Puffed up magnets

Energy-harvesting magnets that change their volume when placed in a magnetic field have been discovered by US researchers. The materials described by Harsh Deep Chopra of Temple University and Manfred Wuttig of the University of Maryland produce negligible waste heat in the process and could displace current technologies and lead to new ones, such as omnidirectional actuators for mechanical devices and microelectromechanical systems (MEMS). [Nature, 2015, 521, 340-343; DOI: 10.1038/nature14459]

All magnets change their shape but not their volume, even auxetic magnets were previously characterized on the basis of volume conserving Joule magnetostriction. This fundamental principle of volume conservation has remained unchanged for 175 years, since the 1840s, when physicist James Prescott Joule found that iron-based magnetic materials would elongate and constrict anisotropically but not change their volume when placed in a magnetic field, so-called Joule magnetostriction.

The work of Chopra, Wuttig changes that observation fundamentally with the demonstration of volume-expanding magnetism. “Our findings fundamentally change the way we think about a certain type of magnetism that has been in place since 1841,” explains Chopra. “We have discovered a new class of magnets, which we call ‘Non-Joulian Magnets,’ that show a large volume change in magnetic fields.” Chopra described the phenomenon to us: "When 'excited' by a magnetic field, they swell up like a puffer fish," he says.

Chopra and Wuttig found that when they thermally treated certain iron-based alloys by heating them in a furnace to approximately 760 Celsius for half an hour and then rapidly cooled them to room temperature, the materials exhibited the non-Joulian behavior. Underlying this phenomenon is the formation of microscopic cellular-like structures that respond to the external magnetic field in a way that has not been observed previously. “Knowing about this unique structure will enable researchers to develop new materials with similarly attractive properties,” adds Wuttig.

The team points out that one area of application lies in the development of compact and efficient magnetic actuators. Conventional magnets can be used as actuator to exert a force and open a valve, for instance, but only operate in one direction since all magnets are limited by Joule magnetostriction. Actuation in two directions under the Joule paradigm requires bulky stacks of magnets, which are inefficient and preclude significant miniaturization. However, the discovery of non-Joulian magnets that can expand in all directions when an external magnetic field is applied it should be possible to construct, compact omnidirectional, the researchers have shown. "Just as in Joule magnetostriction, non-Joulian magnetostriction also occurs in simple iron based alloys at ambient temperatures so they are easy to implement into applications," Chopra told Materials Today.

Moreover, the fact that this phenomenon leads only to negligible heating effects also means that a new generation of sensors and actuators with vanishingly small heat signatures should be plausible. They could find use as compact micro-actuators for aerospace, automobile, biomedical, space and robotics applications. They might also be used in ultra-low thermal signature actuators for sonar and defense applications.

Another important point regarding the discovery, is that these new alloys of Fe–Ga, Fe–Al and Fe–Ge  lack expensive rare-earth elements and so could be inexpensive as well as more robust than their predecessors.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".