External Static Magnetic Field Research Articles

Magneto-optical responses of microcavity-integrated graphene photonic crystals in the infrared spectral region

The magneto-optical responses and photonic band gap properties of the microcavity-integrated graphene photonic crystals were numerically studied. The structure consists of a graphene sheet embedded between two mirror symmetric Bragg reflectors, under the influence of an external static magnetic field. The properties of the microcavity resonance mode were investigated, considering the right- and left-handed circular polarization transmission coefficients and their phases, together with the Faraday rotation angle and ellipticity of the output light. The effects of the repetition number of the Bragg reflectors, thickness of the microcavity central layer and refractive indices of the graphene adjacent layers were considered. The obtained results revealed that a pure linear polarized output light with no ellipticity and high transmittance enhanced Faraday rotation can be achieved. These results can be utilized in designing a variety of graphene based photonic devices and magneto-optical integrated elements, such as miniaturized isolators or circulators.

Instability of surface electron cyclotron TM-modes influenced by non-monochromatic alternating electric field

The influence of non-monochromaticity of an external alternating electric field on excitation of TM eigenmodes at harmonics of the electron cyclotron frequency is considered here. These TM-modes propagate along the plasma interface in a metal waveguide. An external static constant magnetic field is oriented perpendicularly to the plasma interface. The problem is solved theoretically using the kinetic Vlasov-Boltzmann equation for description of plasma particles motion and the Maxwell equations for description of the electromagnetic mode fields. The external alternating electric field is supposed to be a superposition of two waves, whose amplitudes are different and their frequencies correlate as 2:1. An infinite set of equations for electric field harmonics of these modes is derived with the aid of nonlinear boundary conditions. This set is solved using the wave packet approach consisting of the main harmonic frequency and two nearest satellite temporal harmonics. Analytical studies of the obtained set of equations allow one to find two different regimes of parametric instability, namely, enhancement and suppression of the instability. Numerical analysis of the instability is carried out for the three first electron cyclotron harmonics.

Chaotic behavior of collective ion dynamics in the presence of an external static magnetic field

The two-dimensional nonlinear collective ion dynamics in the presence of external magnetic field in an electron-ion plasma is investigated. The analysis is performed for traveling plane waves to elucidate the various aspects of the phase-space dynamics. The presence of magnetic field makes the dynamics of the nonlinear wave complex with a complicated phase-space behavior. Thus, the nonlinear wave supports a wide class of nonlinear structures viz., single soliton, multi-soliton, periodic, and quasi-periodic oscillations depending on the values of M (Mach number) and Ω (the ratio of ion gyro-frequency to the ion plasma frequency). The computational results predict the chaotic behavior of the nonlinear wave and the transition to chaos takes place when Ω≳0.35 depending on the direction of propagation and the value of M. The amplitude of the wave depends on the obliqueness of the propagation and Mach number, whereas the magnetic field changes the dispersion properties of the wave.

Linear and nonlinear magneto-optical rotation on the narrow strontium intercombination line

In the presence of an external static magnetic field, an atomic gas becomes optically active, showing magneto-optical rotation. In the saturated regime, the coherences among the excited substates give a nonlinear contribution to the rotation of the light polarization. In contrast with the linear magneto-optical rotation, the nonlinear counterpart is insensitive to Doppler broadening. By varying the temperature of a cold strontium gas, we observe both regimes by driving the $J=0\rightarrow J=1$ transition on the intercombination line. For this narrow transition, the sensitivity to the static magnetic field is typically three orders of magnitude larger than for a standard broad alkali transition.

Enhanced terahertz radiation generation by two-color laser pulses propagating in plasma

AbstractA one-dimensional (1D) numerical model for studying enhanced terahertz (THz) radiation generation by mixing of ordinary and extraordinary modes of two-color laser pulses propagating in magnetized plasma has been presented. The direction of the static external magnetic field is such that one of the two laser pulses propagates in the extraordinary mode, while the other pulse propagates in the ordinary mode, through homogeneous plasma. A transverse electromagnetic wave with frequency in the THz range is generated due to the presence of the external magnetic field. It is observed that larger amplitude THz radiation can be generated by mixing of the ordinary and extraordinary modes of the two-color laser pulses as compared with the single laser pulse propagating in the extraordinary mode. Further, 2D simulations using the XOOPIC code show that the fields obtained via simulation study are compatible with those obtained from the numerical model.

