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Register a new userQuantum Coherence in Fe8 Molecular Nanomagnets
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Added by
Joan Manel Hernandez Ferras
Publication date
1998
fields
Physics
and research's language is
English
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We report observation of coherent quantum oscilations in spin-10 Fe8 molecular clusters. The powder of magnetically oriented Fe8 crystallites was placed inside a resonator, in a dc magnetic field perpendicular to the magnetization axis. The field dependence of the ac-susceptibility was measured up to 5 T, at 680 MHz, down to 25 mK. Two peaks in the imaginary part of the susceptibility have been detected, whose positions coincide, without any fitting parameters, with the predicted two peaks corresponding to the quantum splitting of the ground state in the magnetic field parallel and perpendicular to the hard magnetization axis.
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Among the factors determining the quantum coherence of the spin in molecular magnets is the presence and the nature of nuclear spins in the molecule. We have explored modifying the nuclear spin environment in Cr$_7$Ni-based molecular nanomagnets by replacing hydrogen atoms with deuterium or the halogen atoms, fluorine or chlorine. We find that the spin coherence, studied at low temperatures by pulsed electron spin resonance, is modified by a range of factors, including nuclear spin and magnetic moment, changes in dynamics owing to nuclear mass, and molecular morphology changes.
Quantum simulators are controllable systems that can be used to simulate other quantum systems. Here we focus on the dynamics of a chain of molecular qubits with interposed antiferromagnetic dimers. We theoretically show that its dynamics can be controlled by means of uniform magnetic pulses and used to mimic the evolution of other quantum systems, including fermionic ones. We propose two proof-of-principle experiments, based on the simulation of the Ising model in transverse field and of the quantum tunneling of the magnetization in a spin-1 system.
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I. S. Tupitsyn Pacific Institute for Theoretical Physics
, n University of British Columbia
2004
The spin coherence phenomena and the possibility of their observation in nanomagnetic insulators attract more and more attention in the last several years. Recently it has been shown that in these systems in large transverse magnetic field there can be a fairly narrow coherence window for phonon and nuclear spin-mediated decoherence. What kind of spin dynamics can then be expected in this window in a crystal of magnetic nanomolecules coupled to phonons, to nuclear spin bath and it to each other via dipole-dipole interactions? Studying multispin correlations, we determine the region of parameters where coherent clusters of collective spin excitations can appear. Although two particular systems, namely crystals of $Fe_8$-triazacyclonane and $Mn_{12}$-acetate molecules, are used in this work to illustrate the results, here we are not trying to predict an existence of collective coherent dynamics in some particular system. Instead, we discuss the way how any crystalline system of dipole-dipole coupled nanomolecules can be analyzed to decide whether this system is suitable for attempts to observe coherent dynamics. The presented analysis can be useful in the search for magnetic systems showing the spin coherence phenomena.
We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6%.
The magnetization of the prototypical molecular magnet Mn12-acetate exhibits a series of sharp steps at low temperatures due to quantum tunneling at specific resonant values of magnetic field applied along the easy c-axis. An abrupt reversal of the magnetic moment of such a crystal can also occur as an avalanche, where the spin reversal proceeds along a deflagration front that travels through the sample at subsonic speed. In this article we review experimental results that have been obtained for the ignition temperature and the speed of propagation of magnetic avalanches in molecular nanomagnets. Fits of the data with the theory of magnetic deflagration yield overall qualitative agreement. However, numerical discrepancies indicate that our understanding of these avalanches is incomplete.
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