264 research outputs found
Simple models for dynamic hysteresis loops calculation: Application to hyperthermia optimization
To optimize the heating properties of magnetic nanoparticles (MNPs) in
magnetic hyperthermia applications, it is necessary to calculate the area of
their hysteresis loops in an alternating magnetic field. The three types of
theories suitable for describing the hysteresis loops of MNPs are presented and
compared to numerical simulations: equilibrium functions, Stoner-Wohlfarth
model based theories (SWMBTs) and linear response theory (LRT). Suitable
formulas to calculate the hysteresis area of major cycles are deduced from
SWMBTs and from numerical simulations; the domain of validity of the analytical
formula is explicitly studied. In the case of minor cycles, the hysteresis area
calculations are based on the LRT. A perfect agreement between LRT and
numerical simulations of hysteresis loops is obtained. The domain of validity
of the LRT is explicitly studied. Formulas to calculate the hysteresis area at
low field valid for any anisotropy of the MNP are proposed. Numerical
simulations of the magnetic field dependence of the area show it follows
power-laws with a large range of exponents. Then, analytical expressions
derived from LRT and SWMBTs are used for a theoretical study of magnetic
hyperthermia. It is shown that LRT is only pertinent for MNPs with strong
anisotropy and that SWMBTs should be used for weak anisotropy MNPs. The optimum
volume of MNPs for magnetic hyperthermia as function of material and
experimental parameters is derived. The maximum specific absorption rate (SAR)
achievable is calculated versus the MNP anisotropy. It is shown that an optimum
anisotropy increases the SAR and reduces the detrimental effects of size
distribution. The optimum anisotropy is simple to calculate and depends on the
magnetic field used in the hyperthermia experiments and on the MNP
magnetization only. The theoretical optimum parameters are compared to the one
of several magnetic materials.Comment: 35 pages, 1 table, 11 figure
A frequency-adjustable electromagnet for hyperthermia measurements on magnetic nanoparticles
We describe a low-cost and simple setup for hyperthermia measurements on
colloidal solutions of magnetic nanoparticles (ferrofluids) with a
frequency-adjustable magnetic field in the range 5-500 kHz produced by an
electromagnet. By optimizing the general conception and each component (nature
of the wires, design of the electromagnet), a highly efficient setup is
obtained. For instance, in a useful gap of 1.1 cm, a magnetic field of 4.8 mT
is generated at 100 kHz and 500 kHz with an output power of 3.4 W and 75 W,
respectively. A maximum magnetic field of 30 mT is obtained at 100 kHz. The
temperature of the colloidal solution is measured using optical fiber sensors.
To remove contributions due to heating of the electromagnet, a differential
measurement is used. In this configuration the sensitivity is better than 1.5
mW at 100 kHz and 19.3 mT. This setup allows one to measure weak heating powers
on highly diluted colloidal solutions. The hyperthermia characteristics of a
solution of Fe nanoparticles are described, where both the magnetic field and
the frequency dependence of heating power have been measured
Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles
We report on transport properties of millimetric super-lattices of CoFe
nanoparticles surrounded by organic ligands. R(T)s follow R(T) =
R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s
follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a
universal curve when shifted by a voltage proportional to the temperature.
Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a
strong voltage-dependence is observed. Its amplitude only depends on the
magnetic field/temperature ratio. Its origin is attributed to the presence of
paramagnetic states present at the surface or between the nanoparticles. Below
1.8 K, this high-field magnetoresistance abruptly disappears and inverse
tunnelling magnetoresistance is observed, the amplitude of which does not
exceed 1%. At this low temperature, some samples display in their I(V)
characteristics abrupt and hysteretic transitions between the Coulomb blockade
regime and the conductive regime. The increase of the current during these
transitions can be as high as a factor 30. The electrical noise increases when
the sample is near the transition. The application of a magnetic field
decreases the voltage at which these transitions occur so magnetic-field
induced transitions are also observed. Depending on the applied voltage, the
temperature and the amplitude of the magnetic field, the magnetic-field induced
transitions are either reversible or irreversible. These abrupt and hysteretic
transitions are also observed in resistance-temperature measurements. They
could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64,
041302 (R), 2001] or could also be interpreted as a true phase transition
between a Coulomb glass phase to a liquid phase of electrons
Room temperature Giant Spin-dependent Photoconductivity in dilute nitride semiconductors
By combining optical spin injection techniques with transport spectroscopy
tools, we demonstrate a spin-photodetector allowing for the electrical
measurement and active filtering of conduction band electron spin at room
temperature in a non-magnetic GaAsN semiconductor structure. By switching the
polarization of the incident light from linear to circular, we observe a Giant
Spin-dependent Photoconductivity (GSP) reaching up to 40 % without the need of
an external magnetic field. We show that the GSP is due to a very efficient
spin filtering effect of conduction band electrons on Nitrogen-induced Ga
self-interstitial deep paramagnetic centers.Comment: 4 pages, 3 figure
Influence of a transverse static magnetic field on the magnetic hyperthermia properties and high-frequency hysteresis loops of ferromagnetic FeCo nanoparticles
The influence of a transverse static magnetic field on the magnetic
hyperthermia properties is studied on a system of large-losses ferromagnetic
FeCo nanoparticles. The simultaneous measurement of the high-frequency
hysteresis loops and of the temperature rise provides an interesting insight
into the losses and heating mechanisms. A static magnetic field of only 40 mT
is enough to cancel the heating properties of the nanoparticles, a result
reproduced using numerical simulations of hysteresis loops. These results cast
doubt on the possibility to perform someday magnetic hyperthermia inside a
magnetic resonance imaging setup.Comment: 6 pages, 3 figure
Magnetic anisotropy determination and magnetic hyperthermia properties of small Fe nanoparticles in the superparamagnetic regime
We report on the magnetic and hyperthermia properties of iron nanoparticles
synthesized by organometallic chemistry. They are 5.5 nm in diameter and
display a saturation magnetization close to the bulk one. Magnetic properties
are dominated by the contribution of aggregates of nanoparticles with respect
to individual isolated nanoparticles. Alternative susceptibility measurements
are been performed on a low interacting system obtained after eliminating the
aggregates by centrifugation. A quantitative analysis using the Gittleman s
model allow a determination of the effective anisotropy Keff = 1.3 * 10^5
J.m^{-3}, more than two times the magnetocristalline value of bulk iron.
