Magnetization and actuation of polymeric microstructures with magnetic nanoparticles for application in microfluidics

An increasing number of lab-on-a-chip devices require advanced fluid manipulations. We intend to address this requirement by incorporating polymeric responsive materials on the walls of the microfluidic channels of such devices. In this paper we present a magnetic polymer made from commercially available functionalized magnetic nanoparticles and PDMS. Loadings of this polymer up to 5% volume of magnetic material were achieved. We report on the Young's modulus of this material and describe its magnetization behavior with a combination of inter-particle interaction and particle cluster demagnetization effects. The magnetic polymer can have a magnetic susceptibility up to 0.5 and by curing in a magnetic field, a magnetic anisotropy of a factor 2 in susceptibility can be created. Finally, a finite element model simulation is performed to quantify the amplitude of motion of a microstructure made of this magnetic polymer, and the local magnetic actuation with a current running in a micro-fabricated wire is discussed.     

Magnetic nanoparticles: Internal probes and heaters within living cells

Tagging living cells with magnetic nanoparticles raised increasing interest in the fields of magnetic resonance imaging, magnetic hyperthermia, cell sorting or tissue engineering. Here, we demonstrate that the confinement of magnetic nanoparticles at the intracellular level, inside endosomes vesicles, allows developing original magnetic manipulations in response to different magnetic field solicitations. These manipulations are used to deform cellular internal membrane, to probe the cell architecture or to heat the cell from its inside.     

On scaling laws in turbulent magnetohydrodynamic Rayleigh-Benard convection

We invoke the concepts of magnetic boundary layer and magnetic Rayleigh number and use the rates of magnetic energy dissipation in the bulk and the boundary layers to derive some scaling laws expressing how the Nusselt number depends on the magnetic Rayleigh number, Prandtl number and magnetic Prandtl number for the simple case of turbulent magnetohydrodynamic Rayleigh-Benard convection in the presence of a uniform vertical magnetic field.     

Magnetic relaxations in amorphous soft magnetic alloys

The paper reviews selected results of the extended experimental investigations of magnetic properties, time–temperature stability and workability of the soft magnetic amorphous alloys controlled by structural magnetic relaxation. Complex approach to the magnetic relaxations in multicomponent amorphous alloys is presented. The transition from magnetic after-effect to a new MAE spectrometry is illustrated on ternary amorphous CoSiB alloy.     

Magnetic field induced incommensurate resonance in cuprate superconductors

The influence of a uniform external magnetic field on the dynamical spin response of cuprate superconductors in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic scattering around low and intermediate energies is dramatically changed with a modest external magnetic field. With increasing the external magnetic field, although the incommensurate magnetic scattering from both low and high energies is rather robust, the commensurate magnetic resonance scattering peak is broadened. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime. The theory also predicts that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying an external magnetic field large enough.     

Magnetoelastic interactions in multilayer microwires

Amorphous magnetic microwires exhibit outstanding magnetic characteristics as giant Barkhausen jump or nearly non-hysteretic behaviour that make them very suitable as sensing elements in various devices. In this article, we overview the different families of microwires and summarise most relevant results in connection with the magnetoelastic interlayer interactions in multilayer microwires. Improved magnetic behaviour can be observed in bi-layer microwires consisting of a magnetic nucleus coated by insulating cover. By employing combined sputtering and electroplating techniques, a novel series of multilayered magnetic microwires have been recently introduced. They consist typically of a magnetic nucleus and several shells having insulating and/or metallic nature, the latter magnetic or not. The magnetic character of the multilayer microwire will then depend on the magnetoelastic coupling between layers. External layers induce compressive stresses on the soft amorphous nucleus resulting in induced axial or circular anisotropies depending on magnetostriction sign. In a similar way, reduction of measuring temperature results in tensile stress as a consequence of different thermal expansion coefficients of various layers.     

Magnetic Fluctuation Measurement in Sino United Spherical Tokamak Plasma

To investigate the magnetic fluctuations and for further transport study, the poloidal and radial magnetic field t is conducted on the Sino United Spherical Tokamak （SUNIST）. Auto-power spectral density indicares that the magnetic fiuctuation energy mainly concentrates in the frequency region lower than lO kttz. The magnetic field oscillations, which are characterized by harmonic frequencies of 40 kHz, are observed in the scrape- off layer; by contrast, in the plasma core, the magnetic fluctuations are of Gaussian type. The time-frequency profiles show that the poloidal magnetic fluctuations are temporally intermittent. The autocorrelation calculation indicates that the fluctuations in decorrelation time vary between the core and the edge.     

