Laser Enables Resolution of Nanomagnet Flipping
At the University of California, Santa Cruz, researchers have demonstrated an optical method to watch nanomagnets flip. Optically tracking changes in the magnetization of the nanostructures could pay off in various ways, explained assistant professor of electrical engineering Holger Schmidt. “The hope would be that, using nanofabrication techniques, we can make nanomagnets with optimized dynamic properties,” he said.
Nanomagnet arrays promise to enable dense data storage. The magnets have one north and one south pole — or one domain — and can be constructed so that their magnetization aligns in one of two directions. That makes the arrays potentially able to store the ones and zeros of digital data, with the capability of switching orientation and stored values in response to an external field. Investigators would like to observe individual nanomagnets as they flip.
The key to the optical technique is to coat the nanomagnets with a thin dielectric layer. That boosts the change in the polarization of a laser beam that reflects off a nanomagnet, which is caused by the magneto-optical Kerr effect.
Working with collaborators at Brigham Young University in Provo, Utah, and at the molecular foundry at Lawrence Berkeley National Laboratory in Berkeley, Calif., they fabricated nickel nanomagnets that measured 150 nm in height and from 50 nm to 5 μm in diameter. Those with diameters of 100 nm or less had one domain that was oriented along the long axis. They manufactured the nanomagnets with a spacing of 2 μm to ensure optical isolation.
The researchers then coated the nickel pillars with a layer of silicon nitride, made 70 nm thick for constructive interference during reflection. They illuminated the sample with a 780-nm laser diode, focusing the beam through a microscope to shine on only one nanomagnet.
They found that the multiple reflection passes of the light through the dielectric material increased the Kerr signal fivefold. Using only a low-aperture and inexpensive objective, Schmidt noted, they thus were able to observe magnets 100 nm or shorter, significantly smaller than the 500 nm possible with uncoated magnets.
Using the technique, the scientists constructed a demonstration near-field scanning system by modifying a scanning probe microscope to hold a tapered optical fiber tip. Schmidt said that a near-field aperture can provide higher sensitivity at smaller diameters and allows the selection of a single nanomagnet in a dense array, enabling the capture of the changes in magnetization of a particular nanomagnet.
“The goal is to observe dynamic processes with subpicosecond time resolution and sub-100-nm sensitivity,” Schmidt said, and the researchers plan to integrate an ultrafast laser into the system to achieve this time resolution. They also hope to implement 10-nm resolution and to add precise tip-sample height feedback control to the near-field scanning system.
Nano Letters, June 22, 2005, online article, doi:10.1021/nl050753p.
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