High resolution electron micrograph of a FePt nanoparticle in L10 phase. We observe the alternation of Fe and Pt layers thanks to their different contrast.
Increasing storage density is an important issue for the magnetic recording industry. Magnetic particles only a few nanometers in diameter offer a promising alternative to the media obtained by "etching" to the scale of a few tens of nanometers. The major difficulty lies in obtaining independent particles and keeping their magnetic orientation at room temperature. Collaboration between SPrAM and SP2M overcame all these challenges.
The Fe50Pt50 alloy exhibits remarkable magnetic properties related to its crystalline phase L10, which looks like a millefeuille “pastry” of pure iron and platinum layers. We know how to chemically synthesize FePt nanoparticles surrounded by organic ligands that prevent their aggregation in liquid phase. But, Fe and Pt atoms are in disorder and the particles have no permanent magnetization. How to put these in order? By heating at 700° C. The unacceptable side effect: the organic ligands are destroyed and the particles aggregate! So, we physically separate the particles before the heat treatment on inert and stable grains of... salt! Upon exiting the oven, the nanoparticles are indeed in the L10 phase with good magnetic properties and remain isolated from each other by salt. But, how do you get them back?
If we dissolve our salty millefeuille “pastry” in water, the particles aggregate together and we do not win much. So we do it in an aqueous solution of cysteine. Inedible, you say! True, but thanks to its S-H thiol function, cysteine binds tightly to the surface of FePt. Our particles are well-dispersed again, and have the same properties as in their salty matrix. The cysteine ligand is not the most suitable for applications, but we managed to substitute a posteriori other ligands best suited to the surface functionalization while keeping independent nanoparticles. Molecular gastronomy thus offers other opportunities than the mere pleasure of the taste buds!
Further reading : A. Delattre et al., Small 6 (2010) 932
Maj : 18/02/2014 (952)