Pierre Capiod1 Olivier Boisron1 Veronique Dupuis1 Damien Le Roy1

1, Institut Lumière Matière, Lyon, , France

Nanocomposite magnets consisting of a fine mixture between a hard magnetic phase and a high saturation magnetization phase would be a promising way to go beyond performances of nowadays single magnetic phase magnets, on top of which NdFeB-based magnets. Theoretical descriptions of nanocomposite magnets [1] revealed the necessity of confining the soft phase in grains of typically less than 10 nm. Yet standard fabrication processes do not permit to have such a control on the microstructure. In this context nanomaterial-dedicated synthesis could permit to realize model films to experimentally explore fine mechanisms that govern magnet performances in such nanocomposite magnets [2].
Here we report on results we obtained from synthesis of Co:FePt nanocomposite by combining low energy cluster beam deposition (LECBD) technique and e-beam evaporation. Indeed to separately adjust size, composition and concentration, two independent beams: one for the mass-selected Co clusters preformed in gas phase and one for the hard L10-FePt matrix produced by alternative electron gun evaporation, are in-situ deposited on a same substrate. Doing so, the soft phase is restricted to the nanocluster size, selected from 2 to 8 nm while the volume fraction of nanoclusters in L10-FePt matrix was varied up to 30%.
In this paper, we present results of standard structural characterizations (XRD, SEM, TEM) and magnetic characterizations (SQUID magnetometry, MFM) performed on our nanocomposite samples. In addition, synchrotron characterizations (EXAFS, XMCD and XLD on the K-edges of Fe and Co) show the fine structure of the nanocomposite. Especially we will discuss specific magnetization reversal of soft and hard phases and the evolution at the interface of Co nanoparticles and the FePt matrix introduced by annealing at high temperature.

[1] Skomski R. and Coey J. M. D. Giant energy product in nanostructured two-phase magnets. Phys. Rev. B 48, 15812–15816 (1993).
[2] Balasubramanian B., Mukherjee P., Skomski R., Manchanda P., Das B. and Sellmyer D. J. Magnetic nanostructuring and overcoming Brown's paradox to realize extraordinary high-temperature energy products. Scientific reports 4, 6265 (2014)