Monte Carlo calculation of the magnetization behavior and the magnetocaloric effect of interacting particles

The magnetization behavior and the magnetic entropy change of a system made up of ferromagnetically interacting particles are calculated by using Monte Carlo simulation. The effect of the magnetic anisotropy of particles and the dipolar–dipolar interaction between particles on the magnetization and the magnetic entropy change of the system are discussed. It is found that there is no spontaneous magnetization, both the magnetic anisotropy of particles and the dipolar–dipolar interaction between particles restrains the system's magnetizing in the external magnetic field. The magnetic entropy change decreases with the increase in temperature in the system without the dipolar–dipolar interaction; however, the dipolar–dipolar interaction between particles makes the magnetic entropy change of the system have maximum value at low temperatures.     

Soft X-ray resonant magnetic scattering study of magnetization reversal in low dimensional magnetic heterostructures

Soft X-ray resonant magnetic scattering (SXRMS) has been used to investigate the microscopic magnetization reversal behavior of complex magnetic systems. SXRMS is a unique technique, providing chemical, spatial and magnetic sensitivity, which is not affected by external magnetic fields. The study of two selected thin magnetic heterostructures is presented, amorphous rare-earth transition metal alloys and perpendicular exchange coupled antiferromagnetic/ferromagnetic films. In the first system, the internal structure of magnetic stripe domains on nanometer length scales is obtained by measuring bi-dimensional (2D) scattering images. In the second system, the element specificity is exploited to identify the role of the uncompensated spins in the antiferromagnetic layer on the exchange coupling phenomena. Future trends are also discussed.     

Tuning the band gap of self-assembled superparamagnetic photonic crystals in colloidal magnetic fluids using external magnetic fields

The model of tunable superparamagnetic photonic crystals self-assembled in colloidal magnetic fluids under externally applied magnetic fields is established. The mechanisms, which are in charge of the tunability of the band gaps with magnetic fields are clarified. The band structures of the triangularly-arrayed two-dimensional photonic crystals with limited heights of magnetic columns are calculated with the experimental data of structures and refractive indices in the literatures. The field-dependent properties of the first band gaps are gained for the z-odd and z-even modes, respectively. Simulation results indicate that the mid frequencies of the first band gaps of the z-odd modes can be easily tuned by the external magnetic fields, while those of the z-even modes bear relatively weak dependence on the external magnetic fields. Simultaneously, the first band gaps of both kinds of modes become wide along with the increase of the magnetic fields. The results presented in this work give a guideline for realizing the tunable photonic crystals with magnetically colloidal materials and magnetic stimuli.     

Evolution of spin dynamics in electron-doped cuprate superconductors

Within the kinetic energy driven superconducting mechanism, the evolution of the magnetic excitations of the electron-doped cuprates in the superconducting state is studied. It is shown that there is a broad commensurate low energy magnetic scattering peak, while the magnetic resonance energy is located among this broad commensurate low energy scattering range. This broad commensurate low energy magnetic scattering disperses outward into a continuous ring-like incommensurate magnetic scattering at high energy.     

Thermal entanglement in the anisotropic Heisenberg XYZ model with different inhomogeneous magnetic fields

Thermal entanglement is investigated in a two-qubit Heisenberg XYZ system with different inhomogeneous magnetic fields. It is found that different magnetic fields have different entanglement and critical values. In addition, according to the relation of spin-spin coupling coefficients, a more efficient control parameter of magnetic field can be obtained by adjusting the direction of external magnetic field.     

Theoretical Study on Ion Escape in Martian Atmosphere

Based on the observation that Martian magnetic moment is gradually reducing from the ancient to the present, we investigate the O^＋ ion flux distribution along magnetic field lines and the ion escaping flux in Martian tail with different assumed Martian magnetic moments. The results show that the O^＋ ion flux along magnetic field lines decreases with distance from Mars; the ion flux along the field line decreases more quickly if the magnetic moment is larger; the larger the magnetic moment, the smaller the ion escaping flux in the Martian tail. The ion escaping flux depends on Z-coordinate in the Martian tail. With decrease of the magnetic moment, the ion escaping flux in the Martian tail increases. The results are significant for studying the water loss from Mars S uFface.     

