FePt Icosahedra with Magnetic Cores and Catalytic Shells

Abstract

Surprisingly oxidation resistant icosahedral FePt nanoparticles showing hard-magnetic properties have been fabricated by an inert-gas condensation method with in-flight annealing. High-resolution transmission electron microscopy (HRTEM) images with sub-Angstrom resolution of the nanoparticle have been obtained with focal series reconstruction, revealing noncrystalline nature of the nanoparticle. Digital dark-field method combined with structure reconstruction as well as HRTEM simulations reveal that these nanoparticles have icosahedral structure with shell periodicity. Localized lattice relaxations have been studied by extracting the position of individual atomic columns with a precision of about (0.002 nm. The lattice spacings of (111) planes from the surface region to the center of the icosahedra are found to decrease exponentially with shell numbers. Computational studies and energy-filtered transmission electron microscopy analyses suggest that a Pt-enriched surface layer is energetically favored and that site-specific vacancies are formed at the edges of facettes, which was experimentally observed. The presence of the Pt-enriched shell around an Fe/Pt core explains the environmental stability of the magnetic icosahedra and strongly reduces the exchange coupling between neighboring particles, thereby possibly providing the highest packing density for future magnetic storage media based on FePt nanoparticles.

Influence of size, shape and core–shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiOx

“…The image corresponds to a McKay icosahedral geometry, where the icosahedron is assembled from single crystal tetrahedra with (111) faces [28,32] (see Figure 2c). This type of structure, as previously observed in other fcc metal NPs [22,23,28,33], can be ascribed to the dynamics of NP growth. In particular, it was found that formation of icosahedra is favoured at fast quenching rates for fcc metal NPs [33][34][35].…”

A guideline for atomistic design and understanding of ultrahard nanomagnets

“…Current efforts further concentrate on improving permanent hard magnets, which are required for future energy-harvesting technologies 3 , while recently also exchange-spring composites based on soft and hard magnetic nanoparticles have been discussed for advanced applications of permanent magnets 4 . Another example is the combination of catalytic activity with magnetic response 5 . Designing new types of bimetallic multifunctional nanoparticles for all of these applications requires the atomistic understanding of the origin of the particles' magnetism, for example, if it is dominated by the large surface fraction of the atoms in a lower coordinated electronic state or dominated by intrinsic lattice strains.…”

“…FePt nanoparticles were synthesized by thermal decomposition of Fe(CO) 5 and Pt(acac) 2 as described elsewhere 23 . The particles are in the chemically disordered state as the formation of the chemically ordered state-which is the equilibrium state at room temperature-is kinetically suppressed.…”

Effect of structure on the magnetic anisotropy ofL10FePt nanoparticles

“…Moreover, several experimental studies support the existence of a minimum size limit below which the L1 0 order can no longer be achieved, 51,52 and other studies have shown the migration of Pt atoms towards the surface in smaller particles. 53,54 However, for more definitive answers concerning feasibility of exploiting FePt nanoparticles in the applications mentioned in the Introduction, detailed and systematic studies are in order.…”