Like a balloon, peculiar magnet grows and shrinks

Alloy of iron and gallium increases volume in a magnetic field


STRETCHY MAGNET  Layers of roughly 40-micrometer-long magnetic cells inside an iron-gallium alloy, seen in this micrograph, enable the material to expand and contract in a magnetic field.

Courtesy of H. Chopra

An unusual magnet expands and contracts when exposed to a magnetic field. While scientists have long known that fields can subtly change magnets’ shape, the alloy described in the May 21 Nature is a rare specimen that experiences significant changes in volume. This material or similar ones could be used to produce actuators that push in all directions or pressure sensors whose magnetism changes as they get compressed.

In 1841, following a tip from a machinist, English physicist James Joule observed that iron exposed to a magnetic field slightly stretched in one direction and constricted in another. The length changes canceled each other out, so the iron’s volume remained the same. Engineers have exploited this phenomenon to produce magnetic actuators that push against a surface or rapidly pulsate to generate sound waves.

Materials scientists Harsh Chopra of Temple University in Philadelphia and Manfred Wuttig of the University of Maryland in College Park studied the magnetic properties of an iron-gallium alloy that was first produced in 2000. The researchers heated small crystals to about 760° Celsius and then, hoping to lock in particular internal properties, quickly cooled the crystals to room temperature. When exposed to a weak magnetic field, the crystals’ volume increased by about several hundredths of a percent. And unlike most shape-shifting magnets, the alloy barely lost any energy as heat. “These materials have different behavior than all other magnets we know of,” Chopra says.

Further analysis revealed that the alloy’s interior consists of folded layers of micrometer-sized magnetized segments. When exposed to magnetic fields aligned in a particular direction, the layers unfold and the material’s volume increases. Thomas Lograsso, a materials scientist at the Ames Laboratory in Iowa, who codiscovered the alloy, says that several materials including gadolinium-silicon-germanium can also expand in magnetic fields. But he agrees that the iron-gallium alloy has many intriguing properties and says the study will help researchers determine how to produce similar materials.

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