Photodetectors Fabricated Using Electrostatic Self-Assembly
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RESEARCH/RESEARCHERS Thermally Assisted Recording Pushes Magnetic Storage Densities beyond Traditional Limits The desire to increase magnetic storage densities requires downscaling of the bit dimensions, which are currently limited to ~50 nm. In the past, this was made possible by reducing the grain size of the recording media. However, the decrease in grain size will eventually reach the superparamagnetic limit, at which thermal energy is sufficient to randomize the magnetization direction. Consequently, further increases in recording densities require materials with higher magnetic anisotropy energy. While these materials are available, they cannot be used because the magnetic field of the recording heads is insufficient to write these materials. Recently, a group of researchers at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y., demonstrated that by thermally assisting the recording process, very highdensity magnetization patterns could be written, therefore breaking the record on magnetic storage density. In an article published in Applied Physics Letters on February 2, researchers H.F. Hamann, Y.C. Martin, and H.K. Wickramasinghe described their experiment in detail. They placed an atomic force microscope tip (radius, ~5 nm) in close vicinity to a magnetic film of TbFe on a mica substrate or a Co/Pt multilayer film, also on a mica substrate. The tip was heated by a laser diode. Some of the energy was transferred on a very local scale from the tip to the magnetic film, which was then heated to close to its Curie temperature. The magnetic bias field was applied to align the magnetization in the heated region. The heated region of the film had a substantially lower coercivity and could be magnetized with a small field. On the basis of magnetic force microscopy measurements, the researchers said that the recorded spot sizes are less than 30 nm in diameter. The magnetization patterns obtained on the thin film correspond to a storage density of ~400 Gbit/in2. The researchers believe this is the highest magnetic storage density reported. Furthermore, by thermal modeling, they predicted that an even smaller heated spot size can be realized. That means, potentially, a recording density of >1 Tbit/in.2 can be achieved. SHIMING WU
Photodetectors Fabricated Using Electrostatic Self-Assembly Organic semiconducting polymers are attractive materials for use in photovoltaic (PV) detectors because of their tunability and flexibility. However, excitons in or136
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