Microscopic mix strengthens magnet

By Eric Smalley, Technology Research News

Work on the nanoscale doesn't have to produce microscopic devices in order to come up with big results.

Researchers from IBM and the Georgia Institute of Technology have caused 4-nanometer metal particles to combine in a way that increases the strength of magnets used in tiny, though not necessarily microscopic, electronic devices. A nanometer is one millionth of a millimeter, or the length of 40 hydrogen atoms.

Permanent magnets are a key element of electric motors, generators, loudspeakers, magnetic separators and magnetic levitation systems, and electric motors are particularly important in small-scale applications. Using stronger permanent magnets could reduce the size of these motors considerably, said Shouheng Sun, a materials scientist at IBM Research.

The researchers' approach melds nanoparticles made from a pair of iron-platinum compounds into a composite magnet that is potentially 50 percent stronger than non-composite iron-platinum compounds, said Sun. The increase in the available energy of the material means the magnet can carry out more work than a similar-size non-composite magnet.

The key to the composite's strength is that the two compounds have different magnetic properties: one is a hard-phase magnet and the other is a soft-phase magnet. Hard-phase magnets hold their own, but aren't very strong: they resist being demagnetized or reoriented by an external magnetic field but tend to have weak magnetic fields. Soft-phase magnets are just the opposite. They tend to have strong magnetic fields but those fields can be easily demagnetized or reoriented by relatively small external magnetic fields.

Coupling hard- and soft-phase materials so their magnetic orientations line up produces an exchange-spring magnet, said Sun. This combination of magnetic resilience and strength harbors more useful available energy, he said.

The researchers made the material by mixing iron-platinum nanoparticles and iron oxide nanoparticles in liquid, causing the nanoparticles to combine by evaporating the liquid, then heating the nanoparticles to 650 degrees Celsius. The heat converted the iron oxide to iron, which mixed with some of the platinum to produce a pair of iron-platinum compounds with different amounts of iron, and different magnetic properties. One material was hard-phase and the other soft-phase. The heat also sintered, or partially blended, the two compounds into an exchange-spring magnet.

This self-assembly process, which causes microscopic bits of material to automatically line up particle by particle, is key when the particles involved are so small. It's hard to make nanoscale composite materials using conventional approaches, said Sun. "With self-assembly, we can start with nano-sized components and engineer them into useful nanostructures," he said.

Exchange-spring magnets are an important new class of permanent magnets, said Caroline Ross, an associate professor of materials science and engineering at the Massachusetts Institute of Technology. This type of magnet has a very high energy product, which makes it ideal for applications like small motors where you need a strong magnet with a minimum volume, she said.

The researchers have shown that nanoparticle synthesis can produce very good exchange-spring magnets, "at least on the small-scale available in the experiment," said Ross. "The key to making these commercially useful is to be able to produce the material in large enough quantities," she said.

The researchers' next step is to control the material's easy-axis orientation, said Sun. A material's easy axis is the direction along which its magnetic field prefers to line up. Controlling the orientation of the easy axis would increase the energy product and thus the strength of the magnet several times, he said.

The nanocomposite self-assembly method could be used to make practical devices within five years, said Sun.

The researchers also plan to use the self-assembly technique to make other kinds of nanocomposite materials for use in data storage and microwave devices, said Sun.

Sun's research colleagues were Hao Zeng of IBM Research and Louisiana Tech University, Jing Li of Louisiana Tech, and J. P. Liu and Zhong Wang of Georgia Institute of Technology. They published the research in the November 28, 2002 issue of Nature. The research was funded by the Defense Advanced Research Projects Agency (DARPA) and IBM.

Timeline:   <5 years
Funding:   Government, Corporate
TRN Categories:   Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Exchange-Coupled Nanocomposite Magnets by Nanoparticle Self-assembly," Nature, November 28, 2002


December 11-25, 2002

Page One

DNA prefers diamond

Material soaks up the sun

Design links quantum bits

Microscopic mix strengthens magnet

Laser pulses could speed memory


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