A New Spin
Computer simulations at ORNL have motivated experiments that may give industry a new way to make next-generation electronic devices.
When the ORNL-Tulane paper, "Spin-dependent Tunneling Conductance of Fe[MgO]Fe Sandwiches," was published, the electronics industry was beginning to use spin valves based on the giant magnetoresistance (GMR) effect for read heads in hard disk drives. Most new computers utilize GMR read heads for storing data.
The electronics industry is shifting its interest, however, from GMR devices to ones based on tunneling magnetoresistance (TMR) for read heads and magnetic random access memory (mRAM). In magnetic memory, data are represented by electron spins instead of electron charges. Such data would not have to be refreshed, eliminating the need for slow rebooting of computers and greatly reducing power consumption. An added benefit would be that data stored in magnetic memory in satellites and elsewhere in outer space cannot be damaged by radiation.
Thomas Schulthess and Xiaoguang Zhang, both in the Computational Materials Sciences Group in ORNL's Computer Science and Mathematics Division, coauthored the 2001 paper with lead writer, Bill Butler, a theorist then at ORNL and now at the University of Alabama, and J. M. MacLaren of Tulane University. Their paper, based on first-principles calculations using ORNL's IBM SP2 supercomputer, suggested that research on experimental TMR devices had been directed at the wrong material.
"Researchers were focusing on aluminum oxide as the insulating barrier layer between iron layers in a thin-film sandwich only 50 nanometers thick," says Zhang. "We explained why they should try looking at magnesium oxide as the barrier."
The ORNL-Tulane group predicted that a device based on magnesium oxide (MgO) instead of aluminum oxide (Al2O3) would be 10 times more sensitive to magnetic data—bits of 1 and 0 stored as nanosized magnetic particles crammed together in a tiny space. A read head could then turn an extremely small magnetic signal into an electrical signal that results from changes in resistivity. In this way, stored data in ever-smaller devices can be copied to computers.
The sensitivity to the magnetic data is measured in the TMR ratio, defined as the ratio between the resistivity values with and without a magnetic field. "Our paper states that because magnesium oxide, unlike aluminum oxide, is crystalline, MgO preserves the pattern by which the electron wave is spread out, boosting the TMR ratio by more than an order of magnitude," Zhang says.
The computer simulation performed at ORNL was funded by the Defense Advanced Research Projects Agency and through cooperative research and development agreements between DOE and IBM and later Seagate Technology, Inc.
In 2004 two experimental groups at IBM and the NanoElectronics Research Institute in Japan published papers that agreed with the ORNL-Tulane prediction. The two groups measured TMR in Fe[MgO]Fe samples and found that the TMR ratio was an order of magnitude higher than that obtained in samples using Al2O3 as the barrier layer.
Because of these experiments, which were motivated by the ORNL-Tulane paper and which validated the accuracy of the ORNL computer simulations, companies such as Motorola and Honeywell are excited by the potential of TMR memory devices for next-generation electronic devices ranging from cell phones to satellites.
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