Materials scientists are always looking for better ways to transform one form of energy into another. In the 11 March Physical Review Letters, theorists predict that a material containing nanoscale regions of a different atomic structure could respond to a magnetic field by changing shape, perhaps stretching 100 times more than traditional materials. Such materials, or cousins that respond to other fields, could be used in devices that sense fields or drive mechanical motion.
Traditional transducer materials, such as the piezoelectrics that convert vibrations to electrical signals, work at the atomic level. Squeezing and stretching a crystal shifts positive and negative ions within each crystalline cell in different directions, creating an electric polarization that leads to a voltage signal. Acoustic guitar pickups and ultrasound imagers both use this effect to detect vibrations. The materials also show the reverse effect, converting electric fields into mechanical motion.
Some magnetic materials can perform a similar trick, turning magnetic fields into mechanical motion or vice versa, but the effect is usually small. Over the past decade or so, researchers have been exploring materials in which internal structural rearrangements lead to much larger responses. The best-known example has an atomic structure where the crystalline cell is, say, longer in one dimension than in the other two–it could be made of blocks shaped like butter sticks, rather than cubes. A large crystal normally has many regions, or domains, each with a different orientation of the long axis. A magnetic field can slightly shift the positions of atoms and favor the growth of the domains best aligned with it, at the expense of non-aligned regions. For example, the field might expand a domain with its long axis aligned along the field and move the domain boundary–called a domain wall–so as to shrink another domain. This domain expansion would stretch the material along the field direction.
These materials have two problems, says Armen Khachaturyan of Rutgers University in Piscataway, New Jersey. One is that an external force pushing in opposition can easily reverse the expansion and move domain walls back, limiting the force the materials can generate in devices. The other is that when domain-wall motions encounter resistance, the material response depends on its previous history, which is bad for some applications.
To solve these problems, Khachaturyan and his colleagues propose embedding nanometer-scale particles of the shape-shifting phase within a distortion-free solid matrix. Such nanocomposites can form spontaneously when, for example, a metal alloy or ceramic is heated, and some of the components cluster into tiny globs with different chemical composition than their surroundings. If the particles are small enough, they will each contain a single domain, and their long axis can easily be changed by an external magnetic field in order to stretch the material.
The team calculates that under some conditions this two-phase nanocomposite could show a large response with no history dependence and could also supply a large force. They suggest that some previously puzzling experiments hint that the effect may already have been seen, but their theoretical framework could allow a more systematic search for practical materials.
“We’re always looking for giant magnetostrictive materials,” says David Laughlin of Carnegie Mellon University in Pittsburgh, Pennsylvania. But he notes that the new work provides a theory that can apply to other kinds of fields as well, such as electric and stress fields. “You get a larger amount of strain right across the board.”