Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles
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Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles Bernd Rellinghaus, Sonja Stappert, Mehmet Acet, and Eberhard F. Wassermann Experimentelle Tieftemperaturphysik and Sonderforschungsbereich 445, Gerhard-Mercator-Universität, D-47048 Duisburg, Germany. ABSTRACT We report on a non-lithographic method for the preparation of self-assembled FePt nanoparticles via inert-gas condensation. Prior to deposition the particles can be sintered in flight at temperatures as high as TS = 1273 K. Whereas un-sintered particles have irregular shapes, particles sintered at elevated temperatures TS ≥ 793 K show a regular faceting. (High resolution) transmission electron microscopy ((HR)TEM) shows that these regularly faceted particles are of icosahedral structure. When being deposited onto amorphous carbon films, the gas-phase sintered particles are found to have a high mobility. In particular, for the high-temperature sintered FePt nanoparticles, we observe that this mobility leads to the formation of particle arrays with hexagonal close-packed arrangements. Within these ordered patches, the particles are separated from one another. Analytical investigations using energy filtered TEM (EFTEM) show that a carbon layer is formed between the particles. Magnetization analyses give results showing that the gas-phase sintered particles are superparamagnetic at room temperature with a blocking temperature of TB = 49K. INTRODUCTION The reduction of size and dimension in solid state materials leads to a significant alteration in their physical properties. Structural, electrical, chemical, optical and magnetic properties may be tailored by simply controlling the size. Owing to the demand for successive integration and miniaturisation in the consumer electronics industry, the magnetism of nanoparticles and nanostructured materials has gained particular importance in recent years. Here, the research on materials, which may serve as information media in high density magnetic data storage, plays a dominant role. Presently, commercial magnetic storage media are nanocrystalline thin films of CoPtTaCr [1]. In this material, the grain size and the (de)coupling between the grains are key parameters for the quality of the material. Whereas in conventional homogeneous media one bit is made up of as many as 100 grains, presently, there are alternative strategies aiming to use single magnetic entities in a regularly arranged array as a bit [2]. Among the methods employed for the preparation of such patterned structures, are optical interference lithography [3], focused ion beam patterning (FIBP) [4], nanoimprint lithography [5], electron beam lithography (EBL) [6], and others. Here, optical lithography provides the advantage of parallel, thus fast structuring of macroscopic samples, while FIBP and EBL suffer from a slow processing due to serial “writing” of individual structures. On the other hand, whereas the smallest feature sizes and periodicities obtained with optical interference methods are in the range 65-125 nm [7], the structures prep
