Magnetic field dependence of the hexagonal to isotropic transition temperature of a single-walled carbon nanotubes dispersed lyotropic liquid crystal

We have carried out electrical conductivity studies on a single-walled carbon nanotubes dispersed lyotropic liquid crystal consisting of 50 wt.% TX-100 in water as a function of magnetic field and temperature. This system exhibits hexagonal and isotropic phases on heating. For all the applied magnetic fields, the temperature dependence of electrical conductivity of the carbon nanotubes dispersed lyotropic liquid crystal system exhibits a discontinuous change at the hexagonal to isotropic transition temperature. We find that the magnetic field dependence of the hexagonal to isotropic transition temperature is similar to that of the viscosity of the system. Using photo images of the sample, we find that the carbon nanotubes in the lyotropic liquid crystal form magnetic field dependent aggregates. We find spherical, rod and hook-like nanotube aggregates for low and high applied magnetic fields respectively. These nanotube aggregates alter the viscosity of our system which in turn alters the transition temperatures.     

Heat generation of aqueously dispersed CoFe2O4 nanoparticles as heating agents for magnetically activated drug delivery and hyperthermia

Self-heating from magnetic nanoparticles under AC magnetic field can be used either for hyperthermia or to trigger the release of an anti-cancer drug, using thermo-responsive polymers. The heat generated by applying an AC magnetic field depends on the properties of magnetic nanoparticles (composition, size, crystal structure) as well as the frequency and amplitude of the magnetic field. Before these systems can be efficiently applied for in vitro or in vivo studies, a thorough analysis of the magnetically induced heating is required. In this study, CoFe₂O₄ nanoparticles were synthesized, dispersed in water, and investigated as heating agents for magnetic thermo-drug delivery and hyperthermia. The temperature profiles and infrared (IR) camera images of heat generation of CoFe₂O₄ nanoparticles under various AC magnetic fields of 127–700 Oe at 195, 231, and 266 kHz were measured using an IR thermacam, excluding the external AC magnetic field interruption. The CoFe₂O₄ nanoparticles were successfully dispersed in water using an 11-mercaptoundecanoic acid ligand exchange method to exchange the solvent used for synthesis of hexane for water. During the heating experiments, each of CoFe₂O₄ nanoparticle solutions reached a steady state where the temperature rose between 0.1 and 42.9 °C above ambient conditions when a magnetic field of 127–634 Oe was applied at 231 or 266 kHz. The heat generation was found to be dependent on the intensity of AC magnetic field and applied frequency. Therefore, the desired heating for magnetically triggered drug delivery or hyperthermia could be achieved in water-dispersed CoFe₂O₄ nanoparticles by adjusting the AC magnetic field and frequency.     

Influence of Co amount on the efficiency of energy absorption of Fe-Co ferrite nanoparticles

We present the chemical synthesis, structural characterization and efficiency of AC field energy absorption of Fe-Co ferrite particles sized between 35 and 100 nm and containing different cobalt amounts up to 3 wt%. Dynamic magnetic measurements in an AC field with frequency of 430 Hz and amplitude of up to 1200 Oe were performed for both solid and liquid dispersions of obtained particles. The energy absorption per one field cycle in function of the field amplitude and Co content was always bigger in liquid systems, under the same conditions, than in solid systems. The obtained results are of significance for the technology of low-frequency ferromagnetic hyperthermia of tumors.     

Imaging magnetic responses of nanomagnets by XPEEM

The Spin-resolved Photoelectron Emission Microscope (SPEEM) is a permanently installed set-up at Helmholtz-Zentrum Berlin (HZB). Due to its specific contrast it is mainly used for magnetic imaging and micro-spectroscopy with quantitative analysis. A crucial point in magnetic imaging is the application of magnetic fields. Many experiments require observation of magnetic responses or the preparation of a certain magnetic state during the measurement. We present a dedicated magnetic sample holder combining magnetic field during imaging with additional temperature control. This set-up enables SPEEM to measure magnetization curves of individual Fe nanocubes (18 nm)³ in size. If additionally alternating magnetic fields are applied we can image the local magnetic AC susceptibility (χ_AC) as a function of temperature. The latter is ideally suited to visualize local variations of the Curie temperature (T_C) in nano- and microstructures.     

Electro-optic study of acrylate based holographic polymer dispersed liquid crystals with broad band photosensitivity

We describe the electro-optic (EO) modulation of diffraction in holographic polymer dispersed liquid crystals, recently developed for broad band photosensitivity, ranging from visible to near infrared. More than 90% EO modulation achieved for relatively low electric fields confirms the phase separation and formation of liquid crystal sub-micrometer sized droplets in the polymer matrix. Molecular reorientation induced by the applied voltage and a residual electrical current induced heating are at the origin of the EO modulation.     

