A new attraction
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Jian Shen, a recipient of the Presidential Early Career Award in Science and Technology in 2004, is a research theme leader at the Department of Energy's Center for Nanophase Materials Sciences. Many predict that Shen's novel techniques for growing and studying magnetic nanostructures will attract a growing number of guest scientists to ORNL's new nanocenter.
Shen and his colleagues in the Low Dimensional Materials by Design Group in ORNL's Condensed Matter Sciences Division have developed novel methods for growing artificially structured materials layer by layer, wire by wire, and dot by dot. These two-dimensional, 1-D, and 0-D structures can be made of either traditional or nontraditional magnetic materials or hybrids of both types.
The physical properties of these nanostructures can be "tuned beyond nature" by controlling the size, shape, and density of each individual layer, wire, or dot in a nanostructure. Thus, iron nanodots and iron horseshoe magnets have dramatically different magnetic properties.
Nanomagnetism research is particularly important to the electronics industry. High-density magnetic data storage devices must have nanometer-sized arrays of magnetic nanodots. Because they are so small, these nanodots usually become magnetic only at very low temperatures. Such data storage devices would be practical only if they could operate at room temperature or higher. "We were able to tune the interaction between nanodots to obtain ferromagnetism well above room temperature," Shen says.
Shen and his colleagues are synthesizing nanostructures from three types of materials and studying the effects of spatial confinement on these materials' magnetic and electron transport properties. They are working with strongly correlated materials, such as compounds containing manganese oxide. These manganites exhibit colossal magnetoresistance--a huge change in electrical resistivity when subjected to a magnetic field. The ORNL team will seek to determine how much the magnetoresistance is affected by the reduced dimensions of manganite nanowires and nanodots.
Shen's group is also studying dilute doped magnetic semiconductors, such as silicon or germanium nanostructures doped with magnetic elements such as manganese atoms. If created properly, these materials could be both ferromagnetic and semiconducting, making them potentially useful for spintronic devices, a new technology that exploits quantum properties of electron spins for a new generation of electronic devices.
Shen and his associates at the University of Tennessee are examining a third class of materials--semiconducting polymers in nanostructures embedded with nanodots or nanowires made of traditional magnetic materials such as iron, cobalt, or nickel. The resulting hybrid material could be used to improve the efficiency of organic light-emitting diodes.
Researchers studying nanomagnetism at ORNL's nanocenter will have a unique collection of tools for synthesizing and characterizing magnetic nanostructures. Electron beam (e-beam) lithography and other e-beam writing tools will be available to synthesize magnetic nanowires and etch nanopatterns to form electrodes.
"These wires are so small that a traditional contact to measure resistance and conductivity would not work," Shen says. "So we must make a nanoscale electrical contact for the outside world to measure the wires' properties."
To image changes in a nanomaterial's magnetic structure as the temperature changes, researchers will use a scanning electron microscope with polarization analysis.
Electron beams are bent by magnetic fields, so ordinary electron microscopes cannot image these fields well. "Our unique microscope will allow researchers to measure magnetic moments in nanosized samples with high resolution," Shen says. "We will also have a scanning tunneling microscope with spin polarization to provide similar information."
The proximity of ORNL's Spallation Neutron Source is ideal for nanomagnetism studies, says Shen, because the intensity of the source's pulsed neutron beams will allow studies of the dynamics and magnetic structure of very small samples of magnetic material.
"Neutrons can penetrate stacked magnetic films used in data storage and provide a depth profile of magnetization in the vertical direction," Shen says. "Neutrons also give chemical information about the structure, allowing magnetic properties to be correlated with chemical composition. We anticipate that this information may guide the design of modular structures with improved magnetic properties needed for high-density data storage and other applications."