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Magnétométrie à micro-SQUID pour l'étude de particules ferromagnétiques isolées aux échelles

Abstract : We present the first magnetization measurements of single submicronic particles at very low temperature (0.1 - 6 K) using micro-bridge-DC-SQUIDs with a flux resolution corresponding to 10^4 µB. We have mainly studied particles patterned by electron-beam lithography (500 - 50 nm), but also wires and clusters.
The angular dependence of the smallest particles, patterned by electron-beam lithography, was close to the model of uniform rotation predicted by Stoner and Wohlfarth. Nevertheless, the magnetisation reversal was triggered by a nucleation process. The temperature (1 to 6 K) and applied field sweeping rate dependence (0.01 to 100 mT/s) of the mean switching field could be described by the simple model of Kurkijärvi based on thermal activated switching. The failure of the model below 1 K can be an indication of macroscopic quantum tunneling. Furthermore we measured the probability for switching at constant applied magnetic field as a function of time and temperature. For the smallest particles it was close to an exponential function. The mean waiting time followed an Arrhenius law, despite temperatures below 1 K a deviation was found which was similar to the one of the mean switching field. Finally, the increase of the width of the switching field distribution at lower temperature is probably due to sample defects (surface roughness, crystalline defects, impurities, etc.) inducing a multi-valley energy landscape revealed at low temperature when the thermal energy is of the same order than the fluctuations of the energy surface.
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https://tel.archives-ouvertes.fr/tel-00010959
Contributor : Wolfgang Wernsdorfer <>
Submitted on : Sunday, November 13, 2005 - 1:54:41 AM
Last modification on : Friday, December 18, 2020 - 1:34:04 PM
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Wolfgang Wernsdorfer. Magnétométrie à micro-SQUID pour l'étude de particules ferromagnétiques isolées aux échelles. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 1996. Français. ⟨tel-00010959⟩

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