510 research outputs found
Quantum nucleation in a single-chain magnet
The field sweep rate (v=dH/dt) and temperature (T) dependence of the
magnetization reversal of a single-chain magnet (SCM) is studied at low
temperatures. As expected for a thermally activated process, the nucleation
field (H_n) increases with decreasing T and increasing v. The set of H_n(T,v)
data is analyzed with a model of thermally activated nucleation of
magnetization reversal. Below 1 K, H_n becomes temperature independent but
remains strongly sweep rate dependent. In this temperature range, the reversal
of the magnetization is induced by a quantum nucleation of a domain wall that
then propagates due to the applied field.Comment: 5 pages, 4 figure
Effect of temperature-dependent shape anisotropy on coercivity with aligned Stoner-Wohlfarth soft ferromagnets
The temperature variation effect of shape anisotropy on the coercivity,
HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni,
fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B)
analysis. An extended N-B equation is thus proposed,by introducing a single
dimensionless correction function, the reduced magnetization, m(\tao) =
MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the
saturation magnetization, and TC is the Curie temperature. The factor, m(\tao),
accounts for the temperature-dependent effect of the shape anisotropy. The
constants, H0 and E0, are for the switching field at zero temperature and the
potential barrier at zero field, respectively. According to this newly derived
equation, the blocking temperature above which the properties of
superparamagnetism show up is described by the expression, TB =
E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The
possible effect on HC(T) and the blocking temperature, TB, attributed to the
downshift of TC resulting from the finite size effect has been discussed also.Comment: 22 pages, 2 figures, 1 table, Accepted by Phys. Rev.
Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge
A first-principles investigation of the anomalous ferromagnetism of a
quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our
calculations show that the symmetry breaking at the step leads to an easy
magnetization axis at an odd angle of {\em towards} the Pt
step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature}
{\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become
noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A
significant enhancement of the Co orbital magnetic moment is achieved when
modest electron correlations are treated within LSDA+ calculations.Comment: Presented at MRS Meeting in Boston, Dec. 2003; 4 pages including 3
figure
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
On alpha stable distribution of wind driven water surface wave slope
We propose a new formulation of the probability distribution function of wind
driven water surface slope with an -stable distribution probability.
The mathematical formulation of the probability distribution function is given
under an integral formulation. Application to represent the probability of time
slope data from laboratory experiments is carried out with satisfactory
results. We compare also the -stable model of the water surface slopes
with the Gram-Charlier development and the non-Gaussian model of Liu et
al\cite{Liu}. Discussions and conclusions are conducted on the basis of the
data fit results and the model analysis comparison.Comment: final version of the manuscript: 25 page
Domain wall structure in magnetic bilayers with perpendicular anisotropy
We study the magnetic domain wall structure in magnetic bilayers (two
ultrathin ferromagnetic layers separated by a non magnetic spacer) with
perpendicular magnetization. Combining magnetic force and ballistic electron
emission microscopies, we are able to reveal the details of the magnetic
structure of the wall with a high spatial accuracy. In these layers, we show
that the classical Bloch wall observed in single layers transforms into
superposed N\'eel walls due to the magnetic coupling between the ferromagnetic
layers. Quantitative agreement with micromagnetic calculations is achieved.Comment: Author adresses AB, SR, JM and AT: Laboratoire de Physique des
Solides, CNRS, Universit\'e Paris Sud, UMR 8502, 91405 Orsay Cedex, France ML
: Laboratoire PMTM, Institut Galil\'ee, CNRS, Universit\'e Paris-13, UPR
9001, 93430 Villetaneuse, Franc
Calculation of coercivity of magnetic nanostructures at finite temperatures
We report a finite temperature micromagnetic method (FTM) that allows for the
calculation of the coercive field of arbitrary shaped magnetic nanostructures
at time scales of nanoseconds to years. Instead of directly solving the
Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any
free parameter by solving a non linear equation, which arises from the
transition state theory. The method is applicable to magnetic structures where
coercivity is determined by one thermally activated reversal or nucleation
process. The method shows excellent agreement with experimentally obtained
coercive fields of magnetic nanostructures and provides a deeper understanding
of the mechanism of coercivity.Comment: submitted to Phys. Rev.
Temperature dependence of the coercive field in single-domain particle systems
The magnetic properties of Cu97Co3 and Cu90Co10 granular alloys were measured
over a wide temperature range (2 to 300K). The measurements show an unusual
temperature dependence of the coercive field. A generalized model is proposed
and explains well the experimental behavior over a wide temperature range. The
coexistence of blocked and unblocked particles for a given temperature rises
difficulties that are solved here by introducing a temperature dependent
blocking temperature. An empirical factor gamma arise from the model and is
directly related to the particle interactions. The proposed generalized model
describes well the experimental results and can be applied to other
single-domain particle system.Comment: 7 pages, 8 figures, revised version, accepted to Physical Review B on
29/04/200
Spin-glass-like behavior of Ge:Mn
We present a detailed study of the magnetic properties of
low-temperature-molecular-beam-epitaxy grown Ge:Mn dilute magnetic
semiconductor films. We find strong indications for a frozen state of
Ge_{1-x}Mn_{x}, with freezing temperatures of T_f=12K and T_f=15K for samples
with x=0.04 and x=0.2, respectively, determined from the difference between
field-cooled and zero-field-cooled magnetization. For Ge_{0.96}Mn_{0.04}, ac
susceptibility measurements show a peak around T_f, with the peak position T'_f
shifting as a function of the driving frequency f by Delta T_f' / [T_f' Delta
log f] ~ 0.06, whereas for sample Ge_{0.8}Mn_{0.2} a more complicated behavior
is observed. Furthermore, both samples exhibit relaxation effects of the
magnetization after switching the magnitude of the external magnetic field
below T_f which are in qualitative agreement with the field- and
zero-field-cooled magnetization measurements. These findings consistently show
that Ge:Mn exhibits a frozen magnetic state at low temperatures and that it is
not a conventional ferromagnet.Comment: Revised version contains extended interpretation of experimental dat
Surface spin-flop phases and bulk discommensurations in antiferromagnets
Phase diagrams as a function of anisotropy D and magnetic field H are
obtained for discommensurations and surface states for a model antiferromagnet
in which is parallel to the easy axis. The surface spin-flop phase exists
for all . We show that there is a region where the penetration length of the
surface spin-flop phase diverges. Introducing a discommensuration of even
length then becomes preferable to reconstructing the surface. The results are
used to clarify and correct previous studies in which discommensurations have
been confused with genuine surface spin-flop states.Comment: 4 pages, RevTeX, 2 Postscript figure
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