Dresden 2003 – wissenschaftliches Programm

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MA: Magnetismus

MA 14: Poster: Schichten(1-31), Spinabh.Trsp.(32-47), Exch.Bias(48-54), Spindyn.(55-64), Mikromag.(65-76), Partikel(77-88), Oflmag.(89-92), Spinelektr.(93-98), Elektr.Theo.(99-103), Mikromag+PhasÜ+Aniso.(104-122), MagnMat.(123-134), Messm+Mol-Mag.(135-139), Kondo(140-151)

MA 14.81: Poster

Dienstag, 25. März 2003, 15:15–19:15, Zelt

Size dependence of the magnetic and decomposition properties of Fe₃C nanoparticles — •Tim Hülser¹, Mehmet Acet¹, Bernd Rellinghaus¹, Eberhard F. Wassermann¹, and Hartmut Wiggers² — ¹Tieftemperaturphysik — ²Institut für Verbrennung und Gasdynamik, Gerhard-Mercator-Universität, 47048 Duisburg

Fe₃C nanoparticles were prepared in a hot wall reactor by the pyrolisis of Fe(CO)₅ and C₂H₄. The yield of Fe₃C was maximized by optimizing the precursor concentration and the reaction pressure and temperature. Transmission electron microscopy shows that the particles have an ellipsoidal geometry with an aspect ratio of approximately 3/4. The average particle size is 30±5 nm. Electron and x-ray diffraction investigations show that the orthorhombic unit cell of the nanoparticles has lattice constants which are about 1% smaller than those of bulk cementite. Magnetization measurements in the temperature range 4 K≤ T ≤ 1000 K give a Curie temperature of 485 K and a saturation magnetization of 130 emu/g, which are values similar to those of the bulk. However, we find also from magnetization measurements that the decomposition of the nanoparticles into ferrite and graphite takes place at roughly T^*=700 K, which is considerably lower than T^*=1130 K for bulk Fe₃C. Since it is known that T^* of bulk Fe₃C is lowered by applying external pressure, it is thought that for nanoparticles, the surface stress is the major cause for the decrease in T^* as well as for the reduction of the lattice constants. Work supported by the Deutsche Forschungsgemeinschaft (SFB 445)