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Direct Laser Interferometry Forms Magnetic Nanodots

Photonics Spectra
Jan 2002
Daniel C. McCarthy

Laser-written compact disks have nibbled at market share for magnetic storage media. Could lasers be aimed next at enabling advanced magnetic media for computer hard drives? Russian and American scientists are collaborating on a way to pattern magnetic nanodot arrays using direct laser interference lithography.

Patterned arrays of C-Co films that are formed easily by interferometric patterns from a XeCl laser could signal development of more thermally stable magnetic storage media. Courtesy of the University of Nebraska.

A computer hard drive measures its capacity by the density of magnetic particles embedded in its surface: 1 Gb is equivalent to 109 grains, and today's commercial hard drives reach capacities of 26 Gb/in.2. If that seems like a lot, the storage industry is aiming for 100 Gb/in.2 or more in several years. After that, it will be increasingly difficult to make media that are both denser and reliable, because of the effects of smaller bit size on thermal stability.

Put simply, smaller bit sizes can contribute to lower thermal stability. Beyond a certain point, thermal fluctuations can flip the magnetic state of small, densely packed grains and decay the data stored on a hard drive disk.

Patterned media, in which each bit has several grains, help control thermal stability because the volume of a whole bit can be much larger than the volume of a single grain. Larger bits can be strongly exchange-coupled, more clearly defined and less likely to flip their magnetic state.

Thus far, the collaborators have produced patterned grains about 250 nm in size that, while relatively large, have excellent thermal stability. Researchers at the University of Nebraska in Lincoln fabricated C-Co films and shipped them to the Institute of Applied Physics and the Institute for Physics of Microstructures, both in Novgorod, Russia. There, scientists applied interfering 308-nm beams from a XeCl laser to pattern two-dimensional arrays of magnetic dots separated by 650-nm spacings. They returned the films to Nebraska for characterization.

"We're at the beginning stages, but it's a single-step fabrication process. We're beginning to get materials that have the right characteristics," said David Sellmyer, a physics professor at the university.

The goal is to get interferometric fringes smaller and smaller to produce denser magnetic dots, which are determined by λ/2. "That simply requires shorter-wavelength lasers or synchrotron sources," he said.

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