1,264 research outputs found
Surface anisotropy in nanomagnets: transverse or N\'eel ?
Through the hysteresis loop and magnetization spatial distribution we study
and compare two models for surface anisotropy in nanomagnets: a model with
transverse anisotropy axes and N\'eel's model. While surface anisotropy in the
transverse model induces several jumps in the hysteresis loop because of the
cluster-wise switching of spins, in the N\'eel model the jumps correspond to
successive {\it coherent partial rotations} of the whole bunch of spins. These
calculations together with experimental results suggest that N\'eel's model for
surface anisotropy is more appropriate.Comment: 12 pages, 6 eps 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
Magnetization of nanomagnet assemblies: Effects of anisotropy and dipolar interactions
We investigate the effect of anisotropy and weak dipolar interactions on the
magnetization of an assembly of nanoparticles with distributed magnetic
moments, i.e., assembly of magnetic nanoparticles in the one-spin
approximation, with textured or random anisotropy.
The magnetization of a free particle is obtained either by a numerical
calculation of the partition function or analytically in the low and high field
regimes, using perturbation theory and the steepest-descent approximation,
respectively. The magnetization of an interacting assembly is computed
analytically in the range of low and high field, and numerically using the
Monte Carlo technique.
Approximate analytical expressions for the assembly magnetization are
provided which take account of the dipolar interactions, temperature, magnetic
field, and anisotropy. The effect of anisotropy and dipolar interactions are
discussed and the deviations from the Langevin law they entail are
investigated, and illustrated for realistic assemblies with the lognormal
moment distribution.Comment: 21 pages, 5 eps figure
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
Phase transition in nanomagnetite
Recently, the application of nanosized magnetite particles became an area of growing interest for
their potential practical applications. Nanosized magnetite samples of 36 and 9 nm sizes were
synthesized. Special care was taken on the right stoichiometry of the magnetite particles. Mössbauer
spectroscopy measurements were made in 4.2–300 K temperature range. The temperature
dependence of the intensities of the spectral components indicated size dependent transition taking
place in a broad temperature range. For nanosized samples, the hyperfine interaction values and their
relative intensities changed above the Verwey transition temperature value of bulk megnetite. The
continuous transition indicated the formation of dendritelike granular assemblies formed during the
preparation of the samples
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
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
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
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
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