The Attraction of New Materials for Data Storage
by David Pescovitz
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Yuri Suzuki joined the College of Engineering last spring after five years as a professor at Cornell University. Her father, Mahiko, is a professor in the Berkeley Physics Department. (Angela Privin photo)
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According to a UC Berkeley study, the world produced approximately 5 exabytes of new information in 2002. That's roughly the equivalent of half a million new libraries the size of the Library of Congress's print collection. Where does it all go? Mostly on hard disks like the one inside your computer.
Fortunately, industry has demonstrated technology that can pack more than 100 gigabits of data into a square inch of magnetic media. Meanwhile, physicist Yuri Suzuki, a UC Berkeley professor of Materials Science and Engineering, is developing new materials and devices that may help push storage density even higher. Someday, this class of new materials could also lead to high-performance computer memory chips that retain their state even when the power is switched off. The key, Suzuki says, is exploiting the unusual magnetic properties that emerge in some materials.
"We understand the magnetism of bulk materials, like the magnets on your refrigerator, but when you're dealing with thin films, some that are just a few atoms thick, the properties change significantly," says Suzuki, who will present her research at the upcoming Berkeley in Silicon Valley Symposium. "We don't want to throw the baby out with the bathwater when we move to such tiny length scales."
If the unique properties of these thin films are carefully controlled, she explains, novel devices that manipulate the magnetic spin as well as the charge of each electron can be fabricated.
"Electrons aren't just charged," Suzuki says. "They have magnetic spin too. And we're not taking full advantage of that."
The spin of an electron, similar to the direction of a rotating top, can either be "spin-up" or "spin-down," Suzuki explains. That magnetic spin can be rotated by a magnetic field. This change in the "magnetization angle" of the material dramatically affects its overall electrical resistance. The changes in resistance can be detected and interpreted as tiny bits of data stored on a piece of magnetic media like a hard disk.
In the last decade, the development of new magnetic materials has enabled industry to produce increasingly sensitive read heads. Most recently, IBM pioneered a new read head technology by alternating thin layers of various metals to achieve what's known as the "giant magnetoresistive" (GMR) effect. When GMR materials are exposed to even weak magnetic fields, their resistance to electrical current plummets, enabling GMR heads to sense much smaller magnetic signals.
"With more sensitive recording heads, we can read denser media," Suzuki says.
Suzuki's magnetic oxide thin films--grown single crystalline--display an even larger "colossal magnetoresistive" (CMR) effect, albeit at low temperatures and high magnetic fields. If those challenges are overcome, she says, CMR heads could be developed that read media that is ten times denser than GMR heads can handle.
To grow the CMR materials, Suzuki uses pulsed laser deposition. The process involves a high-powered laser that blasts away at a target containing particles of magnetic oxides like lanthanum manganese oxide. The plume of particles then condenses on a substrate near the target to form a thin film.
The thin films' properties could also be the basis of a new form of Magnetoresistive Random Access Memory (MRAM). While traditional RAM stores data in as a charge, MRAM converts the bits into magnetic spins. As a result, MRAM is non-volatile--it can retain its state whether the device is powered on or not.
The building blocks of MRAM are magnetic tunnel junctions, two layers of magnetic thin films separated by an insulating layer. The magnetic state of the two layers can be switched between parallel and anti-parallel, representing a zero or one in the binary language of computers. While a one megabyte MRAM chip would contain millions of magnetic tunnel junctions, Suzuki is currently experimenting with single devices to study the novel magnetic phenomena that occur in the nanoworld.
"As a physicist, I get most excited when we observe a property that's completely unexpected," she says. "But I enjoy engineering because the sorts of problems we're working on have immediate relevance."
Yuri Suzuki's home page
Suzuki Research Group
"New faculty profile: Physicist Suzuki joins MSE" by Angela Privin (Forefront, Fall 2003)
How Much Information? 2003 UC Berkeley study
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