Eur. Phys. J. D 9, 501–504 (1999) T HE E UROPEAN P HYSICAL J OURNAL D EDP Sciences  c Societ`a Italiana di Fisica Springer-Verlag 1999 Magnetic properties of nanosized transition metal colloids: the influence of noble metal coating P.M. Paulus 1 , H. B¨ onnemann 2 , A.M. van der Kraan 3 , F. Luis 1,a , J. Sinzig 1 , and L.J. de Jongh 1 1 Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands 2 Max Planck Institut f¨ ur Kohlenforschung, Postfach 101353, D-45466 M¨ ulheim/Ruhr, Germany 3 Interfacultair Reactor Instituut, Delft University of Technology, 2629 JB Delft, The Netherlands Received: 1 September 1998 / Received in final form: 18 November 1998 Abstract. We present a detailed experimental study of Fe and Co colloidal particles stabilized by organic ligands. Comparison is made between the magnetic properties of pure and gold-coated particles. In all cases, evidence of superparamagnetic behaviour is found from dc susceptibility and hysteresis loops, with blocking at quite different temperatures. The magnetic anisotropy, as obtained from the blocking temperature and the coercive field, is larger for the pure particles than for the bulk material. Saturation magnetization values obtained at low temperatures are smaller than the bulk. We attribute these effects to surface pinning by the ligands. The magnetic anisotropy of the cobalt particles coated with a gold layer is found to be highly re- duced, to a value very close to the bulk. By contrast, the anisotropy of iron colloids coated by gold remains larger than the bulk value, probably because of the formation of an inhomogeneous Fe/Au alloy. PACS. 75.30.Gw Magnetic anisotropy – 82.70.Dd Colloids – 75.30.Pd Surface magnetism The study of fine particles of transition metals can be ap- plied to the development of materials for technological ap- plications such as catalysis and magnetic recording. For in- stance, a good understanding of the physical origin of mag- netic anisotropy can help to further decrease the size of the unit of information. In this paper, we study the magnetic properties of Fe and Co colloids stabilized by organic lig- ands. The ligands can substantially modify the anisotropy and magnetic moment of the metal atoms located at the surface of the particles. In its turn, the influence of sur- face pinning may be modified if the magnetic particles are coated by a noble metal layer, which separates the mag- netic core from the ligand shell. The metal colloids used in our research were prepared by reduction of a metal salt with tetraalkylammonium hydrotriorganoborates, using the method described in [1]. The long-chain tetraalkylammonium salts formed directly at the reduction center act as very effective protecting agents for preventing the particles from coalescence. In a later stage, colloids dispersed in toluene can be treated with AuCl 3 , which slowly dissolves, according to the reac- tion (for the Co colloid) 12 Co(0) + 2 AuCl 3 → Co 9 Au 2 + 3 CoCl 2 . (1) This process is expected to coat the metal particle with Au. The magnetic signal of the paramagnetic CoCl 2 that a Corresponding author. e-mail: fluis@phys.leidenuniv.nl Table 1. Physical parameters of the six samples investigated. x 3d and x Au stand for the concentration in mass of transition metal (Fe or Co) and gold, respectively. The size distribution has been obtained by TEM. The average diameter hdi and the width σ follow from a fit of this distribution to a Gaussian function. Sample x 3d x Au hdi(nm) σ(nm) Co (A1) 0.0382 0 3.0(1) 1.0(1) Co/Au (A2) 0.028 2.8(1) Fe (B1) 0.02 0 3.5(4) 1.0(4) Fe/Au (B2) 0.0158 0.0168 2.0(1) 0.9(1) Fe (C1) 0.58 0 5.4(2) 0.8(2) Fe/Au (C2) 0.0138 0.0222 5.1(2) 1.8(2) is also formed during this process has been subtracted from the data (shown below). The size distribution was obtained directly from the analysis of TEM images. Mag- netic measurements were performed using a commercial SQUID magnetometer. The magnetic susceptibility was obtained by the application of a field H = 100 Oe after the sample was cooled in either zero field (ZFC)or in the pres- ence of the same field (FC). 57 Fe M¨ ossbauer spectra were also obtained for some of the samples at several tempera- tures between 8 K and 200 K. The composition and sizes of the particles are given in Table 1 for the different colloids studied. We first discuss the results obtained on two samples of pure (A1) and gold-coated (A2) Co colloids of an average of 3 nm in diameter. Magnetic susceptibility data for these