867 research outputs found
Field sweep rate dependence of the coercive field of single-molecule magnets: a classical approach with applications to the quantum regime
A method, based on the Neel-Brown model of thermally activated magnetization
reversal of a magnetic single-domain particle, is proposed to study the field
sweep rate dependence of the coercive field of single-molecule magnets (SMMs).
The application to Mn12 and Mn84 SMMs allows the determination of the important
parameters that characterize the magnetic properties: the energy barrier, the
magnetic anisotropy constant, the spin, tau_0, and the crossover temperature
from the classical to the quantum regime. The method may be particularly
valuable for large SMMs that do not show quantum tunneling steps in the
hysteresis loops.Comment: 6 pages, 6 figure
Temperature dependence of antiferromagnetic susceptibility in ferritin
We show that antiferromagnetic susceptibility in ferritin increases with
temperature between 4.2 K and 180 K (i. e. below the N\'{e}el temperature) when
taken as the derivative of the magnetization at high fields (
Oe). This behavior contrasts with the decrease in temperature previously found,
where the susceptibility was determined at lower fields ( Oe). At
high fields (up to Oe) the temperature dependence of the
antiferromagnetic susceptibility in ferritin nanoparticles approaches the
normal behavior of bulk antiferromagnets and nanoparticles considering
superantiferromagnetism, this latter leading to a better agreement at high
field and low temperature. The contrast with the previous results is due to the
insufficient field range used ( Oe), not enough to saturate the
ferritin uncompensated moment.Comment: 7 pages, 7 figures, accepted in Phys. Rev.
Understanding and engineering phonon-mediated tunneling into graphene on metal surfaces
Metal-intercalated graphene on Ir(111) exhibits phonon signatures in
inelastic elec- tron tunneling spectroscopy with strengths that depend on the
intercalant. Extraor- dinarily strong graphene phonon signals are observed for
Cs intercalation. Li interca- lation likewise induces clearly discriminable
phonon signatures, albeit less pronounced than observed for Cs. The signal can
be finely tuned by the alkali metal coverage and gradually disappears upon
increasing the junction conductance from tunneling to con- tact ranges. In
contrast to Cs and Li, for Ni-intercalated graphene the phonon signals stay
below the detection limit in all transport ranges. Going beyond the
conventional two-terminal approach, transport calculations provide a
comprehensive understanding of the subtle interplay between the
graphene{electrode coupling and the observation of graphene phonon
spectroscopic signatures
Kondo effect of a Co atom on Cu(111) in contact with an Fe tip
Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with
Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe
tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly
broadened. In contrast, for Cu-covered W tips, the resonance width remains
almost constant throughout the tunneling and contact ranges. The distinct
changes of the line width are interpreted in terms of modifications of the Co d
state occupation owing to hybridization with the tip apex atoms.Comment: 4 pages, 3 figure
Effect of exchange interaction on superparamagnetic relaxation
We use Langer's approach to calculate the reaction rate of a system of two
(classical) spins interacting via the exchange coupling in a magnetic field
, with uniaxial anisotropy of constant .
We find a particular value of the exchange coupling, that is , where , which separates two regimes
corresponding to a two-stage and one-stage switching.
For the N\'eel-Brown result for the one-spin problem is recovered.Comment: 7 pages, 2 eps figures, fig.1 of better quality can be provided upon
reques
Quantized Conductance of a Single Magnetic Atom
A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is
contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling
microscope. When the Co atom bridges two non-magnetic electrodes conductances
of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h
is observed which may indicate fully spin-polarized transport.Comment: 3 pages, 2 figure
Antiferromagnetism in NiO Observed by Transmission Electron Diffraction
Neutron diffraction has been used to investigate antiferromagnetism since
1949. Here we show that antiferromagnetic reflections can also be seen in
transmission electron diffraction patterns from NiO. The diffraction patterns
taken here came from regions as small as 10.5 nm and such patterns could be
used to form an image of the antiferromagnetic structure with a nanometre
resolution.Comment: 10 pages, 7 figures. Typos corrected. To appear in Physical Review
Letter
Theoretical simulation of the anisotropic phases of antiferromagnetic thin films
We simulate antiferromagnetic thin films. Dipole-dipole and antiferromagnetic
exchange interactions as well as uniaxial and quadrupolar anisotropies are
taken into account. Various phases unfold as the corresponding parameters, J, D
and C, as well as the temperature T and the number n of film layers vary. We
find (1) how the strength Delta_m of the anisotropy arising from dipole-dipole
interactions varies with the number of layers m away from the film's surface,
with J and with n; (2) a unified phase diagram for all n-layer films and bulk
systems; (3) a layer dependent spin reorientation (SR) phase in which spins
rotate continuously as T, D, C and n vary; (4) that the ratio of the SR to the
ordering temperature depends (approximately) on n only through (D+Delta/n)/C,
and hardly on J; (5) a phase transformation between two different magnetic
orderings, in which spin orientations may or may not change, for some values of
J, by varying n.Comment: 10 LaTeX pages, 13 eps figures. Submitted to PRB on 30 June 2006.
Accepted on 10 October 200
Magnetic relaxation of exchange biased (Pt/Co) multilayers studied by time-resolved Kerr microscopy
Magnetization relaxation of exchange biased (Pt/Co)5/Pt/IrMn multilayers with
perpendicular anisotropy was investigated by time-resolved Kerr microscopy.
Magnetization reversal occurs by nucleation and domain wall propagation for
both descending and ascending applied fields, but a much larger nucleation
density is observed for the descending branch, where the field is applied
antiparallel to the exchange bias field direction. These results can be
explained by taking into account the presence of local inhomogeneities of the
exchange bias field.Comment: To appear in Physical Review B (October 2005
Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles
We use a Monte Carlo approach to simulate the influence of the dipolar
interaction on assemblies of monodisperse superparamagnetic
nanoparticles. We have identified a critical
concentration c*, that marks the transition between two different regimes in
the evolution of the blocking temperature () with interparticle
interactions. At low concentrations (c < c*) magnetic particles behave as an
ideal non-interacting system with a constant . At concentrations c > c*
the dipolar energy enhances the anisotropic energy barrier and
increases with increasing c, so that a larger temperature is required to reach
the superparamagnetic state. The fitting of our results with classical particle
models and experiments supports the existence of two differentiated regimes.
Our data could help to understand apparently contradictory results from the
literature.Comment: 13 pages, 7 figure
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