Dresden 2006 – scientific programme

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MA: Magnetismus

MA 36: Magnetic Coupling Phenomena / Exchange-Bias

MA 36.8: Talk

Friday, March 31, 2006, 12:30–12:45, HSZ 401

Magnetic Properties of partly oxidized Ni Nanoparticles — •U. Kreibig¹, V. Schneider¹, A. Reinholdt¹, T. Weirich², A. Tillmanns³, H. Krenn⁴, K. Rumpf⁴, and P. Granitzer⁴ — ¹I.Institute of Physics (IA) RWTH Aachen University D-52056 Aachen, Germany — ²Gemeinschaftslabor fuer Elektronenmikroskopie RWTH Aachen University D-52056 Aachen, Germany — ³II.Institute of Physics (IIA) RWTH Aachen University D-52056 Aachen, Germany — ⁴Institute of Experimental Physics Karl-Franzens-University A-8010 Graz, Austria

Ni nanoparticles (mean diameters 5 to 10 nm) were produced by our high efficiency laser evaporation source LUCAS. They were deposited and stepwise oxidized in UHV. Electron microscopic analysis proved the formation of oxide shells around a Ni core. Magnetization was measured by SQUID in the temperature range 5 to 400 K. Alternatively two kinds of hysteris loops were observed in different samples: 1) A single, uniform loop which is strongly shifted by exchange bias. 2) A double hysteresis, consisting of a superposition of two loops, without any exchange bias shift. A qualitative explanation will be given, based upon the magnetic properties of the Ni-oxide shell which deviates from a perfect antiferromagnet. The strength of the remaining anisotropy field of the shell probably depends on the structural and chemical details of the core-shell interface. Within the investigated temperature range no hint of superparamagnetism was observed.