Layer Resolved Structural Relaxation at the Surface of Magnetic FePt Icosahedral Nanoparticles R. M. Wang, 1,2, * O. Dmitrieva, 3 M. Farle, 3,† G. Dumpich, 3 H. Q. Ye, 4 H. Poppa, 2 R. Kilaas, 2 and C. Kisielowski 2,‡ 1 School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA 3 Fachbereich Physik and Center for Nanointegration, Universita ¨t Duisburg-Essen, 47048 Duisburg, Germany 4 Department of Physics, Peking University, Beijing 100871, People’s Republic of China (Received 10 July 2007; published 10 January 2008) 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 Fe 52 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 Fe=Pt core. DOI: 10.1103/PhysRevLett.100.017205 PACS numbers: 75.50.Bb, 75.25.+z, 75.50.Ss, 75.75.+a The structure of small particles is a topic which has attracted interest for more than a century [1]. Icosahedral-shaped particles can be described as a platonic solid in which each tetrahedron is distorted. Such non- crystallographic structures were called multiply twinned particles or MTPs. There are substantial discussions as to their growth mechanism [2]. Recently, increasing evidence was gathered that nanoparticles and clusters possess ‘‘shell periodicity’’ and grow by the accretion of atomic layers. The shell periodicity imposes certain restrictions on the symmetry of the clusters following Plato’s five geometric bodies [3 – 5]. Excellent examples of such shell structures are Mackay icosahedra [6], which are composed of 20 twin-related tetrahedra packed along (111) faces as is shown in Fig. 1(a). It is seen from the geometrical model that an elastic deformation of the individual tetrahedra is required, if merged into icosahedra, which is why one expects that such particles are compressed. However, de- spite numerous studies on their structure (see, for example, [3 – 5]), no direct experimental evidence of such shell for- mation or compression from atomically resolved images has been reported. A topic which is of similar interest — but one that has been hardly addressed — is the question of whether surface reconstructions occur on nanoparticle sur- faces the same way as in the bulk [7]. Localized lattice relaxations or surface reconstructions of individual nano- particles have not been reported either because of limita- tions of previous transmission electron microscopy (TEM) studies [8 –10]. Recent advances in TEM imaging ap- proaches such as the reconstruction of electron exit waves from focal series of lattice images extend the resolution to the sub-angstrom regime to correct for dominant aberra- tions of the objective lens [11,12]. This has enabled us to directly explore the lattice structure and reconstruction in the surface region of FePt icosahedra with lateral dimen- sions of only 9 to 11 atoms. For our investigation we chose the bimetallic alloy FePt, which — due to its large magnetic anisotropy density — has been identified as the best candidate for the fabrication of future ultrahigh density magnetic recording media based on self-assembled arrays of magnetic nanoparticles (NPs). These have been synthesized using organometallic chem- istry or gas phase condensation methods [13]. The latter method affords the possibility to form differently shaped nanoparticles such as icosahedra, decahedra, and cubocta- hedra and also larger facetted spheres [14]. The transfor- mation to the L1 0 phase in FePt NPs smaller than 4 nm has never been unambiguously identified, and the possibility of its formation in small NPs has been questioned based on the influence of surface composition and segregation [15]. Commonly in these discussions, a uniform crystal structure across the NP is assumed; that is to say, the influence of a FIG. 1. (a) Formation of an icosahedron by a suitable combi- nation of 20 tetrahedra, compressive strain is required to accom- modate the tetrahedra. (b) Distribution of the FePt nanoparticles in standard bright field TEM. PRL 100, 017205 (2008) PHYSICAL REVIEW LETTERS week ending 11 JANUARY 2008 0031-9007= 08=100(1)=017205(4) 017205-1  2008 The American Physical Society