arXiv subjects

W. Wernsdorfer

Publications and source records attributed to W. Wernsdorfer.

79 records · Page 5Linked to original sources

arXiv1999

Classical and quantum magnetisation reversal studied in single nanometer-sized particles and clusters using micro-SQUIDs

Recent progress in experiment on quantum tunnelling of the magnetic moment in mesoscopic systems will be reviewed. The emphasis will be made on measurements of individual nanoparticles. These nanomagnets allow one to test the border between classical and quantum behaviour. Using the micro-SQUID magnetometer, waiting time, switching field and telegraph noise measurements show unambiguously that the magnetisation reversal of small enough single crystalline nanoparticles is described by a model of thermal activation over a single-energy barrier. Results on insulating BaFeO nanoparticles show strong deviations from this model below 0.4 K which agree with the theory of macroscopic quantum tunnelling in the low dissipation regime.

cond-mat.mes-hall↗

arXiv1999

Quantum Hole Digging in Magnetic Molecular Clusters

Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum relaxation regime. We showed recently that the predicted ``square-root time'' relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measured the distribution P(H) of molecules which are in resonance at the applied field H. Tunnelling initially causes rapid transitions of molecules, thereby ``digging a hole'' in P(H). For small initial magnetisation values, the hole width shows an intrinsic broadening which may be due to nuclear spins. We present here hole digging measurements in the thermal activated regime which may allow to study the effect of spin-phonon coupling.

cond-mat.mes-hall↗

arXiv1999

Quantum Phase Interference in Magnetic Molecular Clusters

The Landau Zener model has recently been used to measure very small tunnel splittings in molecular clusters of Fe8, which at low temperature behaves like a nanomagnet with a spin ground state of S = 10. The observed oscillations of the tunnel splittings as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum interference of two tunnel paths of opposite windings. Transitions between quantum numbers M = -S and (S - n), with n even or odd, revealed a parity effect which is analogous to the suppression of tunnelling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry or Haldane phase) in a magnetic system. We show here that the quantum interference can also be measured by ac susceptibility measurements in the thermal activated regime.

cond-mat.mes-hall↗

arXiv1999

Single nanoparticle measurement techniques

Various single particle measuring techniques are briefly reviewed and the basic concepts of a new micro-SQUID technique are discussed. It allows measurements of the magnetization reversal of single nanometer-sized particles at low temperature. The influence of the measuring technique on the system of interest is discussed.

cond-mat.mes-hall↗

arXiv1999

Landau Zener method to study quantum phase interference of Fe8 molecular nanomagnets

We present details about an experimental method based on the Landau Zener model which allows to measure very small tunnel splittings $Δ$ in molecular clusters Fe8. The measurements are performed with an array of micro-SQUIDs. The observed oscillations of Delta as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Transitions between M = -S and (S - n), with n even or odd, revealed a parity (symmetry) effect which is analogous to the suppression of tunneling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system. The influence of intermolecular dipole interactions on the measured tunnel splittings $Δ$ are shown.

cond-mat.mes-hall↗

arXiv1999

Nuclear spin driven resonant tunnelling of magnetisation in Mn12 acetate

Current theories still fail to give a satisfactory explanation of the observed quantum phenomena in the relaxation of the magnetisation of the molecular cluster Mn12 acetate. In the very low temperature regime, Prokof'ev and Stamp recently proposed that slowly changing dipolar fields and rapidly fluctuating hyperfine fields play a major role in the tunnelling process. By means of a faster relaxing minor species of Mn12ac and a new experimental 'hole digging' method, we measured the intrinsic line width broadening due to local fluctuating fields, and found strong evidence for the influence of nuclear spins on resonance tunnelling at very low temperatures (0.04 - 0.3K). At higher temperature (1.5 - 4K), we observed a homogeneous line width broadening of the resonance transitions being in agreement with a recent calculation of Leuenberger and Loss.

cond-mat.mes-hall↗

arXiv1999

Observation of the distribution of molecular spin states by resonant quantum tunneling of the magnetization

Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(ξ_H) of magnetic energy bias ξ_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(ξ_H) (around the resonant condition ξ_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.

cond-mat.mes-hall↗