Dispersion regions overlapping for bulk and surface polaritons in a magnetic-semiconductor superlattice.

Extraordinary dispersion features of both bulk and surface polaritons in a finely stratified magnetic-semiconductor structure which is influenced by an external static magnetic field in the Voigt geometry are discussed in this Letter. It is shown that the conditions for total overlapping dispersion regions for the simultaneous existence of bulk and surface polaritons can be reached, providing a conscious choice for the constitutive parameters and material fractions for both magnetic and semiconductor subsystems.

On the in-plane uniaxial anisotropy formation by using Fe–Co–Zr–N films: A theoretical and experimental investigation

In the present paper a simple theoretical approach for the in-plane uniaxial anisotropy evolution in thin films is introduced. In order to show, what are the conditions for a uniaxial anisotropy formation during annealing a ferromagnetic film in an external static magnetic field, a Hamiltonian, i.e., mean energy balances were established with introducing their annealing temperature dependence. At this point, a 1-dimesional chain-like arrangement of Fe and Co elements for an “isotropic” and uniaxial anisotropy state for the numerical computation was assumed. It was shown that a critical energy and annealing temperature (temperature threshold) can be attained from which a uniaxial anisotropy arises. Comparatively, calculations according to the Neél theory delivered the activation energy for inducing a uniaxial anisotropy. The experimental verification of the calculations, by using Fe40Co37Zr11N12 films which were produced by reactive magnetron sputtering, yielded the activation energy of about 250meV. Annealing temperatures above 473K (200°C) enabled marked uniaxial anisotropies. This correlated with the numerical quantum mechanical estimations which yielded a critical annealing temperature of approximately 449K (176°C). The calculated critical energy of 243meV was in a good agreement with the verified experiments if one assumes a short range order of at least 10 ferromagnetic atoms in line (5Fe+5Co) for computation.

Observation of superconductivity in hydrogen sulfide from nuclear resonant scattering.

High-temperature superconductivity remains a focus of experimental and theoretical research. Hydrogen sulfide (H2S) has been reported to be superconducting at high pressures and with a high transition temperature. We report on the direct observation of the expulsion of the magnetic field in H2S compressed to 153 gigapascals. A thin (119)Sn film placed inside the H2S sample was used as a sensor of the magnetic field. The magnetic field on the (119)Sn sensor was monitored by nuclear resonance scattering of synchrotron radiation. Our results demonstrate that an external static magnetic field of about 0.7 tesla is expelled from the volume of (119)Sn foil as a result of the shielding by the H2S sample at temperatures between 4.7 K and approximately 140 K, revealing a superconducting state of H2S.

Microstructural Control of Colloidal‐Based Ceramics by Directional Solidification Under Weak Magnetic Fields

The use of weak magnetic fields to control the microstructural evolution of colloidal‐based systems in conjunction with directional solidification is demonstrated as a convenient processing route to fabricate anisotropic ceramic scaffolds with complex microarchitectures. A variety of graded and aligned microstructures were formed by applying external static magnetic fields oriented radially, axially, and transversely with respect to the solidification direction of freezing slurries containing micro/nanoparticles of ZrO2 and Fe3O4. The graded structures, formed by the radial and axial fields, resemble core–shell architectures composed of dense outer perimeters surrounding porous inner cores. The aligned structures, formed by transverse fields, exhibit two modes of microstructural alignment: lamellar walls aligned by the growing ice crystals and mineral bridges aligned by the magnetic fields. The alignment of mineral bridges that connect adjacent lamellae, provide these scaffolds enhanced strength and stiffness when compressed parallel to their orientation (parallel to the direction of the magnetic field).

