Ferromagnets are solids, in pretty nearly every instance I can recall (though I suppose it's not impossible to imagine an itinerant Stoner magnet that's a liquid below its Curie temperature, and here is one apparent example). There's a neat paper in Science this week, reporting liquid droplets that act like ferromagnets and can be reshaped.
The physics at work here is actually a bit more interesting than just a single homogeneous material that happens to be liquid below its magnetic ordering temperature. The liquid in this case is a suspension of magnetite nanoparticles. Each nanoparticle is magnetic, as the microscopic ordering temperature for Fe3O4 is about 858 K. However, the individual particles are so small (22 nm in diameter) that they are superparamagnetic at room temperature, meaning that thermal fluctuations are energetic enough to reorient how the little north/south poles of the single-domain particles are pointing. Now, if the interface at the surface of the suspension droplet confines the nanoparticles sufficiently, they jam together with such small separations that their magnetic interactions are enough to lock their magnetizations, killing the superparamagnetism and leading to a bulk magnetic response from the aggregate. Pretty cool! (Extra-long-time readers of this blog will note that this hearkens waaaay back to this post.)
2 comments:
I was confused by the hype about this, what makes this significantly better than a ferrofluid that we show kids in school?
Anon, so was I for a while. I think the novelty here is that you can have droplets of fluid that can be configured "on the fly" into shapes that then take on a net magnetization and hold onto it and their shapes. Ferrofluids (also suspensions of ferromagnetic nanoparticles) are neat, and take on all kinds of rich shapes when pushed by an externally applied field, but they can't be locked into those spatial or magnetic configurations.
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