Non-Joulian Magnets: Why Do They Expand?

[sanman]

I've just read about the discovery of non-Joulian magnets, which are supposed to exhibit a new type of behavior by expanding when exposed to a magnetic field. They do not follow the rules of classical magnetorestriction:

http://www.mse.umd.edu/news/news_story.php?id=9072

How can a material volumetrically expand like that due to magnetic field? What is happening at the small scale? Is it that bond-lengths are somehow changing? Or are atoms somehow rearranging themselves in some way?

What are the applications for this? Could they be useful for hydraulic actuators, for example? I'm picturing that such magnets would be placed inside a piston chamber filled with hydraulic fluid, and the expansion of the magnet would then displace more fluid to cause hydraulic actuation.

Or maybe such a magnet could be used to open or close a valve.

Or could the expansion result in a density change for variable buoyancy?

The article says that the volumetric expansion is great, but how much is it in quantitative terms?

 

[stedwards]

Nice. http://www.nature.com/nature/journal/v521/n7552/full/nature14459.html in nature goes into more detail.

"[...]NJM is caused by facile (low-field) reorientation of magnetoelastically and magnetostatically autarkic (self-sufficient) rigid micro-‘cells’, which define the adaptive structure, the origin of which is proposed to be elastic gradients ultimately caused by charge/spin density waves."
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...whatever that means. I can't decode it.

​

 

[sanman]

So the driving force behind this volumetric change is magnetism? Does that mean the force/pressure of the volumetric expansion can be correlated to the strength of the magnetic field? Is there any kind of Hooke's Law here, whereby the force gives out when you cross a certain threshold?

I'm wondering what the key metrics or properties are here for this phenomenon - Newtons/Tesla?