Rashba spin–orbit interaction effect on the optical properties of a disk-like quantum dot

Using the density matrix approach and iterative method, we have theoretically investigated the optical properties of a disk-like quantum dot (QD) with hard-wall confining potential subjected to an external static magnetic field in the presence of a Rashba spin–orbit interaction. The results of numerical calculations for the typical InAs QD show that the linear, nonlinear and total absorption coefficients (ACs) and refractive index (RI) changes are sensitive to the parameters of the magnetic field and dot size. Moreover, the resonance peaks of ACs and RI changes redshift when increasing the Rashba spin–orbit interaction strength. The role of this interaction as a control parameter on the ACs and RI changes has been demonstrated in detail.

Determination of the exchange constant of Tb0.3Dy0.7Fe2 by broadband ferromagnetic resonance spectroscopy.

We present measurements of the exchange stiffness D and the exchange constant A of a sputtered 80 nm Tb0.3Dy0.7Fe2 film. Using a broadband ferromagnetic resonance setup in a wide frequency range from 10 GHz to 50 GHz, multiple perpendicular standing spin-wave resonances were observed with the external static magnetic field applied in-plane. The field corresponding to the strongest resonance peak at each frequency is used to determine the effective magnetization, the g-factor and the Gilbert damping. Furthermore, the dependence of spin-wave mode on field-position is observed for several frequencies. The analysis of spin-wave resonance spectra at multiple frequencies allows precise determination of the exchange stiffness D = (2.79 ± 0.02) × 10-17 T · m2 for an 80 nm thick film. From this value, we calculated the exchange constant A = (9.1 ± 0.1) pJ · m-1.

Magnetism of tryptophan and walk memory of proteins

Biased or correlated random walk of proteins in the intracellular compartment may shape various cellular responses. We report sensitivity of protein walk to external static magnetic field that is equivalent to subjecting the protein solution to a spin perturbation. We explain this spin-response by super-paramagnetism of individual tryptophan residues present in a set of randomly chosen proteins. The correlated randomwalk system can thus facilitate magnetosensing in biological system, a subject that is of some recent interest.

Magnetic nanocomposite scaffolds combined with static magnetic field in the stimulation of osteoblastic differentiation and bone formation

Magnetism has recently been implicated to play significant roles in the regulation of cell responses. Allowing cells to experience a magnetic field applied externally or scaffolding them in a material with intrinsic magnetic properties has been a possible way of utilizing magnetism. Here we aim to investigate the combined effects of the external static magnetic field (SMF) with magnetic nanocomposite scaffold made of polycaprolactone/magnetic nanoparticles on the osteoblastic functions and bone formation. The SMF synergized with the magnetic scaffolds in the osteoblastic differentiation of primary mouse calvarium osteoblasts, including the expression of bone-associated genes (Runx2 and Osterix) and alkaline phosphatase activity. The synergism was demonstrated in the activation of integrin signaling pathways, such as focal adhesion kinase, paxillin, RhoA, mitogen-activated protein kinase, and nuclear factor-kappaB, as well as in the up-regulation of bone morphogenetic protein-2 and phosphorylation of Smad1/5/8. Furthermore, the SMF/magnetic scaffold-stimulated osteoblasts promoted the angiogenic responses of endothelial cells, including the expression of vascular endothelial growth factor and angiogenin-1 genes and the formation of capillary tubes. When the magnetic scaffolds were implanted in mouse calvarium defects, the application of SMF significantly enhanced the new bone formation at 6 weeks, as revealed by the histological and micro-computed tomographic analyses. Current findings suggest that the combinatory application of external (SMF) and internal (scaffold) magnetism can be a promising tool to regenerative engineering of bone.

Electron spin rotations induced by oscillating Rashba interaction in a quantum wire

A novel method and nanodevice are introduced that allows to rotate the single electron spin confined in a gated electrostatic InSb nanowire quantum dot. Proposed method does not require application of any (oscillating or static) external magnetic fields. Our proposal instead employs spatial and time modulation of confining potential induced by electric gates, which, in turn leads to oscillating Rashba type spin-orbit coupling. Moving electron back and forth in such a variable Rashba field allows for realization of spin rotations around two different axes separately without using an external magnetic field. The results are supported by realistic three-dimensional time dependent Poisson-Schr\"{o}dinger calculations for systems and material parameters corresponding to experimentally accessible structures.

