653 research outputs found
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.Comment: 7 pages, 6 figures, submitted to Europhys. Let
Exchange couplings in the magnetic molecular cluster Mn12Ac
The magnetic properties of the molecular cluster Mn12Ac are due to the four
Mn3+ ions which have spins S=3/2 and the eight Mn4+ ions with spins S=2. These
spins are coupled by superexchange mechanism. We determine the four exchange
couplings assuming a Heisenberg-type interaction between the ions. We use exact
diagonalization of the spin Hamiltonian by a Lanczos algorithm and we adjust
the couplings to reproduce the magnetization curve of Mn12Ac. We also impose
the constraint of reproducing a gap of 35K between a S=10 ground state and a
first excited state with S=9. We predict that there is an excited level with
S=8 at 37K above the ground state, only slightly above the S=9 excited state
which lies at 35K and the next excited state is a S=9 multiplet at 67K above
the S=10 ground state.Comment: 15 pages, 6 figures, submitted to Phys Rev B, corrected a misTeX:
values of J1, J2 have changed, refs update
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.Comment: 3 pages, 2 figures, conference proceedings of LT22 (Helsinki,
Finland, August 4-11, 199
Spin-Wave Description of Nuclear Spin-Lattice Relaxation in Mn_{12}O_{12} Acetate
In response to recent nuclear-magnetic-resonance (NMR) measurements on the
molecular cluster Mn_{12}O_{12} acetate, we study the nuclear spin-lattice
relaxation rate 1/T_1 developing a modified spin-wave theory. Our microscopic
new approach, which is distinct from previous macroscopic treatments of the
cluster as a rigid spin of S=10, not only excellently interprets the observed
temperature and applied-field dependences of 1/T_1 for ^{55}Mn nuclei but also
strongly supports the ^{13}C NMR evidence for spin delocalization over the
entire molecule.Comment: to be published in Phys. Rev. Lett., 4 pages, 4 figures embedde
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 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 are shown.Comment: 6 pages, 14 figures, conference proceedings of MMM 1999, San Jose,
15-18 Nov., session number CD-0
The effects of nuclear spins on the quantum relaxation of the magnetization for the molecular nanomagnet Fe_8
The strong influence of nuclear spins on resonant quantum tunneling in the
molecular cluster Fe_8 is demonstrated for the first time by comparing the
relaxation rate of the standard Fe_8 sample with two isotopic modified samples:
(i) 56_Fe is replaced by 57_Fe, and (ii) a fraction of 1_H is replaced by 2_H.
By using a recently developed "hole digging" method, we measured an intrinsic
broadening which is driven by the hyperfine fields. Our measurements are in
good agreement with numerical hyperfine calculations. For T > 1.5 K, the
influence of nuclear spins on the relaxation rate is less important, suggesting
that spin-phonon coupling dominates the relaxation rate at higher temperature.Comment: 4 pages, 5 figure
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
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