Scientists have created the first example of a new type of material known as a half-metallic ferromagnet, and researchers say the development will provide invaluable help to an effort already underway to revolutionize the way computer memory works.
"This type of material may eventually allow us to create non-volatile computer memory systems," says Chia-Ling Chien, professor of physics in the Krieger School of Arts and Sciences at The Johns Hopkins University. "Currently, computers use a technology called dynamic random access memory, or DRAM, and data is lost if the power supply is cut off. The new technology under development is called magnetic random access memory or MRAM, and it will be able to retain data even when power is lost."
Chien and colleagues at Brown University, IBM and the Naval Research Laboratory published a paper on their half-metallic ferromagnet, chromium dioxide (CrO2), in the June 11 issue of "Physical Review Letters."
"Half-metallic materials will also allow scientists and engineers to make superior magnetic sensors. With these sensors, more information can be stored in hard drives of computers," says Gang Xiao, professor of physics at Brown University. Xiao's lab, with help from IBM, developed the single crystal films of the new material. The secret to the potential of MRAM and other related technologies lies in harnessing spin, a characteristic of electrons that conventional electronic circuits do not use.
"Electrons have both a charge and spin, and during the last decade a new generation of technologies known as spintronic or magneto-electronic devices have begun to emerge," says Chien, director of the National Science Foundation Materials Research Science and Engineering Center at Hopkins. "These devices manipulate both the charge of electrons and their spin."
Chien cites giant magneto-resistance read-heads for hard drives as an example. "These simple spintronic devices, invented recently, were so advantageous that they're now found in practically all hard drives in computers," says Chien. Electron spin and a material's magnetic properties are linked.
The spin of each electron behaves like a tiny magnet with north and south poles, according to Chien. Scientists describe an electron's spin, and the orientation of the magnet, as "up" or "down." They use the percentage of electrons in a metal with spin up and the percentage with spin down to determine a property known as "spin polarization." Copper, for example, has zero spin polarization. Common magnets have 40 percent spin polarization.
Theorists posited the possibility of a material with 100 percent spin polarization approximately 15 years ago, Chien says, but the idea wasn't taken very seriously by scientists until the emergence of magneto-electronics in the last decade.
All the electrons in a 100 percent spin-polarized material have the same spin orientation -- either all up or all down. By definition, that means one of the two possibilities for electron spin, which scientists call a spin band, is absent, leaving only one spin band. In a normal metallic compound, by contrast, both spin bands are present. That's why scientists call a 100 percent spin-polarized material "half-metallic."
Xiao and colleagues at Brown and IBM used a technique called chemical vapor deposition to grow single-crystal films of chromium dioxide. Using a superconductor, scientists at JHU measured the spin polarization of the chromium dioxide films and found that they were at least 96 percent spin polarized.
"We have spent more than two years to finally develop a method to make this material with high yield and perfection. Chromium dioxide is a gift of nature to us; simple, elegant and rich in physics and exciting properties," Xiao says. "The superior quality of the materials that Brown and IBM produced is essential for the high spin polarization," Chien says.
Researchers are already working to incorporate the chromium dioxide films into a structure at the heart of MRAM technology known as a magnetic tunnel junction. The structure consists of an insulator sandwiched between two electrodes.
By controlling the orientation of the magnetization of each electrode, scientists can make the junction switch between high resistance to electricity and low resistance. Using the new half-metallic ferromagnet for the electrodes should make it possible to increase the high-resistance configuration to an insulating configuration, an accomplishment that will make MRAM technology much more feasible.
Scientists hope one day to allow computers to use magnetic tunnel junctions in the same way that they use tiny capacitors in current memory systems. "The difference, though, would be that the capacitors in current memory systems leak and have to be electronically refreshed periodically to prevent data loss," says Chien. "Because MRAM relies on magnetic orientations, loss of power would not mean loss of the data it stores."
Other authors on the paper are Yi Ji, Fengyuan Yang, and Gustav Strijkers, of Hopkins; Jeffrey Byers of the Naval Research Laboratory; Alexander Anguelouch of Brown; and Arunava Gupta of IBM.
The research was funded in part by the National Science Foundation through the http://www.pha.jhu.edu/groups/mrsec/main.html. Materials Research Science and Engineering Center at Johns Hopkins.
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