Elaboration of Nanomagnet Arrays: Organization and Magnetic Properties of Mass-Selected FePt Nanoparticles Deposited on Epitaxially Grown Graphene on Ir(111)

The moiré pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moiré pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to $700°\mathrm{C}$ where the hard magnetic phase of FePt is obtained.

Figure 1

(a) GISAXS intensities (at room temperature, after deposition at $150°\mathrm{C}$) along the $⟨100⟩$, $⟨010⟩$, and $⟨1\text{−}10⟩$ directions of the hexagonal moiré lattice. The presence of a correlation peak (highlighted in red) in those directions reflects the hexagonal organization of FePt nanoparticles on the moiré lattice. Out of azimuth, no correlation is visible. (b) GISAXS intensity in the $⟨100⟩$ direction at $700°\mathrm{C}$. (c) GISAXS intensity profile (fixed ${\alpha }_f$) of the correlation peak for three different temperatures with their respective fits represented with the black line. The curves have been shifted for clarity. The fraction $f_{\text{order}}$ of particles coherently located on the moiré lattice remains constant around 35% $\pm 3\%$.

Figure 2

Top (a) and (b) are, respectively, before and after annealing XAS spectrum at the Fe $L_{3,2}$ edges taken at 4 K for light circularly polarized left ($+$) and right ($-$) with the isotropic XAS signal shown in dotted line. Both averaged XAS signals have been normalized for a direct comparison of the XMCD spectrum. The XMCD spectrum [bottom (a) and (b), respectively before and after annealing] is the difference between the two XAS spectra recorded with opposite orientation of the magnetic field and the x-ray polarization (respectively, blue and red for left and right helicity). The dotted lines are the integral of the XMCD spectrum to highlight the modification of the intensity as well as the evolution of the $m_L/m_S$ ratio. (c) Hysteresis loops taken at 4 K for two incidence angles 0° (normal to the surface, in red) and 60° from the normal (in blue), before annealing (top) and after annealing (bottom). The fits are pictured in green for both phase of the nanoparticles.