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Effets quantiques mésoscopiques d'ions de terres rares faiblement couplés

Abstract : Mesoscopic quantum behavior of diluted rare-earth compounds. In a weakly Ho$^(3+)$-doped non magnetic single-crystal, namely LiYF$_4$, the usual spin-lattice relaxation time $T_1$ may be much longer than the experimental time window at very low temperature. Below T=200mK, staircase-like hysteresis loops are observed at slow field-sweep rates, showing that quantum tunneling of the magnetic moment occurs at resonant applied fields. This mesoscopic behavior is reminiscent of low-frequency quantum fluctuations at avoided level crossings in the electro-nuclear Zeeman diagram of a nearly-isolated single-ion (strong hyperfine coupling). The dynamics is associated with the coherent rotation of the rare-earth electronic momentum and nuclear spin, although the whole process is incoherent due to weak hyperfine couplings related to diamagnetic near-neighbor anions (the nuclear spin bath). Because of a phonon-bottleneck regime, hysteresis loops are strongly modified at faster field-sweep rates, leading to additional magnetization steps due to spin-spin cross-relaxation tunneling processes. Based on a two-ion representation, it is shown that two-ion co-tunneling and dipolar biased-tunneling events now dominate the relaxation of the magnetization, in agreement with ac-susceptibility measurements in a higher temperature regime. This many-body behavior may have some connections with the quantum spin glass problem. In addition to molecular magnets, quantum tunneling of the magnetic moment can now be investigated in details in diluted rare-earth compounds showing a large uniaxial anisotropy. Moreover, such single crystals may become very suitable systems to observe quantum coherence in magnetism and to study dissipative/decoherent phenomena in the framework of mesoscopic magnets.
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https://tel.archives-ouvertes.fr/tel-00007452
Contributor : Romain Giraud <>
Submitted on : Thursday, November 18, 2004 - 7:49:59 PM
Last modification on : Friday, December 18, 2020 - 1:22:02 PM
Long-term archiving on: : Friday, April 2, 2010 - 9:15:36 PM

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Romain Giraud. Effets quantiques mésoscopiques d'ions de terres rares faiblement couplés. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2002. Français. ⟨tel-00007452⟩

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