Magnetic properties of Co-doped SnO2 at different carrier concentrations

Magnetic properties of Co-doped wide-gap semiconductor SnO₂ were studied theoretically by using the PPLCAO first-principles computational scheme. Since the carrier plays an important role on magnetic properties about diluted magnetic semiconductors (DMS) materials, we discuss the origin of magnetic moments and the magnetic ordering mechanism with different carrier concentration in Co-doped SnO₂ based on calculated spin density distribution. It is found that, the RKKY interaction is dominated in the magnetic coupling in Co-doped SnO₂.     

Mach–Zehnder interferometer by utilizing phase modulation of transmitted light through magnetic fluid films possessing tunable refractive index

Due to its diverse applications in photonics, bio-sensors, mechanics, etc., Mach–Zehnder interferometer becomes one of important devices. Hence, lots of efforts have been paid to develop advanced Mach–Zehnder interferometers. In this work, we explore new-model Mach–Zehnder interferometer, in which one of arm is consisted of magnetic fluid films. By utilizing the tunable refractive index of magnetic fluid films under external magnetic fields, the traveling phase of a propagating light through the magnetic fluid film is changed. This could lead to a variation in the interfered intensity of the Mach–Zehnder interferometer when an external magnetic field is applied. The modulation in the interfered intensity by the external magnetic field is demonstrated experimentally, and the relevant physical origin is also discussed.     

From bubble to Skyrmion: Dynamic transformation mediated by a strong magnetic tip

Skyrmions in thin metallic ferromagnetic films are stable due to competition between the RKKY interaction and uniaxial magnetic anisotropy. We study static nonlinear excitations in magnetic film in the presence of strong cylindrical magnetic tip of nanometer size. We mimic the RKKY interaction by the next-nearest-neighbors ferromagnetic and antiferromagnetic exchange interactions. We demonstrate analytically and numerically dissipative transformation of a bubble created by a strong magnetic tip into a stable Skyrmion.     

Spontaneous and field-induced magnetic transitions in YBaCo2O5.5

A detailed study of magnetic properties of cobaltite YBaCo₂O_5.5 has been performed in high (up to 35 T) magnetic fields and under hydrostatic pressure up to 0.8 GPa. The temperatures of paramagnet-ferromagnet (PM-FM) and ferromagnet-antiferromagnet (FM-AF) phase transitions and their pressure derivatives have been determined. It has been revealed that in the compound with yttrium, in contrast to those with magnetic rare earth atoms, the AF-FM field-induced magnetic phase transition is accompanied by a considerable field hysteresis below 240 K, and the magnetic field of 35 T is not sufficient to complete this transition at low temperatures. The hysteresis value depends on the magnetic field sweep rate, which considered as an evidence of magnetic viscosity that is especially strong in the region of coexistence of the FM and AF phases. High values of susceptibility for the field-induced FM phase show that Co spin state in these compounds changes in strong magnetic field.     

Direct comparison of the aging and memory effects of magnetic nanoclusters and nanoparticles

The magnetic characteristics of a dense magnetic nanoparticle system and a spin glass system consisting of magnetic nanoclusters are compared. Zero field cooled and field cooled magnetization measurements, including aging and memory experiments, of the nanoparticle and the magnetic cluster systems show similar characteristics, suggesting a common origin for the spin glass-type behavior of the magnetic nanoparticle and nanocluster systems.     

Magnetic field effects on the optical properties of an azo-dye doped liquid crystal

Homeotropically aligned nematic liquid crystals doped with azo-dye were subjected both to a linear polarized light of a He–Ne laser and to a magnetic field perpendicular to the incident light beam. We found that the emerging light was elliptically polarized when using magnetic field strengths higher than the threshold value for the magnetic Freedericksz transition. The light transmission, the rotatory power (induced by azo-dye) and the ellipticity varied quasiperiodically when increasing magnetic field strength. The number and positions of maxima and minima depend on the cell thicknesses. Changes in the phase difference between the emergent ordinary and extraordinary rays were computed from the experimental data and the magnetic field dependence of the birefringence was determined.     

Magnetic particle separation using controllable magnetic force switches

Magnetic particle separation is very important in biomedical applications. In this study, a magnetic particle microseparator is proposed that uses micro magnets to produce open/closed magnetic flux for switching on/off the separation. When all magnets are magnetized in the same direction, the magnetic force switch for separation is on; almost all magnetic particles are trapped in the channel side walls and the separation rate can reach 95%. When the magnetization directions of adjacent magnets are opposite, the magnetic force switch for separation is off, and most magnetic particles pass through the microchannel without being trapped. For the separation of multi-sized magnetic particles, the proposed microseparator is numerically demonstrated to have high separation rate.