1,604 research outputs found
Composition dependent magnetic properties of iron oxide - polyaniline nanoclusters
Gamma - Iron Oxide prepared by sol -gel process was used to produce
nanocomposites with polyaniline of varying aniline concentrations. TEM shows
the presence of chain like structure for lower polyaniline concentration. The
room temperature hysteresis curves show finite coercivity of 160 Oe for all the
composites while the saturation magnetization was found to decrease with
increasing polymer content. ZFC - FC magnetisation measurements indicate high
blocking temperatures. It is believed that this indicates a strongly
interacting system, which is also shown by our TEM results. Monte Carlo
simulations performed on a random anisotropy model with dipolar and exchange
inteactions match well with experimental results.Comment: 9 (nine) pages, 6 figures (jpeg and eps
Magnetic relaxation in finite two-dimensional nanoparticle ensembles
We study the slow phase of thermally activated magnetic relaxation in finite
two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles
whose easy axes of magnetization are perpendicular to the distribution plane.
We develop a method to numerically simulate the magnetic relaxation for the
case that the smallest heights of the potential barriers between the
equilibrium directions of the nanoparticle magnetic moments are much larger
than the thermal energy. Within this framework, we analyze in detail the role
that the correlations of the nanoparticle magnetic moments and the finite size
of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure
Role of dipolar interactions in a system of Ni nanoparticles studied by magnetic susceptibility measurements
The role of dipolar interactions among Ni nanoparticles (NP) embedded in an
amorphous SiO2/C matrix with different concentrations has been studied
performing ac magnetic susceptibility Chi_ac measurements. For very diluted
samples, with Ni concentrations < 4 wt % Ni or very weak dipolar interactions,
the data are well described by the Neel-Arrhenius law. Increasing Ni
concentration to values up to 12.8 wt % Ni results in changes in the
Neel-Arrhenius behavior, the dipolar interactions become important, and need to
be considered to describe the magnetic response of the NPs system. We have
found no evidence of a spin-glasslike behavior in our Ni NP systems even when
dipolar interactions are clearly present.Comment: 7 pages, 5 figures, 3 table
Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles
We have studied the magnetic behavior of dextran-coated magnetite
(FeO) nanoparticles with median particle size \left=8 .
Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed
that the magnetic moment of the particles was much smaller than the bulk
material. However, we found no evidence of magnetic irreversibility or
non-saturating behavior at high fields, usually associated to spin canting. The
values of magnetic anisotropy from different techniques indicate that
surface or shape contributions are negligible. It is proposed that these
particles have bulk-like ferrimagnetic structure with ordered A and B
sublattices, but nearly compensated magnetic moments. The dependence of the
blocking temperature with frequency and applied fields, ,
suggests that the observed non-monotonic behavior is governed by the strength
of interparticle interactions.Comment: 11 pages, 7 figures, 3 Table
Magnetic properties of polypyrrole - coated iron oxide nanoparticles
Iron oxide nanoparticles were prepared by sol -gel process. Insitu
polymerization of pyrrole monomer in the presence of oxygen in iron oxide
ethanol suspension resulted in a iron oxide - polypyrrole nanocomposite. The
structure and magnetic properties were investigated for varying pyrrole
concentrations. The presence of the gamma - iron oxide phase and polypyrrole
were confirmed by XRD and FTIR respectively. Agglomeration was found to be
comparatively much reduced for the coated samples, as shown by TEM. AC
susceptibility measurements confirmed the superparamagnetic behaviour.
Numerical simulations performed for an interacting model system are performed
to estimate the anisotropy and compare favourably with experimental results.Comment: 11 pages,8 figure
Tailoring Fe/Ag Superparamagnetic Composites by Multilayer Deposition
The magnetic properties of Fe/Ag granular multilayers were examined by SQUID
magnetization and Mossbauer spectroscopy measurements. Very thin (0.2 nm)
discontinuous Fe layers show superparamagnetic properties that can be tailored
by the thickness of both the magnetic and the spacer layers. The role of
magnetic interactions was studied in novel heterostructures of
superparamagnetic and ferromagnetic layers and the specific contribution of the
ferromagnetic layers to the low field magnetic susceptibility was identified.Comment: 5 pages and 3 figure
Geometric diagnostics of complex patterns: Spiral defect chaos
Motivated by the observation of spiral patterns in a wide range of physical, chemical, and biological systems, we present an automated approach that aims at characterizing quantitatively spiral-like elements in complex stripelike patterns. The approach provides the location of the spiral tip and the size of the spiral arms in terms of their arc length and their winding number. In addition, it yields the number of pattern components (Betti number of order 1), as well as their size and certain aspects of their shape. We apply the method to spiral defect chaos in thermally driven Rayleigh- Bénard convection and find that the arc length of spirals decreases monotonically with decreasing Prandtl number of the fluid and increasing heating. By contrast, the winding number of the spirals is nonmonotonic in the heating. The distribution function for the number of spirals is significantly narrower than a Poisson distribution. The distribution function for the winding number shows approximately an exponential decay. It depends only weakly on the heating, but strongly on the Prandtl number. Large spirals arise only for larger Prandtl numbers. In this regime the joint distribution for the spiral length and the winding number exhibits a three-peak structure, indicating the dominance of Archimedean spirals of opposite sign and relatively straight sections. For small Prandtl numbers the distribution function reveals a large number of small compact pattern components
Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles
Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of
reverse micelles, used as nanoreactors to obtain average particle sizes
2 to 4 nm. The blocking temperatures extracted from
magnetization data increased from to 20 K for increasing particle
size. Low-temperature \MOS measurements allowed to observe the onset of
differentiated contributions from particle core and surface as the particle
size increases. The magnetic properties measured in the liquid state of the
original emulsion showed that the \FH phase is not present in the liquid
precursor, but precipitates in the micelle cores after the free water is
freeze-dried. Systematic susceptibility \chi_{ac}(\emph{f},T) measurements
showed the dependence of the effective magnetic anisotropy energies
with particle volume, and yielded an effective anisotropy value of kJ/m.Comment: 8 pages, 10 figures. Nanotechnology, v17 (Nov. 2006) In pres
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