Self-field AC losses of textured Bi2Sr2CaCu2O8+δ thin rods

Self-field AC losses of polycrystalline Bi-2212 thin rods textured by a Laser Floating Zone (LFZ) melting technique have been measured at 77 K. With the optimal processing parameters, these rods, of 1.6–2 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm² in the self-field which decreases to about 1.5 kA/cm² in fields of 0.02 T applied perpendicular to the rod axis. The self-field AC losses have been measured in DC magnetic fields up to 0.03 T. The measurements in zero field show that for a large current range the losses are dominated by hysteresis losses as described by the Critical State Model for a cylinder. For the measurements in DC fields the losses show an increasingly resistive-like dependence with current, while the hysteretic component expected from the CSM becomes less important. Measurements at different frequencies also indicated that the loss per cycle in fields is strongly frequency dependent.     

AC losses in multifilamentary Bi(2223) tapes with an interfilamentary resistive carbonate barrier

For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO₃ barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.     

Absence of field anisotropy in the intrinsic ferromagnetic signals of highly oriented pyrolytic graphite

We have measured the magnetization of bulk samples of highly oriented pyrolytic graphite (HOPG) at magnetic fields applied parallel and perpendicular to the graphene layers. Within experimental error the intrinsic ferromagnetic signals of the samples show similar magnetic moments at saturation for the two magnetic field directions, in contrast to recently published data (J. ?ervenka et al., Nat. Phys. 5 (2009) 840). To check that the SQUID device provides correctly the small ferromagnetic signals obtained after subtracting the 100 times larger diamagnetic background, we have prepared a sample with a superconducting Pb-film deposited on one of the HOPG surfaces. We show that the field dependence of the measured magnetic moment and after the background subtraction is highly reliable even in the sub-μ emu range providing the real magnetic properties of the embedded small ferromagnetic and superconducting signals.     

Polarization and transmission of microwaves in the magnetic suspension in the applied magnetic field

The production method of magnetic suspension consisting of ferromagnetic particles dispersed in cedarwood oil is presented at the beginning of this article. Next, the set-up for microwaves generation using a klystron is described. The main part of this paper concerning microwave transmission and polarization during its passage in samples of the produced magnetic suspension placed in a magnetic field is based on the following parameters: induction of this field, filling factor of magnetic suspension by ferromagnetic particles, dimensions of particles, viscosity of liquid carrier, and ratio of the magnetic field changes. Conducted investigations show that microwaves are damped and polarized in these magnetic suspensions. Obtained results are discussed and observed effects are explained by ordering of ferromagnetic particles in magnetic suspension by applied magnetic field.     

Continuous production of inorganic magnetic nanocomposites for biomedical applications by laser pyrolysis

Magnetic composites of Fe-based nanoparticles encapsulated in carbon/silica (C/SiO₂@Fe) or carbon (C@Fe) matrices were prepared by laser-induced pyrolysis of aerosols. The powders were dispersed in aqueous solutions at pH 7 resulting in biocompatible colloidal dispersions with a high resistance to biodegradation. Structural and magnetic properties and the suitability of aqueous dispersions as contrast agent for MRI were analyzed. The results of these characterizations and the NMR relaxivity data are very encouraging for application of laser pyrolysis products in the field of living tissues.     

Microhydrodynamical modeling of transverse susceptibility of magnetic inks (Theory of ‘DIMAG’)

The time and field dependence of the transverse susceptibility, χ_t, of a suspension of single domain ferromagnetic Brownian particles in a viscous fluid is calculated. Magnetic interactions are taken into account through the mean field model. The model is used to interpret (so far qualitatively) the so-called DIMAG experiment, in which a DC field is applied to the ink sample for a short period of time and a small perpendicular AC field probes χ_t before, during and after the DC field impact. The relative increase in χ_t, after the DC field is switched on was used earlier to characterize the dispersion quality of inks. We show that in the systems with negative interactions (acicular particles) this increase has a maximum when plotted as a function of the DC field. The maximum shifts to higher fields as the interaction increases, which corresponds to what happens in the DIMAG testing of less dispersed inks.     

AC loss measurements on model Bi-2223 conductors

Measurements of the AC loss in applied magnetic fields at 77 K have been made on model composite Bi-2223 conductors. A vibrating sample magnetometer (VSM) and a dual Hall sensor magnetometer (HSM) were used to cover the frequency range from below 0.01 Hz to over 250 Hz at AC fields up to 0.05 T rms. The VSM was limited to the frequency range below 0.2 Hz. A comparison of the two measurement techniques was possible at intermediate frequencies. The samples consisted of vertical stacks of well separated flat filaments of superconductor in Ag and Ag-alloy matrix, allowing a range of filament coupling conditions to be explored.     

Biocompatible high-moment FeCo-Au magnetic nanoparticles for magnetic hyperthermia treatment optimization

A great deal of attention has been paid to the use of magnetite nanoparticles as heating elements in the research of magnetic fluid hyperthermia. However, these particles have a relatively low magnetization and as a result, have low heating efficiency as well as difficulties in detection applications. To maximize heating efficiency we propose and show the use of high-moment Fe(Co)-Au core-shell nanoparticles. Using a physical vapor nanoparticle-deposition technique the high-moment nanoparticles were synthesized. The water-soluble particles were placed in an AC magnetic field of variable magnetic field frequencies. The temperature rise was measured and compared to theory.     

