Magnetic transistor means changeable chips

By Kimberly Patch, Technology Research News

Transistors allow electrons to flow through them in a controlled way. It is this controlled flow of electrons that provides the on-off signals that are the basis of computing.

Electronic transistors control this flow of electrons by putting a barrier, or junction in front of them, and then adding current to allow them to hop over it.

But the flow of electrons doesn't necessarily have to be controlled by adding current. Physicists from the Universities of Iowa and Missouri are proposing a type of transistor that uses the magnetic spin of electrons to switch the flow of electrons on and off.

Magnetic spin transistors would work in a way similar to today's electronic transistors, with one potentially large advantage: they could be created as easily as writing information onto a magnetic disk. Computer chips made with magnetic spin transistors could therefore be reconfigured.

Ordinary electronic transistors have three electrodes, which carry current. The electrons enter through one of the electrodes, exit through another, and the third controls the electron flow through the other two.

The key to an electronic transistor is its n-p-n junction -- the way the electrons are arranged around the atoms of the p portion of the material provides a barrier that electrons can pass through only when they have enough energy. Putting current through the controlling electrode allows the electrons to flow over the junction.

The plans for a magnetic spin transistor call for using the difference between spin up and spin down electrons to control electron flow. Electrons can be thought of as tiny spinning tops with a magnetic field oriented along the spin axis. The electrons can be in one of two states -- spin up or spin down.

In the spin transistor, the barrier would be a portion of the material whose magnetic domain has the opposite magnetization as the carriers, or electrons with one of the two spin and orientations. "The thing about magnetic semiconductors is they have different energies for carriers that spin up and spin down. We're using that to [change] the barrier, said Michael Flatté, an associate professor of physics at the University of Iowa.

The idea behind the spin transistor is not to use it in place of electric transistors, but to use it where it can provide something more, said Flatté.

Because the barriers in spin transistors are magnetic, they can be changed, enabling, in effect a chip configuration that can be changed as easily as rewriting data on a magnetic disk. "You could have in some sense a universal chip [that] you can configure any way you want," said Flatté.

For instance, a certain region of this type of chip could start out as a wire, with the same magnetization all across the region. Then, by reorienting the magnetization of the middle part of the region to have the opposite magnetization "you create a domain wall and now you have essentially a transistor," he said.

Shuffling around the wires and transistors on a chip could be done in two distinct ways, Flatté said. "A structure similar to a write head for a hard disk could do this... or you could have built-in writing regions, which would be faster but it involves a lot more processing -- you would have to integrate writing wires into the system as well," he said.

Spin transistors could also be used as nonvolatile memory, Flatté said. Magnetic nonvolatile memory could turn out to be cheaper than today's standard nonvolatile memory because the signals could be detected without having to use additional transistors to amplify them, he said.

The idea of a magnetic transistor is not entirely new, but the way the researchers have worked out the fundamental research is good, said Manijeh Razeghi, a professor of electrical and computer engineering and director of the Center for Quantum Devices at Northwestern University.

Going from this proposal to an actual spin transistor is a potentially difficult road, however, she said. "The idea is good but the challenging part is the materials and the fabrication," said Razeghi.

There is potential on the materials side, according to Flatté. A new material that looks like it could meet the spin transistor requirements was made by a team of researchers based at the Tokyo Institute of Technology, who reported their results in the February 2, 2001 issue of Science. The researchers doped titanium dioxide with cobalt to make an n-doped ferromagnetic semiconductor.

There are still unknowns about using this material for spin transistors, he said. One key question is how distinct the opposite spin positions of the electrons are in the material. Another is how thin the junction between the spin up and spin down sections of the transistor can be made. The thinner the barrier, the more easily the transistor can be controlled.

If the titanium dioxide cobalt material turned out to work reasonably well, a magnetic transistor based on this idea could be produced in less than five years, Flatté said.

The researchers are continuing to look for appropriate materials to use in magnetic transistors. They are also working out how far apart the spin directions need to be in order to be reliably detected, he said.

Flatte's research colleague was Giovanni Vignale of the University of Missouri. They published the research in the February 26, 2001 issue of Applied Physics Letters. The research was funded by the National Science Foundation (NSF).

Timeline:   > 5 years
Funding:   Government
TRN Categories:  Semiconductors and Materials
Story Type:   News
Related Elements:  Technical paper, "Unipolar Spin Diodes and Transistors," Applied Physics Letters, February 26, 2001; Technical Paper, "Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide," Science, February 2, 2001.


March 21, 2001

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