Europhys. Lett.
Volume 47, Number 1, July 1999
Page(s) 122 - 127
Section Condensed matter: electronic structure, electrical, magnetic and optical properties
Published online 01 September 2002
DOI: 10.1209/epl/i1999-00361-2

Europhys. Lett, 47 (1), pp. 122-127 (1999)

Dynamical properties of non-interacting Co nanoparticles

M. Respaud 1, M. Goiran 1, J. M. Broto 1, F. Lionti 2, L. Thomas 2, B. Barbara 2
T. Ould Ely 3, C. Amiens 3 and B. Chaudret 3

1 LPMC-SNCMP, Insa, Complexe Scientifique de Rangueil
31077 Toulouse Cedex, France
2 Laboratoire Louis Néel - 25 avenue des Martyrs, 38042 Grenoble Cedex, France
3 Laboratoire de Chimie de Coordination
205 route de Narbonne, 31077 Toulouse Cedex, France

(received 4 December 1998; accepted in final form 20 April 1999)

PACS. 75.50Tt - Fine-particle systems.
PACS. 76.60Es - Relaxation effects.


The dynamical properties of the magnetic moment of non-interacting cobalt nanoparticles dispersed in a polymer were investigated by ac-susceptibility ($10\;{\rm Hz} < f < 10^4\,{\rm Hz}$). We thus estimated the relaxation time of the magnetisation ($\tau$) associated to the superparamagnetic properties of the particles. $\tau$ was found to follow
the general Néel-Brown relaxation dynamics, with a pre-exponential relaxation time $\tau_0$ of $4 (\pm 2) \cdot 10^{-12}\,{\rm s}$ for temperatures between 10 and 20 K. By combining the experimental results from ferromagnetic resonance, ac-susceptibility, and magnetisation, measurements giving the microscopic and macroscopic parameters, we demonstrate for the first time that the Coffey theoretical expressions were able to predict both $\tau$ and $\tau_0$ values. Finally, the thermal dependence of $\tau_0$ is discussed.


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