Jahresbericht 2003 der Arbeitsgruppe Experimentalphysik Prof. Dr. Michael Farle

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1 Self-assembly of Fe x Pt 1-x nanoparticles. M. Ulmeanu, B. Stahlmecke, H. Zähres and M. Farle Institut für Physik, Universität Duisburg-Essen, Lotharstr. 1, Duisburg Future magnetic storage media are expected to consist of the smallest possible magnetic particles, which are periodically arranged in a 2D array. Each particle may then serve as individual bit. Two major problem are to be solved to realize this concept. In order to overcome the superparamagnetic limit, the magnetic materials must have a very high magnetic anisotropy, as it is, e.g. found in L1 0 ordered FePt [1]. Also, the preparation method must be suitable to produce large area 2D periodic particle arrangements. Wet chemical routes have proven to allow for the preparation of such periodic particle arrangements. The goal of this work is to develop a better understanding of the self-assembling of aprox. 3 nm monodisperse Fe x Pt 1-x nanoparticles, provided by S. Sun (IBM). The preparation procedure and the structural investigations carried on these systems are described in detail elsewhere [2]. Amorphous carbon covered Cu grid (conventional transmission electron microscopy (TEM) grids) and native oxidized Si substrates were used for the TEM and scanning electron microscopy (SEM) investigations. The Fe x Pt 1-x colloidal suspension was diluted with hexane and the drying process was carried out in air at room temperature. For the Si substrates a simple procedure was employed in order to overcome the wettability phenomena due to the small contact angle that the hexane solvent is presenting in interaction with a Si oxide native layer. In the Figure 1(a) is presented a schematic diagram of a pinned droplet of liquid with a contact angle θ between 0 and 90 (the drying process of this kind of droplet involves the appearance of the so-called coffee rings effect). The figure 1(b) represent the solution that we used which let the free ( unpinned ) droplet to pass over the substrate area (by simply using a larger volume of solution), providing the possibility of drying like a very thin film. In this simple way, well ordered self-assembled monolayer of nanoparticles can be created on native oxidized Si substrates in a reproducible way. AG Farle Seite -1-

2 Figure 1. Schematic diagram of the wetting behaviour of a pinned droplet (A) and free unpinned droplet (B) of a solution containing hexane and Fe x Pt 1-x nanoparticles drying in air at room temperature on a native oxidized Si substrate. This simple and effective solution has less or no effect on the conventional TEM grids since the time of drying is much smaller than in the case of Si substrate (a few seconds, comparative with tens of seconds for the Si substrate) due to a very thin substrate and a high diffusion of the liquid solution on this kind of substrates. We present in Figure 2 two comparative images of nanoparticles on the amorphous carbon TEM grid. While the Figure 2(a) presents a 2D hexagonal close packed (hcp) arrangement of a monolayer of nanoparticles, in Figure 2(b) we present a typical 2D arrangement of a double layer of nanoparticles. We would like to point out that the disturbance of the hcp arrangement is a good indication of the presence of more than one layer of nanoparticles presented on the substrate. Figure 2. TEM micrograph of the Fe 43 Pt 57 nanoparticles dried on an amorphous carbon covered Cu grid. (a) the hexagonal closed packed (hcp) arrangement of a monolayer of nanoparticles (b) the disturbed hcp arrangement for a double layer of nanoparticles.

3 To obtain ordered Fe x Pt 1-x nanoparticles on Si substrates we employed the free unpinned droplet method. The figures 3, 4 and 5 are high resolution (HR) SEM images of Fe x Pt 1-x nanoparticles deposited onto monolayers on Si substrates. In order to assure large coverage areas on the substrate an optimum equilibrium between the concentration of the nanoparticles and the volume of the hexane solvent has to be found. The only difference between the above mentioned figures is the concentrations of the nanoparticle in the volume of liquid. A solution of tee-grey colour was initial prepared, this corresponding to the Figure 3. Then additional concentrated solution was added, until the colour became dark grey, corresponding to the Figures 4 and 5. The variation of the concentration induces low coverage areas with holes (like in the Figure 3 (a), (b) and (c)) with short range hcp ordering, areas with tens of micrometers uniformly covered with nanoparticles, as presented in Figure 4 and finally areas with long range ordering of the nanoparticles as presented in the Fourier transformation image of Figure 5 (d). Figure 3. (a) SEM image of 3.6 nm Fe 43 Pt 57 nanoparticles deposited on Si substrate (b) and (c) magnification of the marked section in the figure (a) indicating an inhomogeneous coverage of the areas (the black areas represents the Si substrate).

4 Figure 4. (a) SEM image of 3.6 nm Fe 48 Pt 52 nanoparticles deposited on Si substrate (b) zoom in the the centrum of the image (a). Figure 5. (a) SEM image of 3.6 nm Fe 48 Pt 52 nanoparticles deposited on Si substrate (native oxidized). (b) and (c) magnification of the marked section in the figure (a) indicating regions with long range hcp arrangement (in the order of ~ 200 nm). The Fourier Transformation presented in image (d) is given to show the ordered structure.

5 This simple and effective method assures good coverage with ordered self-assembling of nanoparticles on Si substrates, allowing us to create reproducible sample with a good knowledge of the topography of the surface. These important aspects can be very useful for future magnetic measurements. In conclusion, it has been shown that the ordering of self-assembling Fe x Pt 1-x nanoparticles by simply drying in air condition on various substrates it is possible. The distribution of the particles on the surface can be described by a hcp structure with local order. The size of the ordered monolayer assembling can be as large as tens of micrometers, while the long range hcp arrangement is observed on areas of hundreds of nanometers. Acknowledgments: This work was supported by EC Human Grant No. HPRN-CT [1] D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, M. F. Toney, M. Schwickert, J. U. Thiele, and M. F. Doerner, IEEE Trans. Magn. 36, (2000). [2] S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, (2000).