750 research outputs found
Dynamical Monte Carlo investigation of spin reversals and nonequilibrium magnetization of single-molecule magnets
In this paper, we combine thermal effects with Landau-Zener (LZ) quantum
tunneling effects in a dynamical Monte Carlo (DMC) framework to produce
satisfactory magnetization curves of single-molecule magnet (SMM) systems. We
use the giant spin approximation for SMM spins and consider regular lattices of
SMMs with magnetic dipolar interactions (MDI). We calculate spin reversal
probabilities from thermal-activated barrier hurdling, direct LZ tunneling, and
thermal-assisted LZ tunnelings in the presence of sweeping magnetic fields. We
do systematical DMC simulations for Mn systems with various temperatures
and sweeping rates. Our simulations produce clear step structures in
low-temperature magnetization curves, and our results show that the thermally
activated barrier hurdling becomes dominating at high temperature near 3K and
the thermal-assisted tunnelings play important roles at intermediate
temperature. These are consistent with corresponding experimental results on
good Mn samples (with less disorders) in the presence of little
misalignments between the easy axis and applied magnetic fields, and therefore
our magnetization curves are satisfactory. Furthermore, our DMC results show
that the MDI, with the thermal effects, have important effects on the LZ
tunneling processes, but both the MDI and the LZ tunneling give place to the
thermal-activated barrier hurdling effect in determining the magnetization
curves when the temperature is near 3K. This DMC approach can be applicable to
other SMM systems, and could be used to study other properties of SMM systems.Comment: Phys Rev B, accepted; 10 pages, 6 figure
Phases of anisotropic dipolar antiferromagnets
We study systems of classical magnetic dipoles on simple cubic lattices with
dipolar and antiferromagnetic exchange interactions. By analysis and Monte
Carlo (MC) simulations, we find how the antiferromagnetic phases vary with
uniaxial and fourfold anisotropy constants, C and D, as well as with exchange
strength J. We pay special attention to the spin reorientation (SR) phase, and
exhibit in detail the nature of its broken symmetries. By mean field theory and
by MC, we also obtain the ratio of the higher ordering temperature to the SR
transition temperature, and show that it depends mainly on D/C, and rather
weakly on J. We find a reverse SR transition.Comment: 10 LaTeX pages, 14 eps figures. Submitted to PRB on 03 October 2005.
Accepted on 13 December 200
Patterning molecular scale paramagnets at Au Surface: A root to Magneto-Molecular-Electronics
Few examples of the exploitation of molecular magnetic properties in
molecular electronics are known to date. Here we propose the realization of
Self assembled monolayers (SAM) of a particular stable organic radical. This
radical is meant to be used as a standard molecule on which to prove the
validity of a single spin reading procedure known as ESR-STM. We also discuss a
range of possible applications, further than ESR-STM, of magnetic monolayers of
simple purely organic magnetic molecule.Comment: This preprint is currently partially under revisio
Butterfly hysteresis loop at non-zero bias field in antiferromagnetic molecular rings: cooling by adiabatic magnetization
At low temperatures, the magnetization of the molecular ferric wheel NaFe
exhibits a step at a critical field due to a field-induced
level-crossing. By means of high-field torque magnetometry we observed a
hysteretic behavior at the level-crossing with a characteristic butterfly shape
which is analyzed in terms of a dissipative two-level model. Several unusual
features were found. The non-zero bias field of the level-crossing suggests the
possibility of cooling by adiabatic magnetization.Comment: 4 pages, 5 figures, REVTEX4, to appear in PR
Mechanisms of decoherence in weakly anisotropic molecular magnets
Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules,
such as V15, are studied. We show that an important decohering factor is the
rapid thermal fluctuation of dipolar interactions between magnetic molecules. A
model is proposed to describe the influence of this source of decoherence.
