December 2, 2011 — Led by University of Maryland (UMd) researchers, a multi-institution team has produced a cobalt/iron alloy that could be the basis for a new class of sensors and micromechanical devices controlled by magnetism. The alloy does not use rare-earth elements to achieve its properties.
Materials scientists at the National Institute of Standards and Technology (NIST) contributed precision measurements of the alloy’s structure and mechanical properties to the project.
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| Figure. Transmission electron microscope (TEM) image taken at NIST of an annealed cobalt iron alloy. The high magnetostriction seen in this alloy is due to the two-phase iron-rich (shaded blue) and cobalt-rich (shaded red) structure and the nanoscale segregation. SOURCE: Bendersky of NIST. |
The alloy exhibits a phenomenon called "giant magnetostriction," an amplified change in dimensions when placed in a sufficiently strong magnetic field. The effect is analogous to the piezoelectric effect. They can be used as sensitive magnetic field detectors and tiny actuators for micromechanical devices. Unlike piezoelectrics, magnetostrictive elements require no wires and can be controlled by an external magnetic field source.
The team used a combinatorial screening technique, fabricating hundreds of 10mm-long test silicon cantilevers and coating them with a thin film of alloy, gradually varying the ratio of cobalt to iron across the array of cantilevers. They also used two different heat treatments, including one in which the alloy was heated to an annealing temperature and then suddenly quenched in water.
Quenching is a classic metallurgy technique to freeze a material’s microstructure in a state that it normally only has when heated. In this case, measurements at NIST and the Stanford Synchrotron Radiation Lightsource (SSRL) showed that the best-performing alloy had a delicate hetereogenous, nanoscale structure in which cobalt-rich crystals were embedded throughout a different, iron-rich crystal structure. Magnetostriction was determined by measuring the amount by which the alloy bent the tiny silicon cantilever in a magnetic field, combined with delicate measurements at NIST to determine the stiffness of the cantilever.
The best annealed alloy showed a sizeable magnetostriction effect in magnetic fields as low as about 0.01 Tesla.
The results, says team leader Ichiro Takeuchi of UMd, are lower than, but comparable to, the values for the best known magnetostrictive material, a rare-earth alloy called Tb-Dy-Fe (Terbium-dysprosium-iron), but with the advantage that the new alloy doesn’t use rare earths. "Freezing in the heterogeneity by quenching is an old method in metallurgy, but our approach may be unique in thin films," he observes.
The quenched alloy might offer both size and processing advantages over more common piezoelectric microdevices, says NIST materials scientist Will Osborn. "Magnetorestriction devices are less developed than piezoelectrics, but they’re becoming more interesting because the scale at which you can operate is smaller," he says. "Piezoelectrics are usually oxides, brittle and often lead-based, all of which is hard on manufacturing processes. These alloys are metal and much more compatible with the current generation of integrated device manufacturing. They’re a good next-generation material for microelectromechanical machines."
The effort also involved researchers from the Russian Institute of Metal Physics, Urals Branch of the Academy of Science; Oregon State University and Rowan University. Funding sources included the Office of Naval Research and the National Science Foundation. SSRL is part of the SLAC National Accelerator Laboratory, operated under the auspices of the U.S. Department of Energy.
The research is reported here: D. Hunter, W. Osborn, K. Wang, N. Kazantseva, J. Hattrick-Simpers, R. Suchoski, R. Takahashi, M.L. Young, A. Mehta, L.A. Bendersky, S.E. Lofland, M. Wuttig and I. Takeuchi. Giant magnetostriction in annealed Co1-xFex thin-films. Nature Communications. Nov. 1, 2011. DOI: 10.1038/ncomms1529. Access it here: http://www.nature.com/ncomms/journal/v2/n10/full/ncomms1529.html
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