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Submicron to Nanometer Size Single Particle Measurements

Published online by Cambridge University Press:  15 February 2011

B. Barbara ,
W. Wernsdorfer ,
E. Bonet Orozco ,
K. Hasselbach ,
A. Benoit ,
D. Mailly ,
N. Demoncy ,
H. Pascard ,
L. Francois  and
N. Duxin
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B. Barbara
Affiliation:
LMLN - CNRS, BP166, 38042 Grenoble Cedex 9, France, [email protected]
W. Wernsdorfer
Affiliation:
LMLN - CNRS, BP166, 38042 Grenoble Cedex 9, France, [email protected]
E. Bonet Orozco
Affiliation:
LMLN - CNRS, BP166, 38042 Grenoble Cedex 9, France, [email protected]
K. Hasselbach
Affiliation:
CRTBT - CNRS, BP166, 38042 Grenoble Cedex 9, France
A. Benoit
Affiliation:
CRTBT - CNRS, BP166, 38042 Grenoble Cedex 9, France
D. Mailly
Affiliation:
L2M - CNRS, 196 Av. H. Ravera, 92220 Bagneux, France
N. Demoncy
Affiliation:
SESI - CEA/CNRS, Ecole Polytechnique, 91128 Palaiseau, France
H. Pascard
Affiliation:
SESI - CEA/CNRS, Ecole Polytechnique, 91128 Palaiseau, France
L. Francois
Affiliation:
SRSI, URA CNRS 1662, BP 52, 4 place Jussieu, 75005, Paris
N. Duxin
Affiliation:
SRSI, URA CNRS 1662, BP 52, 4 place Jussieu, 75005, Paris
M.P. Pileni
Affiliation:
SRSI, URA CNRS 1662, BP 52, 4 place Jussieu, 75005, Paris
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Abstract

Low temperature magnetization measurements of individual ferromagnetic particles and wires are presented (0.1 < T(K) < 6). The detector was a Nb micro-bridge-DC-SQUID, fabricated using electron-beam lithography. The angular dependence of the switching field could be explained approximatively by simple classical micromagnetic concepts (uniform rotation, curling…). However, dynamical measurements evidenced nucleation and propagation of domain walls, except for the smallest particles of about 20 nm. The variation of the mean switching field distribution (as a function of temperature and field sweeping rate) and of the probabilities of switching (as a function of temperature and the applied field) allowed to study in details the dynamics of magnetization reversal of individual particles. For sub-micron particles, we found that above a crossover temperature of 1K, the mean switching field and the switching probability follow a thermally activated model. For temperatures below IK, the dynamics of magnetization reversal becomes temperature independent which is interpreted in terms of deviations from the Néel-Brown model of magnetization reversal due to surface roughness and oxidazation. Although this crossovei temperature is much too large to be interpreted with current models of quantum tunneling, such an effect cannot be excluded. Measurements performed on ferromagnetic nanoparticles of good quality (single crystalline and with a diameter smaller than 25 nm), allowed us to show for the first time that the magnetization reversal can be described by thermal activation over the anisotropy energy barrier, as originally proposed by Néel. The observation of telegraph noise strengthens these results. Our measurements open the door to the observation of macroscopic quantum tunneling oí the magnetization in an individual particle containing 103-105 spins.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 475: Symposium M – Magnetic Ultrathin Films, Multilayers, and Surfaces – 1997 , 1997 , 265
Copyright
Copyright © Materials Research Society 1997

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References

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