275 research outputs found
Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe to Fe
Using an efficient numerical scheme that exploits spatial symmetries and
spin-parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for ferric wheels up to Fe. The largest calculation
involves the Fe ring which spans a Hilbert space dimension of about 145
million for M=0 subspace. Our calculated gaps from the singlet ground state
to the excited triplet state agrees well with the experimentally measured
values. Study of the static structure factor shows that the ground state is
spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be
viewed as quantized states of a rigid rotor with the gap between the ground and
the first excited state defining the inverse of moment of inertia. We have
studied the quantum dynamics of Fe as a representative of ferric wheels.
We use the low-lying states of Fe to solve exactly the time-dependent
Schr\"odinger equation and find the magnetization of the molecule in the
presence of an alternating magnetic field at zero temperature. We observe a
nontrivial oscillation of magnetization which is dependent on the amplitude of
the {\it ac} field. We have also studied the torque response of Fe as a
function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure
Macroscopic quantum coherence in mesoscopic ferromagnetic systems
In this paper we study the Macroscopic Quantum Oscillation (MQO) effect in
ferromagnetic single domain magnets with a magnetic field applied along the
hard anistropy axis. The level splitting for the ground state, derived with the
conventional instanton method, oscillates with the external field and is
quenched at some field values. A formula for quantum tunneling at excited
levels is also obtained. The existence of topological phase accounts for this
kind of oscillation and the corresponding thermodynamical quantities exhibit
similar interference effects which resembles to some extent the electron
quantum phase interference induced by gauge potential in the Aharonov-Bohm
effect and the -vacuum in Yang-Mills field theory..Comment: 12 pages, 4 figures, to appear in Phys. Rev.
Quantum-Classical Phase Transition of Escape rate in Biaxial Spin Particles
The escape rates of the biaxial single domain spin particles with and without
an applied magnetic field are investigated. Using the strict potential field
description of spin systems developed by Ulyanov and Zaslavskii we obtain new
effective Hamiltonians which are considered to be in exact spin-coordinate
correspondence unlike the well studied effective Hamiltonians with the
approximate correspondence. The sharp first-order transition is found in both
cases. The phase diagram of the transitions depending on the anisotropy
constant and the external field is also given.Comment: 15 pages, 8 figure
Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac
The crossover from thermal hopping to quantum tunneling is studied. We show
that the decay rate with dissipation can accurately be determined near
the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin
(WKB) exponent, we also calculate contribution of the fluctuation modes around
the saddle point and give an extended account of a previous study of crossover
region. We deal with two dangerous fluctuation modes whose contribution can't
be calculated by the steepest descent method and show that higher order
couplings between the two dangerous modes need to be taken into considerations.
At last the crossover from thermal hopping to quantum tunneling in the
molecular magnet Mn_{12}Ac is studied.Comment: 10 pages, 3 figure
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
Nonadiabatic Pauli susceptibility in fullerene compounds
Pauli paramagnetic susceptibility is unaffected by the electron-phonon
interaction in the Migdal-Eliashberg context. Fullerene compounds however do
not fulfill the adiabatic assumption of Migdal's theorem and nonadiabatic
effects are expected to be relevant in these materials. In this paper we
investigate the Pauli spin susceptibility in nonadiabatic regime by following a
conserving approach based on Ward's identity. We find that a sizable
renormalization of due to electron-phonon coupling appears when
nonadiabatic effects are taken into account. The intrinsic dependence of
on the electron-phonon interaction gives rise to a finite and negative isotope
effect which could be experimentally detected in fullerides. In addition, we
find an enhancement of the spin susceptibility with temperature increasing, in
agreement with the temperature dependence of observed in fullerene
compounds. The role of electronic correlation is also discussed.Comment: Revtex, 10 pages, 8 figures include
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
Magnetic Field Dependence of Macroscopic Quantum Tunneling and Coherence of Ferromagnetic Particle
We calculate the quantum tunneling rate of a ferromagnetic particle of diameter in a magnetic field of arbitrary angle. We consider the
magnetocrystalline anisotropy with the biaxial symmetry and that with the
tetragonal symmetry. Using the spin-coherent-state path integral, we obtain
approximate analytic formulas of the tunneling rates in the small -limit for the magnetic field normal to the easy axis (), for the field opposite to the initial easy axis (),
and for the field at an angle between these two orientations (). In addition, we obtain numerically the tunneling rates for
the biaxial symmetry in the full range of the angle of the magnetic
field (), for the values of \epsilon =0.01 and
0.001.Comment: 25 pages of text (RevTex) and 4 figures (PostScript files), to be
published in Phys. Rev.
Tunneling of a large spin via hyperfine interactions
We consider a large spin \bf S in the magnetic field parallel to the uniaxial
crystal field, interacting with N >> 1 nuclear spins \bf I_i via Hamiltonian
\cal H = -DS_z^2 - H_zS_z+ A{\bf S}\cdot \sum_{i=1}^N {\bf I}_i with A << D, at
temperature T. Tunneling splittings and the selection rules for the resonant
values of H_z are obtained perturbatively. The quantum coherence exists at T <<
ASI while at T >= ASI the coherence is destroyed and the relaxation of \bf S is
described by a stretched dependence which can be close to log t under certain
conditions. Relevance to Mn-12 acetate is discussed.Comment: 5 PR pages, 4 figures, submitted to PR
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
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