Exciting breakthrough: Researchers have for the first time converted an iron-containing liquid into a true permanent magnet – something that was previously considered impossible. The new liquid magnet is still magnetic even when no external magnetic field is applied. This breakthrough opens up a whole new field of research and could find numerous applications, as the researchers in the journal "Science" report.
So far, the matter seemed clear: only solids can be real permanent magnets – materials that react magnetically even without an external magnetic field. Only their ordered structure ensures that the uniformly aligned atomic spins remain in their position. This is different with ferrous liquids: Because in such ferrofluids, the atoms and molecules move around disorderly, they retain their magnetization only as long as an external magnetic field is applied ̵
Drops with crust of nanoparticles
Things are different, as Thomas Russell from the University of Massachusetts and his team now prove. For the first time, you have succeeded in turning a liquid into a permanent magnet. "We were wondering what we could do to make a ferrofluid not just temporary but permanently magnetic," explains Russell. "The fluid would have to behave like a solid magnet, but still remain liquid."
In search of a solution, the researchers experimented with various solutions of iron oxide-containing nanoparticles. Such nanoparticles are typically only paramagnetic. But that changed when Russell and his team put these particles into an emulsion of an oil and a liquid polymer. As they observed, minute droplets formed in the solution, at the boundary layer of which the nanoparticles were close together – like a dense crust.
The surprising thing, however: when the researchers applied a magnetic field to this solution and removed shortly thereafter, the droplets with the particle crust remained magnetic. "We almost could not believe it," Russel says. The originally paramagnetic ferrofluid droplets had been transformed into true ferromagnets. Even if the droplets were deformed, the nanoparticles in the solution retained their magnetic alignment permanently, the researchers noted.
But how is this possible? As the scientists discovered, the accumulation of nanoparticles at the droplet interface plays a crucial role in this. The iron-containing particles are so tightly packed together that they can no longer rotate freely. This maintains their once imposed magnetic order, even if the droplet deforms as a whole. Only when a magnetic field is applied again does the magnetic orientation of the particles change again.
New material – new applications
"With this we have created a new material that is both fluid and magnetic – nobody has ever done that before observed, "notes Russel. The ferromagnetic droplets have the characteristics of a fluid, but the magnetic properties of a solid. Thus, this solution forms a liquid permanent magnet. "This opens the door to a whole new field of research," says Russell.
Possible applications would be, for example, printable, flexibly deformable magnets or robotic fluids that can be controlled and controlled with magnetic fields. But magnetic sponges or elastic magnetic materials could also be made soon using this technique. "This is a milestone in the development of magnetic materials," says Russell.
"What began as a strange observation has become a whole new field of science," adds first author Xubo Liu of the Lawrence Berkeley Laboratory. (Science, 2019; doi: 10.1126 / science.aaw8719)
Source: DOE / Lawrence Berkeley National Laboratory