•Keir Foster¹, Jörg Weissmüller^1,2, Raghavan Nadar Viswanath¹, Dominik Kramer¹, Branko Stahl³ und Jens Ellrich^3
1Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe, Germany
²Universität des Saarlandes, Fachrichtung Technische Physik, Saarbrücken, Germany
³U Darmstadt, Materials Science Dep., Petersenstr. 23, 64287 Darmstadt, Germany

A small-angle X-ray (SAXS) study and magnetic characterization of dilute and non-dilute samples of FePt nanoparticles suspended in hexane have been conducted. Particle-size distributions were derived from the SAXS data for dilute samples via a maximum entropy method. A mean particle size of 4.0 nm was obtained, in good agreement to that determined by transmission electron microscopy. SAXS measurements on more condensed samples show a series of clear maxima for q = 1-4nm^-1 indicating a degree of local ordering is present. Magnetic characterization by vibrating sample magnetometry (VSM) and AC susceptibility measurements indicate superparamagnetic behaviour with a blocking temperature of T_B = 13,1 +/- 0,3 K. The real component of the susceptibility showed frequency dependent maxima for measurement frequencies of 0,1-10 kHz and were successfully analysed in terms of the Neel theory of superparamagnetism. The crucial role of sample concentration on the inter-particle distance, and hence on the dipole-dipole interactions between particles, and how this effects the observed magnetic properties will be discussed.