Calculation of AC losses in HTSC wires with arbitrary current voltage characteristics

A method for the calculation of magnetic field dynamics and AC losses in superconductors with smooth current–voltage characteristics is described. It is based on an integral equation for the current density, recently used by Brandt for magnetic relaxation. Brandt's equation is generalized to include arbitrary external magnetic fields and transport currents. One of the benefits of the integral equation formulation is that no boundary conditions ‘at infinity' are required, thus restricting the calculation region to the conductor cross section. The method is applied to superconducting tapes in oblique external fields. A further extension of the theory is shown to be applicable to the calculation of coupling losses in twisted multifilamentary superconductors.     

直流电对金属液电磁净化的影响

推导了矩形管通电金属液的感应磁场,基于磁流体力学方程组,数值计算了二维金属液诱生流场,并追踪了夹杂物颗粒在流场中的运动轨迹.结果表明,金属液的感应磁场不可忽视,其诱生流场干扰了外加磁场下夹杂物的正常迁移,电磁净化宜采用扁管和圆管;同时,利用感应磁场的自约束性,单电流可实现金属液的电磁净化.     

AC-driven anomalous stochastic diffusion and chaotic transport in magnetic superlattices

We study regular and chaotic motion of the charge carriers in a two-dimensional electron gas subject to a spatially periodic magnetic field and to an AC electric field. We show how the interplay between the time-periodic electric and the spatially-periodic magnetic field leads to dynamical chaos and to fast stochastic diffusion of the electrons. The cases of a one-dimensional magnetic modulation with AC pumping and of a pure two-dimensional magnetic modulation are compared. We find the direct effect of anomalous diffusion and Lévy flights on the conductivity of a sample.     

Tribological behaviors of dialkyldithiophosphate-encapsulated Ni nanoparticles prepared by microwave-assisted reaction

We applied the microwave-assisted heating method to prepare the pyridine dialkyldithiophosphate-encapsulated Ni (Ni-DDP) nanoparticles. The surface-encapsulated Ni particles were characterized by Fourier transform infrared spectrometer, thermogravimetric analysis, X-ray diffraction, and transmission electron microscopy. Their tribological behavior in lubricate base oil was investigated on four-ball machines. The results show that uniform Ni-DDP nanoparticles about 12 nm can be obtained using microwave heating device and also they exhibit good tribological behaviors dispersed in liquid paraffin.     

Magnetic nanoparticles for application in cancer therapy

Magnetic particles play nowadays an important role in different technological areas with potential applications in fields such as electronics, energy and biomedicine. In this report we will focus on the hyperthermia properties of magnetite nanoparticles and the effect of several chemical/physical parameters on their heating properties. We will discuss about the need of searching new smaller magnetic systems in order to fulfill the required physical properties which allow treating tumoral tissues more efficiently by means of magnetically induced heat. Preliminary results will be shown about the effect of a biocompatible shell of core–shell magnetite NPs on the heating properties by application of a RF magnetic field.     

Anisotropy and relaxation processes of uniaxially oriented CoFe2O4 nanoparticles dispersed in PDMS

When a uniaxial magnetic field is applied to a non-magnetic dispersive medium filled with magnetic nanoparticles, they auto-assemble into thin needles parallel to the field direction, due to the strong dipolar interaction among them. We have prepared in this way magnetically oriented nanocomposites of nanometer-size CoFe₂O₄ particles in a polydimethylsiloxane polymer matrix, with 10% w/w of magnetic particles. We present the characteristic magnetic relaxation curves measured after the application of a magnetic field forming an angle α with respect to the needle direction. We show that the magnetic viscosity (calculated from the logarithmic relaxation curves) as a function of α presents a minimum at α=0, indicating slower relaxation processes associated with this configuration of fields. The results seems to point out that the local magnetic anisotropy of the nanoparticles is oriented along the needles, resulting in the macroscopic magnetic anisotropy observed in our measurements.     

Ultrawide frequency linewidth of the permeability spectra of FeCoN thin film sputtered on the flexible substrate

Analysis shows that it is possible to make use of dispersed magnetic ripple fields to obtain a wide frequency linewidth of permeability spectra of soft magnetic thin films. As-sputtered FeCoN thin film sputtered on flexible Kapton substrate is studied as an example. It has ultrawide frequency linewidths of its resonance peaks in the permeability spectra, compared to its counterpart deposited on Si substrate. The frequency linewidth of FeCoN on Kapton substrate decreases with external magnetic field, showing a different field dependence from that of FeCoN on Si substrate. The ultrawide frequency linewidth and its decrease with external magnetic field are ascribed to the dispersed magnetic ripple fields caused by the flexible substrate. This work shows that the flexible substrate is effective in obtaining a wide frequency linewidth of the permeability spectra of soft magnetic thin films.