Alloy sensitive to magnetic fields could produce new MEMS devices

Alloy sensitive to magnetic fields could produce new MEMS devices

A rare-earth-element-free alloy of cobalt and iron could be the basis for a new class of sensors and micromechanical system (MEMS) devices controlled by magnetism. The alloy was created by a team led by researchers from the University of Maryland (UMd) and including researchers from Oregon State University, Rowan University, the National Institute of Standards and Technology (NIST), and the Russian Institute of Metal Physics, Urals Branch of the Academy of Science.

The Co-Fe alloy exhibits giant magnetostriction, the phenomenon in which an amplified change in dimensions occurs when the material is placed in a sufficiently strong magnetic field. The effect can be used in various ways, such as for creating sensitive magnetic-field detectors and micromechanical actuators. Unlike piezoelectrics, magnetostrictive elements can be controlled by an external magnetic field and don’t need to be wired up.

The alloy might also offer both size and processing advantages over 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.”

To find the best mixture of metals and processing, the team fabricated hundreds of 10-mm-long test cantilever and coated them with a thin film of alloy, gradually varying the ratio of cobalt to iron across the array of cantilevers. The researchers found that by heating the alloy to annealing temperature and then quenching it, they created a delicate, heterogeneous, nanoscale structure in which cobalt-rich crystals were embedded throughout an iron-rich crystal structure (see Fig. 1 ).

Fig. 1: This TEM image taken at NIST shows the two-phase iron-rich (shaded blue) and cobalt-rich (shaded red) structure of an annealed cobalt-iron alloy. The high magnetostriction of the alloy is due to this structure and the nanoscale segregation.

The best annealed alloy showed a sizeable magnetostriction effect in magnetic fields as low as about 0.01 Tesla. Magnetostriction was determined by measuring the amount by which the alloy bent the tiny silicon cantilever in a magnetic field,

According to team leader, Professor Ichiro Takeuchi of UMd, the results are lower than, but comparable to, the values for the best known magnetostrictive material (terbium-dysprosium-iron, or Tb-Dy-Fe) but with the advantage that the new alloy doesn’t use any rare-earth elements, which are sometimes difficult to obtain. “Freezing in the heterogeneity by quenching is an old method in metallurgy, but our approach may be unique in thin films,” he observes. “That’s the beauty: a nice, simple technique, but you can get these large effects.” For further information, contact Ichiro Takeuchi at
, or 301-405-6809.

Richard Comerford