Multiply Twinned Structures in Gas-Phase Sintered Stoichiometric FePt Nanoparticles
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Multiply Twinned Structures in Gas-Phase Sintered Stoichiometric FePt Nanoparticles Sonja Stappert, Bernd Rellinghaus, Mehmet Acet, and Eberhard F. Wassermann Experimentelle Tieftemperaturphysik and Sonderforschungsbereich 445, Gerhard-Mercator Universität, D-47048 Duisburg, Germany. ABSTRACT Stoichiometric FePt nanoparticles with sizes in the range 3 – 6 nm were prepared in the gas-phase and thermally sintered prior to deposition. Whereas unsintered particles exhibit irregular shapes and a low degree of crystallinity, the majority of the sintered particles are multiply twinned and have predominantly icosahedral structure. There is no indication for the occurance of L10 ordering in the gas-phase sintered particles, although previous post-deposition annealing experiments of unsintered particles had shown the occurrence of partial formation of the L10 FePt intermetallic phase. On the other hand, analysis of the structural data obtained from (high resolution) electron microscopy shows that the relative amount of icosahedral particles increases with increasing sintering temperature. This result is discussed on the basis of a structure model of an irregular icosahedron, which is built up from distorted tetrahedral building blocks. The distortion in each tetrahedron is in accordance with the tetragonally compressed unit cell of the L10-phase. INTRODUCTION Nanostructured arrays of ferromagnetic FePt alloys with concentrations around the stoichiometric composition are promising candidates for magnetic media in high density magnetic data storage [1]. The thermodynamic equilibrium structure of FePt is the chemically ordered tetragonal phase L10 which exhibits a large magneto-crystalline anisotropy. This gives the possibility to prepare ultra-small and thermally stable nano-magnets [1,2]. Several methods have been applied for preparing nano-crystalline or nano-particulate films of L10-ordered FePt, FePd, or CoPt. Besides DC-sputtering of thin films [3], another promising approach is to prepare nanoparticles surrounded by an organic shell using a wet-chemical method [4]. Particles produced by this technique broaden the application potential, since the organic shell allows for large scale regular arrangement of the FePt nanoparticles via self-organization. In bulk FePt alloys, the formation of the L10 phase is kinetically suppressed, and thermal annealing at temperatures as high as about 1000°C [2] is needed to obtain the L10 tetragonal phase. In FePt thin films, on the other hand, the required annealing temperatures for the formation of the ordered phase is found to be significantly reduced [3]. This is ascribed to the enhanced diffusivity in these low dimensional systems. So far, FePt nanoparticles obtained from various techniques described in the literature do not have the L10-ordered state in the as-prepared state, but are rather in the disordered face-centered cubic (fcc) structure. Thus, post-preparation annealing at temperatures around Tann=500°C is necessary to induce L10 ordering [4]. However, depending on the details o
