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New Alloy Could Replace Rare Earths in Novel Micromechanical Devices

Posted on: 11/22/2011
Led by a group at the University of Maryland (UMd), a multi-institution team of researchers has combined modern materials research and an age-old metallurgy technique to produce an alloy that could be the basis for a new class of sensors and micromechanical devices controlled by magnetism. The alloy, a combination of cobalt and iron, is notable, among other things, for not using rare-earth elements to achieve its properties.

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 more familiar piezoelectric effect that causes certain materials, like quartz, to compress under an electric field. They can be used in a variety of ways, including as sensitive magnetic field detectors and tiny actuators for micromechanical devices. The latter is particularly interesting to engineers because, unlike piezoelectrics, magnetostrictive elements require no wires and can be controlled by an external magnetic field source.

To find the best mixture of metals and processing, the team used a combinatorial screening technique, fabricating hundreds of tiny test 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 quenching, a classic metallurgy technique to freeze a material's microstructure in a state that it normally only has when heated.

In the best results from the study, the annealed alloy showed a sizeable magnetostriction effect in magnetic fields as low as about 0.01 Tesla. This is lower than, but comparable to, the values for the best known magnetostrictive material, a rare-earth alloy called Tb-Dy-Fe

The quenched alloy might also offer both size and processing advantages over more common piezoelectric microdevices, since it is metal and much more compatible with the current generation of integrated device manufacturing.

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