface determines whether spinup or spin down electrons are allowed to pass through it."
According to his research team, such a "spin reversal"can be used to control current flow.
Significance of the discovery
"Electron spins are oriented at random in an ordinary electriccircuit, and, therefore, do not affect current flow,"explained Yan Li, the first author of the research paper, who madethe discovery. "But if spin is polarized, that is, aligned inone direction, you can manipulate the flow of current and thetransport of information -- a feature that would be of greatinterest to the semiconductor industry. What is amazing is thatonly a couple of atomic layers of MgO can completely reverse thespin selection of the interface. This is unexpected because MgO isnot a magnetic material."
Li, a graduate student in the Department of Physics and Astronomyworking toward her doctorate in physics, said the research teamwill work next on making electronic devices based on the spinreversal. "This will not only test its feasibility forapplications, but also help determine the cause of the spinreversal, which is still unclear," she said.
Kawakami's lab is one of very few labs in the world that performboth the advanced material synthesis and pulsed laser measurementsneeded for experiments with FM/SC structures.
"Without the strong interplay between the materialsdevelopment and optical measurements, the type of discovery we madeprobably would not have been possible," Kawakami said.
A new area of research, spintronics already has helped developdisk-drive read heads and non-volatile memory chips. Researchersbelieve spintronics also will make "instant-on" computersone day, as well as chips that can store and process data.
Kawakami, who also is a member of UCR's Center for NanoscaleScience and Engineering, and Li were joined by UCR's Y. Chye, Y.F.Chiang, K. Pi and W. H. Wang; and UC Santa Barbara's J.M. Stephens,S. Mack and D.D. Awschalom.
Grants from the Office of Naval Research, the National ScienceFoundation and the Center for Nanoscience Innovation for Defensesupported the two-year study.