Journal of Materials Research

Articles

Magnetization of carbon-coated ferromagnetic nanoclusters determined by electron holography

S. Seraphina1 c1, C. Beelia2 p1, J-M. Bonarda3, J. Jiaoa4 p2, P. A. Stadelmanna5 and A. Châtelaina6

a1 Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721

a2 Centre Interdépartemental de Microscopie Electronique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

a3 Institut de Physique Expérimentale, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

a4 Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721

a5 Centre Interdépartemental de Microscopie Electronique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

a6 Institut de Physique Expérimentale, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

Abstract

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.

(Received November 30 1998)

(Accepted April 02 1999)

Correspondence:

c1 Address all correspondence to this author. e-mail: seraphin@u.arizona.edu

p1 Present address: Laboratory of Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland.

p2 Present address: Department of Physics, Portland State University, Portland, OR 97207, U.S.A.

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