Structure and Magnetism of Co and CoAg Nanocrystals
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Structure and Magnetism of Co and CoAg Nanocrystals Marina Spasova, Tamara Radetic1, Nelli S. Sobal2, Michael Hilgendorff2, Ulf Wiedwald, Michael Farle, Michael Giersig2, Uli Dahmen1 Institut für Halbleiterphysik und Optik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany 1 NCEM, Lawrence Berkeley Laboratory,1 Cyclotron Road, Berkeley, California 94720 U.S.A. 2 Hahn-Meitner-Institut, Glienicker Strasse 100, 14109 Berlin, Germany ABSTRACT Monodisperse, air-stable Co/CoO and CoAg55 nanoparticles with a mean diameter of about 11 nm have been synthesized using methods of colloidal chemistry. High resolution transmission electron microscopy (TEM) and Electron Energy-Loss Spectroscopy (EELS) element-specific TEM images reveal a multiply-twinned fcc Co metallic core covered with a 2-2.5 nm thick CoO shell. The lattice parameters are in agreement with those of bulk Co and CoO. A shift of the hysteresis loop of 0.4 T, induced by field cooling of the Co/CoO particles, indicates a strong unidirectional exchange anisotropy due to the interaction between the ferromagnetic Co core and the antiferromagnetic CoO shell. CoAg55 composite particles consist of grains of fcc Co and fcc Ag. No evidence for alloy formation was observed. Electron energy-loss and X-ray microanalysis indicate that Co is predominantly found in the surface region of the particles. SQUID magnetometry shows that at room temperature the CoAg55 particles are superparamagnetic while at 90 K a hysteresis loop was detected with a coercive field of 0.07 T and a remanent magnetization of 32 % of the saturation value. INTRODUCTION Two-dimensional lattices of monodisperse ferromagnetic nanoscale particles present a promising approach to the manufacturing of future ultra-high-density magnetic recording media. Arrays of oriented single-domain nanocrystals with large intrinsic magnetic anisotropy may provide magnetic recording at areal densities in the terabits-per-square-inch regime. The selfassembly of colloidal nanoparticles [1-5] provides a low-cost approach for such type of media. Magnetic properties are extremely sensitive to crystal structure, composition of the particles and their surface properties. Nanoparticles consisting of a ferromagnetic (FM) core covered with an antiferromagnetic (AFM) shell exhibit unidirectional exchange anisotropy due to exchange coupling at the FM/AFM interface [6, 7]. In nanoparticles consisting of a dia- or paramagnetic core covered with a FM shell, the ratio of surface (interface)-to-bulk atoms is larger than that in thin films and in monoelement magnetic nanoparticles. Hence, it is an interesting system to study surface and interfacial magnetism such as for example, induced polarization effects in noble metals in contact with 3d FM metals [8]. Here, we report detailed transmission electron microscopy (TEM) studies of crystal structure and chemical composition and also determine the magnetic properties of two types of core-shell colloidal nanoparticles: Co covered with a CoO shell and diamagnetic Ag c
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