Researchers have discovered a new class of magnet that increases in volume when placed in a magnetic field and generates only negligible amounts of heat in the process. These properties could transform many existing technologies and enable a few new ones.

Harsh Deep Chopra, Professor and Chair of Mechanical Engineering at Temple University, and Manfred Wuttig, Professor of Materials Science and Engineering at the University of Maryland, published their findings in the May 21st issue of the journal Nature.

Important work
Magnets and magnet operation are central to a lot of modern technology. Magnets are used to:

  • Sense a mechanical input and convert it to an electrical signal (sensors)
  • Use an electrical signal to displace a mechanical structure (actuators)
  • Generate power from a mechanical input (generators)

It’s no surprise, then, that an advance in magnet performance could have a big impact on a wide range of activities.

Breaking an old rule
A conventional iron-based magnet will change shape, but not volume, when placed in a magnetic field. As a magnet grows longer, it also grows thinner so that its total volume remains unchanged. This principle – magnetostriction – was first described by physicist James Prescott Joule in the 1840s. Conventional magnets, therefore, can only exert force in the single direction of their shape change, which limits what they can do to as energy conversion tools. If a device must move in more than one direction, bulky stacks of magnets are required.

Chopra and Wuttig discovered that some thermally-treated iron-based alloys could be freed from Joulian magnetostriction. The researchers baked the alloy at 1400o F (760o C) for 30 minutes and then rapidly cooled it to room temperature. The treatment changed its molecular structure and introduced a microscopic cell-like organization into the material that had never been seen before. The structural change was also accompanied by an important change in behavior.

The altered alloy significantly increased its volume in all directions when exposed to a magnetic field, and returned to its original volume when the field was removed. A non-Joulian material, showing non-Joulian magnetostriction (NJM), had been created.

Magnified image of a non-Joulian iron-gallium alloy, showing the periodic magnetic “cells” (Harsh Deep Chopra – Temple University)

Magnified image of a non-Joulian iron-gallium alloy, showing the periodic magnetic “cells”
(Harsh Deep Chopra – Temple University)

NJM alloys adapt to magnetic fields by reorienting their micro-cell structures. Magnetization within the cells isn’t uniform, however, but is modulated across cell boundaries in the material. The strain gradients between these cell regions relax in the presence of a magnetic field, allowing a uniform volume change.

“Our findings fundamentally change the way we think about a certain type of magnetism that has been in place since 1841,” said Chopra in a recent press release.

New technology tools
Non-Joulian magnets offer some major advantages over conventional magnets. Omnidirectional devices, for example, could be easily designed without the need for stacked magnet assemblies.

Movement of non-Joulian magnets generates very little heat, which makes them ideal candidates for low-thermal systems such as sonars. Non-Joulian alloys are also free of rare-earth elements which gives them improved mechanical properties, and lower cost, compared to conventional magnets.

The insights gained from this work will likely foster further advances in magnetic materials and processes.

“Knowing about this unique structure will enable researchers to develop new materials with similarly attractive properties,” said Wuttig.

Certainly, the Chopra and Wuttig work confirms the continued value of basic research, even involving processes that we thought we understood for 175 years.

As Wutting observed, “The response of these magnets differs fundamentally from that likely envisioned by Joule.”


Primary Source:
New class of swelling magnets to boost energy efficiency