Layer Resolved Structural Relaxation at the Surface of Magnetic FePt Icosahedral Nanoparticles

The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of ${\mathrm{Fe}}_{52}{\mathrm{Pt}}_{48}$. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich $\mathrm{Fe}/\mathrm{Pt}$ core.

Figure 1

(a) Formation of an icosahedron by a suitable combination of 20 tetrahedra, compressive strain is required to accommodate the tetrahedra. (b) Distribution of the FePt nanoparticles in standard bright field TEM.

Figure 2

Experimental exit wave phase images of the same representative FePt nanoparticle exposed to a current density of $\sim 20\mathrm{A}/{\mathrm{cm}}^2$ at 300 kV for 0, 20, and 30 min [panels (a), (b), and (c), respectively]. White arrows mark partially occupied shells. Dark arrows mark edge columns that are commonly missing. The inset of (a) is a Fourier transform of the phase image showing an information limit of 0.090 nm. Note the successive removal of the surface layers effectively reducing the size of the particle. Panel (d) shows line profiles taken from the indicated areas of the real space images. Details as narrowly spaced as 0.09 nm can be resolved.

Figure 3

Modeling and simulations. (a) Proposed icosahedron model with 10 179 randomly distributed Fe and Pt atoms. (b) Simulated phase image from the proposed model matches the experiments from Fig. 2.

Figure 4

Quantitative data analyses. (a) Column position map of the particle shown in Fig. 2a. (b) Shell spacing of Fig. 2 as function of shell number. Solid lines are fitted exponential functions. The symbols I, II, and III refer to Figs. 2a, 2b, 2c, respectively. The smaller particle shows the largest outward $d_{111}$ relaxation. The bulk value of ${\mathrm{Fe}}_{52}{\mathrm{Pt}}_{48}$ is $d_{111}=0.2198\mathrm{nm}$. The inset compares the average shell and column spacing that increase as the particle shrinks.