50,284 research outputs found
Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles
The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles
Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn(x)Fe(3-x)O(4)/Fe(x)Mn(3-x)O(4), core/shell heterostructured nanoparticles
Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn xFe 3-xO 4/Fe xMn 3-xO 4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific application
Sensors and Biosensors Related to Magnetic Nanoparticles
This book describes interesting examples of magnetic materials with magnetic nanoparticles or compact devices using composites with nanoparticles, including new engineering solutions and theoretical contributions on the magnetic biosensing of soft matter composites. Authors from different countries formed international team of experts sharing 10 contributed papers, 1 feature paper, and 1 topical review
Magnetic nanocomposites at microwave frequencies
Most conventional magnetic materials used in the electronic devices are
ferrites, which are composed of micrometer-size grains. But ferrites have small
saturation magnetization, therefore the performance at GHz frequencies is
rather poor. That is why functionalized nanocomposites comprising magnetic
nanoparticles (e.g. Fe, Co) with dimensions ranging from a few nm to 100 nm,
and embedded in dielectric matrices (e.g. silicon oxide, aluminium oxide) have
a significant potential for the electronics industry. When the size of the
nanoparticles is smaller than the critical size for multidomain formation,
these nanocomposites can be regarded as an ensemble of particles in
single-domain states and the losses (due for example to eddy currents) are
expected to be relatively small. Here we review the theory of magnetism in such
materials, and we present a novel measurement method used for the
characterization of the electromagnetic properties of composites with
nanomagnetic insertions. We also present a few experimental results obtained on
composites consisting of iron nanoparticles in a dielectric matrix.Comment: 20 pages, 10 figures, 5 table
Correlation between tunneling magnetoresistance and magnetization in dipolar coupled nanoparticle arrays
The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled
anisotropic magnetic nanoparticles is studied using a resistor network model
and a realistic micromagnetic configuration obtained by Monte Carlo
simulations. Analysis of the field-dependent TMR and the corresponding
magnetization curve shows that dipolar interactions suppress the maximum TMR
effect, increase or decrease the field-sensitivity depending on the direction
of applied field and introduce strong dependence of the TMR on the direction of
the applied magnetic field. For off-plane magnetic fields, maximum values in
the TMR signal are associated with the critical field for irreversible rotation
of the magnetization. This behavior is more pronounced in strongly interacting
systems (magnetically soft), while for weakly interacting systems (magnetically
hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in
contrast to the situation for non-interacting nanoparticles or in-plane fields
(Hmax=Hc). The relation of our simulations to recent TMR measurements in
self-assembled Co nanoparticle arrays is discussed.Comment: 21 pages, 8 figures, submitted to Physical Review
Exchange-spring behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite
In this work we report a study of the magnetic behavior of ferrimagnetic
oxide CoFe2O4 and ferrimagnetic oxide/ferromagnetic metal CoFe2O4/CoFe2
nanocomposites. The latter compound is a good system to study hard
ferrimagnet/soft ferromagnet exchange coupling. Two steps were used to
synthesize the bimagnetic CoFe2O4/CoFe2 nanocomposites: (i) first preparation
of CoFe2O4 nanoparticles using the a simple hydrothermal method and (ii) second
reduction reaction of cobalt ferrite nanoparticles using activated charcoal in
inert atmosphere and high temperature. The phase structures, particle sizes,
morphology, and magnetic properties of CoFe2O4 nanoparticles have been
investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS),
transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM)
with applied field up to 3.0 kOe at room temperature and 50K. The mean diameter
of CoFe2O4 particles is about 16 nm. Mossbauer spectra reveal two sites for
Fe3+. One site is related to Fe in an octahedral coordination and the other one
to the Fe3+ in a tetrahedral coordination, as expected for a spinel crystal
structure of CoFe2O4. TEM measurements of nanocomposite show the formation of a
thin shell of CoFe2 on the cobalt ferrite and indicate that the nanoparticles
increase to about 100 nm. The magnetization of nanocomposite showed hysteresis
loop that is characteristic of the exchange spring systems. A maximum energy
product (BH)max of 1.22 MGOe was achieved at room temperature for CoFe2O4/CoFe2
nanocomposites, which is about 115% higher than the value obtained for CoFe2O4
precursor. The exchange-spring interaction and the enhancement of product
(BH)max in nanocomposite CoFe2O4/CoFe2 have been discussed.Comment: 9 pages, 10 figure
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