Organization and magnetic properties of mass-selected FePt nanoparticles deposited on epitaxially grown graphene on Ir(111)

The FePt alloy, when chemically ordered in the L1₀ phase, is among the magnetic materials displaying the highest magnetic anisotropy constant. Therefore it is a perfect candidate for ultra-high density magnetic storage applications, provided nanoparticles can be prepared in such a high anisotropy phase and organized in a 2D array. One path of bottom-up elaboration following a physical route consists in using template surfaces with specific sites regularly distributed. Such a 2D lattice can be obtained with the moiré (hexagonal lattice of 2.5 nm cell parameter) displayed by a graphene layer epitaxially grown on a Ir(111) surface [2]. For the first time, we have characterized the organization and the magnetic properties of FePt nanoparticles on such a moiré pattern.
FePt/graphene/Ir(111) samples have been prepared using Low Energy Cluster Beam Deposition [3] of preformed size-selected FePt nanoparticles (around 2 nm diameter). We will discuss the organization of such particles on specific sites of the moiré lattice, as determined by grazing incidence x-ray scattering measurements [4]. The deposited nanoparticles are sensitive to the moiré pattern and we find that the resulting organization can be preserved up to temperatures around 700°C. Using X-ray Magnetic Circular Dichroism measurements, we will report a clear evolution of the magnetic properties of the FePt nanoparticles induced by annealing (anisotropy modification, interface effects between FePt and the graphene...), while the particles keep their individuality (no layer formation is observed).
[1] P. Andreazza et al., Surface Science Reports 70, 188 (2015)
[2] A. T. N’Diaye et al., New Journal of Physics 10, 043033 (2008)
[3] V. Dupuis et al., Phys. Chem. Chem. Phys. 17, 27996 (2015)
[4] S. Linas et al., Scientific Reports 5, 13053 (2015)