Magnetization of carbon-coated ferromagnetic nanoclusters determined by electron holography
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Magnetization of carbon-coated ferromagnetic nanoclusters determined by electron holography S. Seraphina) Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721
C. Beelib) Centre Interde´partemental de Microscopie Electronique, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
J-M. Bonard Institut de Physique Expe´rimentale, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
J. Jiaoc) Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721
P.A. Stadelmann Centre Interde´partemental de Microscopie Electronique, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
A. Chaˆtelain Institut de Physique Expe´rimentale, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland (Received 30 November 1998; accepted 2 April 1999)
The magnetic properties of carbon-coated Co and Ni nanoparticles aligned in chains were determined using transmission electron holography. The measurements of the phase change of the electron wave due to the magnetization of the sample were performed. The ratio of remnant magnetization to bulk saturation magnetization Mr /Ms of Co decreased from 53% to 16% and of Ni decreased from 70% to 30% as the particle diameter increased from 25 to 90 nm. It was evident that the inhomogenous magnetic configurations could diminish the stray field of the particles. After being exposed to a 2-Tesla external magnetic field, the Mr /Ms of Co increased by 45% from the original values with the same dependency on the particle size. The Mr /Ms of Ni particles, on the other hand, increased only 10%. The increased magnetization could be attributed to the merging of small domains into larger ones after the exposure to the external magnetic field. The validity of the interpretation of the holograms was established by simulation.
I. INTRODUCTION
Ferromagnetism is a collective phenomenon which is caused by the mutual interaction of many atoms and their electrons. As a consequence, the size, composition, microstructure, and morphology of a ferromagnetic sample influence its magnetic properties. Various models have been used to interpret different aspects of ferromagnetism.1 The interaction can be considered limited to nearest neighbors, placing emphasis on the properties of
a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Laboratory of Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland. c) Present address: Department of Physics, Portland State University, Portland, OR 97207, U.S.A. J. Mater. Res., Vol. 14, No. 7, Jul 1999
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the atoms involved (Heisenberg model). Other models require that the itinerant properties of the electrons are considered (bandstructure model). The reality lies in between both models, dependent on the size and microstructure of the sample. The understanding gained new inputs by the p
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