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Ferromagnetism revealed in suspensions of magnetic nanoplatelets in liquid crystal

Published online by Cambridge University Press:  12 February 2014

Dirk Wouters

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MRS Bulletin , Volume 39 , Issue 2: Elastic Strain Engineering , February 2014 , pp. 102
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Copyright © Materials Research Society 2014 

Inspired by their beautiful color patterns viewed in an optical microscope and their intriguing behavior, Alenka Mertelj from the J. Stefan Institute in Ljubljana, Slovenia, developed an interest in the dynamics of complex fluids. Now she and her colleagues at the Institute—Darja Lisjak, Miha Drofenik, and Martin Čopič—have added a new dimension to that complexity: ferromagnetism.

Transmission electron microscope image of magnetic nanoplatelets. Image credit: Alenka Mertelj.

Previous to their work, ferromagnetic complex fluids had only been observed at either liquid helium temperatures or above 1000 K. By mixing nanoparticles in a nematic liquid crystal, the research team succeeded in making a room-temperature fluid ferromagnetic phase, thereby solving a 40-year-old problem. The issue at hand: avoid aggregation of the nanoparticles while maintaining sufficient magnetic interaction between them. Their trick: the use of magnetic nanoplatelets. As reported in the December 12, 2013 issue of Nature (DOI:10.1038/nature12863; p. 237), the researchers found that the platelet shape of barium hexaferrite particles allows for a suitable interplay between the magnetic and nematic-elastic interactions, and combined with quench-cooling of the suspension from the isotropic into the nematic phase, stable, aggregate-free samples are formed.

Proving that the concept works, their samples showed hysteretic magnetization switching at very low magnetic fields. Furthermore, depending on the fabrication procedure employed, mono-domain behavior (for samples cooled in a magnetic field) or multi-domain behavior with domain wall motion was observed.

Adding ferromagnetism to the well-established electro-optical properties of liquid crystals means these materials represent a new class of multiferroics. They may enable many new applications, especially in magneto-optic devices. Mertelj said, “Whereas in liquid crystals one can control the propagation of light by an electric field, this new material may allow similar control by a magnetic field. Or they can be used to image the magnetic field with a liquid crystal.”

The use of other (chiral or smectic order) types of liquid crystals has the potential to further open up a completely new research field.