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William Atkins
Tuesday, 23 January 2007 00:16
Researchers at the U.S. National Institute of Standards and Technology (NIST), Gaithersburg, Maryland, and the University of Arizona (UA), Tucson, Arizona, are making nanodot arrays that react to strong magnetic fields. These nanodots might eventually end up within future commercial hard drives.
The nanodots produced so far are 50 nanometers in diameter (about one-thousandth the diameter of a human hair) where one nanometer is equal to one-billionth the length of one meter. They are made with a magnetic force microscope using the process of electron beam lithography.
A magnetic force microscope—a type of scanning probe microscope—maps the changes found within a magnetic field by measuring the magnetic interaction between a sample and a magnetic tip on the microscope. Electron beam lithography is the process of generating patterns on a surface with the use of a beam of electrons.
Using the binary numeral system (base 2) of zeros and ones commonly used in computer systems, nanodots are built so they can switch back and forth between a north pole and a south pole, depending on the strength of the magnetic field. A dark colored pole, as seen under a microscope, is magnetized in the ‘up” direction, which represents 1 in binary code, and a light colored pole is magnetized in the ‘down’ direction, which corresponds to 0.
The key to making nanodots usable is to make them switch between the poles without much variability—what is called variation in nanodot switching response. This control was difficult in the past due to a lack of understanding about the basic nature of this variability. However, variation has been successfully controlled by the NIST/UA team to less than 5% of the average switching field. They have also been able to identify the reasons why variability is introduced into the nanodots, which primarily involves the design of the multilayer material.
To minimize variability, the researchers—Justin M. Shaw, W.H. Rippard, S.E. Russek, T. Reith, and C.M. Falco—design nanodots with a thin layer (film) of the element tantalum (Ta) only a few nanometers in thickness, what they call a ‘seed layer’. Then, a multilayer film of alternating layers of the elements cobalt (Co) and palladium (Pd) is applied on a silicon (Si) wafer.
Within the January 15, 2007 issue of the Journal of Applied Physics, the researchers state within their paper (“Origins of switching field distributions in perpendicular magnetic nanodot arrays”): “The seed layer can alter the strain, orientation or texture of the film. By making and comparing different types of multilayer stacks, it was possible to isolate the effects of different seed layers on switching behaviour.”
Nanodot technology is considered one of two primary ways to increase the density of magnetic data storage in the future. The other way is to use a laser beam to heat and switch bits of data. Some scientists currently think that a combination of the two methods may best result in reducing the size of magnetic data storage. Extensive research and development, however, is still needed within both technologies before anything is introduced in the marketplace.
Additional information on the National Institute of Standards and Technology and the University of Arizona is found at their home Web pages, respectively: http://www.nist.gov/ and http://www.arizona.edu/.
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