90 research outputs found
Application of the discrete Wentzel-Kramers-Brillouin method to spin tunneling
A discrete version of the WKB method is developed and applied to calculate
the tunnel splittings between classically degenerate states of spin
Hamiltonians. The results for particular model problems are in complete accord
with those previously found using instanton methods. The discrete WKB method is
more elementary and also yields wavefunctions.Comment: RevTex, 2ps figures, incorporates errata (typos and omitted
reference) in JMP pape
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
Scaling analysis of a divergent prefactor in the metastable lifetime of a square-lattice Ising ferromagnet at low temperatures
We examine a square-lattice nearest-neighbor Ising quantum ferromagnet
coupled to -dimensional phonon baths. Using the density-matrix equation, we
calculate the transition rates between configurations, which determines the
specific dynamic. Applying the calculated stochastic dynamic in Monte Carlo
simulations, we measure the lifetimes of the metastable state. As the magnetic
field approaches at low temperatures, the lifetime prefactor diverges
because the transition rates between certain configurations approaches zero
under these conditions. Near and zero temperature, the divergent
prefactor shows scaling behavior as a function of the field, temperature, and
the dimension of the phonon baths. With proper scaling, the simulation data at
different temperatures and for different dimensions of the baths collapse well
onto two master curves, one for and one for .Comment: published versio
Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others
The dynamical theory of thermally activated resonant magnetization tunneling
in uniaxially anisotropic magnetic molecules such as Mn_12Ac (S=10) is
developed.The observed slow dynamics of the system is described by master
equations for the populations of spin levels.The latter are obtained by the
adiabatic elimination of fast degrees of freedom from the density matrix
equation with the help of the perturbation theory developed earlier for the
tunneling level splitting [D. A. Garanin, J. Phys. A, 24, L61 (1991)]. There
exists a temperature range (thermally activated tunneling) where the escape
rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias
field due to tunneling at the top of the barrier. At lower temperatures this
regime crosses over to the non-Arrhenius law (thermally assisted tunneling).
The transition between the two regimes can be first or second order, depending
on the transverse field, which can be tested in experiments. In both regimes
the resonant maxima of the rate occur when spin levels in the two potential
wells match at certain field values. In the thermally activated regime at low
dissipation each resonance has a multitower self-similar structure with
progressively narrowing peaks mounting on top of each other.Comment: 18 pages, 8 figure
Quantum Fluctuations in Large-Spin Molecules
A new type of mesoscopic quantum effect in large-spin molecules possessing
easy-axis anisotropy, such as Mn12, is predicted. The response of such a system
to an external field applied perpendicular to the easy axis is considered. It
is shown that the susceptibility of this system exhibits a peculiar peak of
purely quantum origin. This effect arises from very general properties of
quantum fluctuations in spin systems. We demonstrate that the effect is
entirely accessible for contemporary experimental techniques. Our studies show
that the many-spin nature of the Mn12 clusters is important for a correct
description of this quantum peak.Comment: REVTeX, 3 pages, 6 figures (postscript
A Distribution of Tunnel Splittings in Mn-Acetate
In magnetic fields applied parallel to the anisotropy axis, the relaxation of
the magnetization of Mn measured for different sweep rates is shown to
collapse onto a single scaled curve. The form of the scaling implies that the
dominant symmetry-breaking process that gives rise to tunneling is a locally
varying second-order anisotropy, forbidden by tetragonal symmetry in the
perfect crystal, which gives rise to a broad distribution of tunnel splittings
in a real crystal of Mn-acetate. Different forms applied to even and
odd-numbered steps provide a distinction between even step resonances
(associated with crystal anisotropy) and odd resonances (which require a
transverse component of magnetic field).Comment: 4 pages, 5 figures. New title; text more clearly writte
Spin Tunneling and Phonon-assisted Relaxation in Mn12-acetate
We present a comprehensive theory of the magnetization relaxation in a
Mn12-acetate crystal in the high-temperature regime (T>1 K), which is based on
phonon-assisted spin tunneling induced by quartic magnetic anisotropy and weak
transverse magnetic fields. The overall relaxation rate as function of the
longitudinal magnetic field is calculated and shown to agree well with
experimental data including all resonance peaks measured so far. The Lorentzian
shape of the resonances, which we obtain via a generalized master equation that
includes spin tunneling, is also in good agreement with recent data. We derive
a general formula for the tunnel splitting energy of these resonances. We show
that fourth-order diagonal terms in the Hamiltonian lead to satellite peaks. A
derivation of the effective linewidth of a resonance peak is given and shown to
agree well with experimental data. In addition, previously unknown spin-phonon
coupling constants are calculated explicitly. The values obtained for these
constants and for the sound velocity are also in good agreement with recent
data. We show that the spin relaxation in Mn12-acetate takes place via several
transition paths of comparable weight. These transition paths are expressed in
terms of intermediate relaxation times, which are calculated and which can be
tested experimentally.Comment: 18 pages, 22 EPS figures, REVTe
Level splittings in exchange-biased spin tunneling
The level splittings in a dimer with the antiferromagnetic coupling between
two single-molecule magnets are calculated perturbatively for arbitrary spin.
It is found that the exchange interaction between two single-molecule magnets
plays an important role in the level splitting. The results are discussed in
comparison with the recent experiment.Comment: 12 pages, to be published in Phys. Rev.
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
Resonant Magnetization Tunneling in Mn12 Acetate: The Absence of Inhomogeneous Hyperfine Broadening
We present the results of a detailed study of the
thermally-assisted-resonant-tunneling relaxation rate of Mn12 acetate as a
function of an external, longitudinal magnetic field and find that the data can
be fit extremely well to a Lorentzian function. No hint of inhomogeneous
broadening is found, even though some is expected from the Mn nuclear hyperfine
interaction. This inconsistency implies that the tunneling mechanism cannot be
described simply in terms of a random hyperfine field.Comment: Some minor revisions, title changed, updated figures, two added
notes, one added reference. RevTeX, 4 pages, 3 postscript figures. Submitted
to Rapid Communication
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