Hyperthermia measurements are performed on agglomerates of nanoparticles at a
magnetic field up to 66 mT and at frequencies in the range 5-300 kHz. Maximum
measured SAR is 280 W/g at 300 kHz and 66 mT. Specific absorption rate (SAR)
displays a square dependence with the magnetic field below 30 mT but deviates
from this power law at higher value. SAR is linear with the applied frequency
for mu_0H=19 mT. The deviations from the linear response theory are discussed.
A refined estimation of the optimal size of iron nanoparticles for hyperthermia
applications is provided using the determined effective anisotropy value
Magnetic hyperthermia in single-domain monodisperse FeCo nanoparticles: Evidences for Stoner-Wohlfarth behaviour and large losses
We report on hyperthermia measurements on a colloidal solution of 15 nm
monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic
field display a sharp increase followed by a plateau, which is what is expected
for losses of ferromagnetic single-domain NPs. The frequency dependence of the
coercive field is deduced from hyperthermia measurement and is in quantitative
agreement with a simple model of non-interacting NPs. The measured losses (1.5
mJ/g) compare to the highest of the literature, though the saturation
magnetization of the NPs is well below the bulk one.Comment: 14 pages, 3 figure
Feasibility of using rural waste products to increase the denitrification efficiency in a surface flow constructed wetland
A surface flow constructed wetland (CW) was set in the Lerma gully to decrease nitrate (NO3 -) pollution from agricultural runoff water. The water flow rate and NO3 - concentration were monitored at the inlet and the outlet, and sampling campaigns were performed which consisted of collecting six water samples along the CW flow line. After two years of operation, the NO3 - attenuation was limited at a flow rate of ~2.5 L/s and became negligible at ~5.5 L/s. The present work aimed to assess the feasibility of using rural waste products (wheat hay, corn stubble, and animal compost) to induce denitrification in the CW, to assess the effect of temperature on this process, and to trace the efficiency of the treatment by using isotopic tools. In the first stage, microcosm experiments were performed. Afterwards, the selected waste material was applied in the CW, and the treatment efficiency was evaluated by means of a chemical and isotopic characterization and using the isotopic fractionation (e) values calculated from laboratory experiments to avoid field-scale interference. The microcosms results showed that the stubble was the most appropriate material for application in the CW, but the denitrification rate was found to decrease with temperature. In the CW, biostimulation in autumn-winter promoted NO3 - attenuation between two weeks and one month (a reduction in NO3 - between 1.2 and 1.5 mM was achieved). After the biostimulation in spring-summer, the attenuation was maintained for approximately three months (NO3 - reduction between 0.1 and 1.5 mM). The e15NNO3/N2 and e18ONO3/N2 values obtained from the laboratory experiments allowed to estimate the induced denitrification percentage. At an approximate average flow rate of 16 L/s, at least 60% of NO3 - attenuation was achieved in the CW. The field samples exhibited a slope of 1.0 for d18O-NO3 - versus d15N-NO3 -, similar to those of the laboratory experiments (0.9â1.2). Plant uptake seemed to play a minor role in NO3 - attenuation in the CW. Hence, the application of stubble in the CW allowed the removal of large amounts of NO3 - from the Lerma gully, especially when applied during the warm months, but its efficacy was limited to a short time period (up to three months). © 2019 Elsevier B.V
Influence of structural and magnetic properties in the heating performance of multicore bioferrofluids
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).Biomedical applications of superparamagnetic iron oxide particles have been of interest for quite a number of years. Recent developments show that multifunctionality can be efficiently achieved using polymers to coat the particles and to provide anchoring elements to their surface. This leads to the formation of nanobeads with a reduced number of particles trapped by the polymeric structure. While the magnetothermic behavior of isolated nanoparticles has been a subject of interest over the past several years, multicore magnetic nanobeads have thus far not received the same attention. The influence of structural and magnetic properties in the hyperthermia performance of a series of magnetic fluids designed for biomedical purposes is studied here. The fluids are made of maghemite multicore polymeric beads, with variable nanoparticle size and hydrodynamic size, dispersed in a buffer solution. The specific loss power (SLP) was measured from 5 to 100 kHz with a field intensity of 21.8 kA/m. SLP increases with increasing magnetic core size, reaching 32 W/g Fe 2O3 at 100 kHz for 16.2 nm. Within the framework of the linear response theory, a graphical construction is proposed to describe the interplay of both size distributions and magnetic properties in the heating performance of such fluids in a given frequency range. Furthermore, a numerical model is developed to calculate the spare contribution of NĂ©el and Brown relaxation mechanisms to SLP, which gives a fair reproduction of the experimental data. © 2013 American Physical Society.R.B. would like to thank ICMA-CSIC for the JAE predoc grant. Financial support from Grant No. MAT2011-25991 is gratefully acknowledged. We acknowledge Fundaçùo para a CiĂȘncia e Tecnologia (FCT, Portugal), COMPETE, and FEDER programs (Pest-C/CTM/LA0011/2013). N.J.O.S. acknowledges FCT for the CiĂȘncia 2008 program.Peer Reviewe
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