FeCo alloys are important soft magnetic materials thanks to their interesting and unique physical properties that include high permeability, very high saturation magnetization and high Curie temperature. This makes FeCo alloy nanoparticles very attractive for a number of device applications.

Although there are various ways to synthesize these materials, it is often difficult to prepare monodisperse FeCo nanoparticles with controllable shape and size (especially for particles smaller than 20 nm in size), and, more importantly, particles that are chemically stable in ambient conditions. Most chemically synthesized FeCo nanoparticles take on a disordered body-centered cubic (bcc) structure and are not stable in air, so the particles generally need to be subjected to a heat treatment to make them stable. Unfortunately, this heat treatment causes the particles to sinter, which leads to unwanted particle agglomeration that destroys the nanoscale morphology of the particles.

Now, J Ping Liu at the University of Texas at Arlington and colleagues have overcome this problem by developing a novel synthesis technique that allows them to control particle morphology and improve the stability of the particles at the same time. The reductive heat treatment of CoFe2O4 nanoparticles by adding finely grounded NaCl salt powder produces air-stable FeCo nanoparticles that retain the same nanoparticle size as their oxide precursors. The salt powder works as a separating medium that keeps the CoFe2O4 nanoparticles apart so that they can undergo a phase transformation to the ordered bcc structured FeCo without sticking together and sintering. NaCl is chemically stable up to the annealing temperatures employed and can easily be removed by simply washing the particles in water. The team has succeeded in producing FeCo particles with diameters of between 8 and 20 nm that have a saturation magnetization as high as 211 emu/g.

“Controlling the particle size of oxide nanoparticles is fairly easy with established synthesis methods. However, to produce FeCo nanoparticles with the desired particle size and size distribution was still challenging,” explained Liu. “With this new technique, the size of FeCo nanoparticles can be finely tuned by just changing the size of the CoFe2O4 nanoparticles. The heat-treated FeCo nanoparticles produced are air stable thanks to the formation of the ordered bcc structure as well as the thin carbon shells formed on the surfaces of FeCo nanoparticles when they are heated.”

“What is more, the method to produce the FeCo particles can be easily scaled up and is very economic and ‘green’, and the technique might be applied to prepare a variety of other nanoparticles,” he added.

These air-stable FeCo nanoparticles could be the ideal building blocks for high-performance nanocomposite permanent magnets and nanostructured magnetic devices, and might also be suitable for biomedical applications.

More information about the research can be found in the journal Nanotechnology 24 345605.

Further reading

Magnets warm up to get stronger (Aug 2007)
High-yield graphite-encapsulated FeCo nanoparticles prepared for MRI test (Sep 2011)