Making Colors with Magnets

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The Riverside researchers found that processing the materials at high temperatures ensured that the 10-nanometer particles formed with a crystalline atomic structure. It also caused the particles to group together to form similarly sized clusters. In contrast, more commonly used room-temperature synthesis results in particles that form irregular agglomerations. The uniformity of the clusters and the crystallinity of the particles seem to improve the magnetic response of the materials, Yin says, although he and his colleagues are still looking into the underlying mechanisms involved.

The materials can switch colors at a rate of twice a second, which is still too slow for use in TVs and computer monitors. Yin hopes to increase switching speeds still more by using smaller amounts of material, perhaps in microscopic capsules. Such small amounts will make it easier to present a uniform magnetic field to the entire sample, potentially aiding the rearrangement of the clusters. Also, such microcapsules could be arranged to form pixels in a display, as is done now with E-Ink, a type of electronic paper used in some electronic book readers and a cell phones. (See "A Good Read.")

But even with faster speeds, Yin doesn't expect the materials to replace current computer-monitor technology. Rather, he has his sights set on larger-scale applications that would take advantage of the low cost of the materials. Examples could include posters that can be rewritten but don't have to change as fast as displays of video.

One significant drawback of the current materials is that they would need a constant power supply to preserve the magnetic field and hold the microcapsules at a set color. Yin's next step is to develop a version of the materials that remains stable after their color is changed--that is, until they're switched to a new color. If this is possible, then a poster could be printed with something like the read-write head on a hard drive, Yin says. It would preserve the image until it's rewritten with another pass of the print head, using no power in between.

"At this stage it's fun to play with," Velev says. "Maybe at later stages it could be used for some decorative purpose, such as paint that changes color, or some new types of labels or display boards. Right now it's a beautiful piece of research."