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Physical Sciences Division
Research Highlights

October 2006

Spin the Electron

The computer Industry is speeding towards a hard, brick wall. Can spintronics stop it?

Contact: Scott Chambers

Results: A new class of materials that transmit data based on an electron's spin could revolutionize the world's computers, thanks to scientists at Pacific Northwest National Laboratory and their collaborators. The team recently demonstrated a key relationship that brings us one step closer to this new class of materials.

Since 2001, certain oxides which are not normally magnetic have been made ferromagnetic by adding a few percent of a magnetic element, such as cobalt. What is new is that this research team has demonstrated for the first time that the magnetism in one such material is directly tied to the presence of additional electrons that convert the material from an insulator to a semiconductor. The material is zinc oxide in which some of the zinc atoms are replaced with cobalt.

Cobalt-doped zinc oxide, or materials like it, could be critical in developing revolutionary computer chips to enable spin electronics, or spintronics. In spintronics, circuits would use the electron spin, rather than the electron charge, to carry signals and process information. To develop such chips, researchers must find semiconductors that remain magnetic at and above room temperature. Magnetic means the spin of the electrons that carry the signal is preferentially oriented in one direction. Other such magnetic semiconductors exist, but these materials lose their beneficial magnetic properties well below room temperature, reducing their usefulness.

Crystal diagram of cobalt-doped zinc oxide
Crystal diagram of cobalt-doped zinc oxide showing the location of the atoms and the movement of spin polarized electrons (yellow). Green balls are cobalt. Blue, zinc. Red, oxygen. [Full Image].

Why it matters: Without spintronics or another revolutionary approach, the computer industry will soon hit a hard, brick wall," said Pacific Northwest National Laboratory Fellow Scott Chambers. In the next decade, the transistors that power computer chips will become so small and tightly packed that they will not function efficiently, and the rapid increases in computing power which we have come to expect will end.

Advances in computer technologies inevitably affect everything from scientific discoveries to consumer electronics. For example, future scientific discoveries depend on ever-faster computers to do calculations that simply can't be done now. If spintronic-based computers could be economically developed, researchers could avoid the brick wall.

Methods: Using the Laboratory's expertise in oxide materials and sophisticated film growth, the research team first prepared crystalline films of cobalt-doped zinc oxide that were insulating, or nonconductive. They then injected zinc metal vapor into these films. This step added extra electrons to the oxide's crystal lattice, giving the material a measure of conductivity. Using sophisticated x-ray methods, the team determined the bonding environment of cobalt atoms in the material. This ruled out certain anomalies that could occur in the doping process.

Once a film was successfully made, the researchers measured the conductivity and magnetism of the material. As the conductivity (the number of free electrons) increased, so did the magnetism. When the electrons were removed, the magnetization decreased. The two properties (conductivity and magnetism) are so tightly correlated that a cause-and-effect relationship seems likely.

What's next: Statistical calculations show that at a 5% cobalt doping level, the material should measure approximately 1.6 Bohr magnetons (a measure of magnetism) for each cobalt atom; however, the material is currently measuring approximately 0.1 Bohr magnetons. The research team wants to know why and how to make the materials more magnetic by adding additional conduction electrons. Doing so will significantly enhance the magnetic signal and shed further light on the mechanism of magnetism.

Collaborators: This work was done in collaboration with the University of Washington and Argonne National Laboratory.

Funding agency: The U.S. Department of Energy Basic Energy Sciences, Material Sciences and Engineering Division funded this research at PNNL and Argonne National Laboratory. The National Science Foundation, Research Corporation, the Dreyfus Foundation, and the Sloan Foundation funded the work at University of Washington.

Citation: Kittilstved KR, DA Schwartz, AC Tuan, SM Heald, SA Chambers, and DR Gamelin. 2006. "Direct kinetic correlation of carriers and ferromagnetism in Co2+:ZnO." Physical Review Letters, 97:037203.

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