Based on the exact solution of this model, we show that at relatively high
temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not
suppressed, and, in principle, can be detected experimentally. Therefore, these
molecules may be suitable prototype systems for study of physical processes
taking place in quantum computers.Comment: 4 pages RevTeX, 1 figure (PostScript
New error bounds for asymptotic approximations of Jacobi polynomials and their zeros
Bounds for the error term of an asymptotic representation of the Jacobi polynomial P(α,β) n (cos ϑ), as n → ∞, are given. The procedure for deriving these bounds is based on a new inequality of Bernstein-type satisfied by P(α,β) n (cos ϑ). Application to the zeros of Jacobi polynomials is considered. Numerical examples are given to illustrate the sharpness of the new results
The Influence of Magnetic Anisotropy on the Kondo Effect and Spin-Polarized Transport through Magnetic Molecules, Adatoms and Quantum Dots
Transport properties in the Kondo regime of a nanosystem displaying uniaxial
magnetic anisotropy (such as a magnetic molecule, magnetic adatom or quantum
dot coupled to a localized magnetic moment) are analyzed theoretically. In
particular, the influence of spin-polarized transport through a local orbital
of the system and exchange coupling of conduction electrons to the system's
magnetic core on the Kondo effect is discussed. The numerical renormalization
group method is applied to calculate the spectral functions and linear
conductance in the case of the parallel and antiparallel configurations of the
electrodes' magnetic moments. It is shown that both the magnetic anisotropy as
well as the exchange coupling between electrons tunneling through the
conducting orbital and magnetic core play an important role in formation of the
Kondo resonance, leading generally to its suppression. Specific transport
properties of such a system appear also as a nontrivial behavior of tunnel
magnetoresistance. It is also shown that the Kondo effect can be restored by an
external magnetic field in both the parallel and antiparallel magnetic
configurations.Comment: 14 pages with 10 EPS figures (version as accepted for publication in
Physical Review B
Fast quantum noise in Landau-Zener transition
We show by direct calculation starting from a microscopic model that the
two-state system with time-dependent energy levels in the presence of fast
quantum noise obeys the master equation. The solution of master equation is
found analytically and analyzed in a broad range of parameters. The fast
transverse noise affects the transition probability during much longer time
(the accumulation time) than the longitudinal one. The action of the fast
longitudinal noise is restricted by the shorter Landau-Zener time, the same as
in the regular Landau-Zener process. The large ratio of time scales allows
solving the Landau-Zener problem with longitudinal noise only, then solving the
same problem with the transverse noise only and matching the two solutions. The
correlation of the longitudinal and transverse noise renormalizes the
Landau-Zener transition matrix element and can strongly enhance the survival
probability, whereas the transverse noise always reduces it. Both longitudinal
and transverse noise reduce the coherence. The decoherence time is inverse
proportional to the noise intensity. If the noise is fast, its intensity at
which the multi-quantum processes become essential corresponds to a deeply
adiabatic regime. We briefly discuss possible applications of the general
theory to the problem of the qubit decoherence and to the spin relaxation of
molecular magnets.Comment: 12 pages, 8 figure
Time Correlation Functions of Three Classical Heisenberg Spins on an Isosceles Triangle and on a Chain: Strong Effects of Broken Symmetry
At arbitrary temperature , we solve for the dynamics of single molecule
magnets composed of three classical Heisenberg spins either on a chain with two
equal exchange constants , or on an isosceles triangle with a third,
different exchange constant . As T\rightrarrow\infty, the Fourier
transforms and long-time asymptotic behaviors of the two-spin time correlation
functions are evaluated exactly. The lack of translational symmetry on a chain
or an isosceles triangle yields time correlation functions that differ
strikingly from those on an equilateral trinagle with . At low ,
the Fourier transforms of the two autocorrelation functions with
show one and four modes, respectively. For a semi-infinite range, one
mode is a central peak. At the origin of this range, this mode has a novel
scaling form.Comment: 9 pages, 14 figures, accepted for publication in Phys. Rev.
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