275 research outputs found
Low-Temperature Features of Nano-Particle Dynamics
In view of better characterizing possible quantum effects in the dynamics of
nanometric particles, we measure the effect on the relaxation of a slight
heating cycle. The effect of the field amplitude is studied; its magnitude is
chosen in order to induce the relaxation of large particles (~7nm), even at
very low temperatures (100mK). Below 1K, the results significantly depart from
a simple thermal dynamics scenario.Comment: 1 tex file, 4 PostScript figure
Laser diagnostics for NTP fuel corrosion studies
Viewgraphs and explanations on laser diagnostics for nuclear thermal propulsion (NTP) fuel corrosion studies are presented. Topics covered include: NTP fuels; U-Zr-C system corrosion products; planar laser-induced fluorescence (PLIF); utilization of PLIF for corrosion product characterization of nuclear thermal rocket fuel elements under test; ZrC emission spectrum; and PLIF imaging of ZrC plume
A new experimental procedure for characterizing quantum effects in small magnetic particle systems
A new experimental procedure is discussed, which aims at separating thermal
from quantum behavior independently of the energy barrier distribution in small
particle systems.
Magnetization relaxation data measured between 60 mK and 5 K on a sample of
nanoparticles is presented. The comparison between experimental data and
numerical calculations shows a clear departure from thermal dynamics for our
sample, which was not obvious without using the new procedure presented here.Comment: LaTeX source, 6 pages, 5 PostScript figure
Disentangling Distribution Effects and Nature of the Dynamics in Relaxation Measurements: the RMR Method
URL: http://www-spht.cea.fr/articles/s00/004 (sur invitation). Comment séparer les effets de distribution et la nature de la dynamique dans des mesures de relaxation magnétiques: la méthode RMRWe discuss here the nature of the low temperature magnetic relaxation in samples of magnetic nanoparticles. In addition to usual magnetic viscosity measurement, we have used the Residual Memory Ratio (RMR) method. This procedure enables us to overcome the uncertainties usually associated with the energy barrier distribution, thus giving a more detailed insight on the nature of the observed dynamics. A custom made apparatus coupling dilution refrigeration and SQUID magnetometry allowed measurements of very diluted samples at temperatures ranging between 60mK and 7K. Two types of particles have been studied: -FeO of moderate anisotropy, and CoFeO of higher anisotropy where quantum effects are more likely to occur. In both cases, the data cannot simply be interpreted in terms of mere thermally activated dynamics of independent particles. The deviation from thermal activation seems to go opposite of what is expected from the possible effect of particle interactions. We therefore believe that it suggests the occurrence of quantum dynamics at very low temperatures
Non-monotonic field-dependence of the ZFC magnetization peak in some systems of magnetic nanoparticles
We have performed magnetic measurements on a diluted system of gamma-Fe2O3
nanoparticles (~7nm), and on a ferritin sample. In both cases, the ZFC-peak
presents a non-monotonic field dependence, as has already been reported in some
experiments,and discussed as a possible evidence of resonant tunneling. Within
simple assumptions, we derive expressions for the magnetization obtained in the
usual ZFC, FC, TRM procedures. We point out that the ZFC-peak position is
extremely sensitive to the width of the particle size distribution, and give
some numerical estimates of this effect. We propose to combine the FC
magnetization with a modified TRM measurement, a procedure which allows a more
direct access to the barrier distribution in a field. The typical barrier
values which are obtained with this method show a monotonic decrease for
increasing fields, as expected from the simple effect of anisotropy barrier
lowering, in contrast with the ZFC results. From our measurements on
gamma-Fe2O3 particles, we show that the width of the effective barrier
distribution is slightly increasing with the field, an effect which is
sufficient for causing the observed initial increase of the ZFC-peak
temperatures.Comment: LaTeX file 19 pages, 9 postscript figures. To appear in Phys. Rev. B
(tentative schedule: Dec.97
Adapting to the digital age: a narrative approach
The article adopts a narrative inquiry approach to foreground informal learning and exposes a collection of stories from tutors about how they adapted comfortably to the digital age. We were concerned that despite substantial evidence that bringing about changes in pedagogic practices can be difficult, there is a gap in convincing approaches to help in this respect. In this context, this project takes a âbottom-upâ approach and synthesises several life-stories into a single persuasive narrative to support the process of adapting to digital change. The project foregrounds the small, every-day motivating moments, cultural features and environmental factors in people's diverse lives which may have contributed to their positive dispositions towards change in relation to technology enhanced learning. We expect that such narrative approaches could serve to support colleagues in other institutions to warm up to ever-changing technological advances
Relaxation and Landau-Zener experiments down to 100 mK in ferritin
Temperature-independent magnetic viscosity in ferritin has been observed from
2 K down to 100 mK, proving that quantum tunneling plays the main role in these
particles at low temperature. Magnetic relaxation has also been studied using
the Landau-Zener method making the system crossing zero resonant field at
different rates, alpha=dH/dt, ranging from 10^{-5} to 10^{-3} T/s, and at
different temperatures, from 150 mK up to the blocking temperature. We propose
a new Tln(Delta H_{eff}/tau_0 alpha) scaling law for the Landau-Zener
probability in a system distributed in volumes, where Delta H_{eff} is the
effective width of the zero field resonance.Comment: 13 pages, 4 postscript figure
By design : negotiating flexible learning in the built environment discipline
The term ‘flexible education’ is now firmly entrenched within Australian higher education discourse, yet the term is a contested one imbued with a multiplicity of meanings. This paper describes a process designed to elucidate how the idea of flexible education can be translated into teaching models that are informed by the specific demands of disciplinary contexts. The process uses a flexible learning ‘matching’ tool to articulate the understandings and preferences of students and academics of the Built Environment to bridge the gap between student expectations of flexibility and their teacher’s willingness and ability to provide that flexibility within the limits of the pedagogical context and teaching resources. The findings suggest an informed starting point for educators in the Built Environment and other creative disciplines from which to traverse the complexities inherent in negotiating flexibility in an increasingly digital world
Millikelvin magnetic relaxation measurements of alpha-Fe2O3 antiferromagnetic particles
In this paper we report magnetic relaxation data for antiferromagnetic
alpha-Fe2O3 particles of 5 nm mean diameter in the temperature range 0.1 K to
25 K. The average spin value of these particles S=124 and the uniaxial
anisotropy constant D=1.6x10^-2 K have been estimated from the experimental
values of the blocking temperature and anisotropy field. The observed plateau
in the magnetic viscosity from 3 K down to 100 mK agrees with the occurrence of
spin tunneling from the ground state Sz = S. However, the scaling M vs Tln(nu
t) is broken below 5 K, suggesting the occurrence of tunneling from excited
states below this temperature.Comment: 4 pages (two columns), 4 figure
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Imaging through turbid media with a nonlinear-optical correlation time gate
Experiments directed towards a clinically useful optical imaging system use long-pulse near-infrared lasers and a correlation time gate based on degenerate four-wave mixing in a nonlinear medium
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