BEAM-FORMING AND BEAM-STEERING CAPABILITIES OF A RECONFIGURABLE PLASMA ANTENNA ARRAY

We present the numerical parametric study of a reconfigurable plasma antenna array (PAA) composed of a metallic half-wavelength dipole and a set of cylindrical plasma discharges arranged in a planar square lattice. Our results, obtained with the linear embedding via Green’s operators (LEGO) method, indicate that beam-forming and beam-steering functionality can be achieved and controlled by appropriately choosing the number and position of the active plasma discharges around the dipole. Furthermore, we show that an external static magnetic field and the plasma density have a noticeable effect on the radiation pattern of the antenna.

Magnetic nanocomposites for tissue regeneration therapies

Event Abstract Back to Event Magnetic nanocomposites for tissue regeneration therapies Rebecca Zhiyu Yuan1, Kaveh Memarzadeh1, Robert A. Brown2 and Jie Huang1 1 University College London, Mechanical Engineering, United Kingdom 2 University College London, Faculty of Medical Sciences, United Kingdom Introduction: The use of tissue engineering scaffolds to repair and promote bone formation has received much clinical attention over the past decades. However, to guide and stimulate cells towards the target site remains challenging. There are increasing efforts to promote current cell therapies by developing new methodologies to target, monitor, and control cells. Recently, magnetic nanoparticles (MNPs) have been identified as being capable of promoting the proliferation and differentiation of osteoblast under static magnetic field (SMF)[1]. Iron oxide nanoparticles (IONPs) are clinically approved and can be injected intravenously, however, the majority of the IONPs are rapidly cleared from the bloodstream by the reticuloendothelial system. IONPs have been encapsulated in a gel matrix; there are concerns of toxic side-effects of organic solvents. Encapsulated electrospun nanofibre web has offered an attractive alternative solution[2]. In this study, IONPs were synthesised and their nanofiber mesh were produced by eletrospinning (ES). A device to support cell culture under SMF was prepared by 3D printing, and used for in vitro study. Materials and Methods: IONPs were prepared by precipitation of iron (III) chloride hexahydrate and Iron (II) chloride tetrahydrate with an addition of ammonia[3]. The microstructure of IONPs was examined by Transmission Electron Microscopy (TEM) and phase purity was determined by X-ray diffraction (XRD). The biocompatibility of IONPs was assessed by in vitro culture using a human osteoblast MG63 cell model. IONPs / polyvinylpyrrolidone (PVP) / ethanol / water suspensions were formulated for ES, and various parameters were employed to control the size of fibres. The microstructure of the nanofibre mesh was examined by Scanning Electron Microscopy (SEM) and Scanning Probe Microscopy (SPM). Results and Discussion: By using TEM examination, the image analysis showed that the average diameter of the IONPs was 30∓ 20 nm. Phase purity was further confirmed by XRD; IONP nanofibre meshes were produced with size of 100 nm to 2 μm. An uniform distribution of IONPs in the nanocomposites was revealed form SPM (Fig. 1). Figure 1. (A) TEM micrograph of IONPs (B) SEM and (C) SPM micrographs of IONPs nanofibre mesh. Alamar Blue assay showed that MG63 cells were able to proliferate when in contact with IONPs coated substrates during 7 days culture. The cell activity was increased significantly under SMF (Fig. 2) indicating a stimulating effect. Figure 2. A comparison of the proliferation of MG63 cells under SMF and non-static magnetic field (NSMF) from alamar blue assay (n=3). Conclusion: Biocompatible iron oxide nanoparticles were successfully prepared and tested. The size and morphology of electrospun IONP nanofibres can be controlled by varying solution properties and processing parameters. With the aid of external static magnetic field, the three-dimensional magnetic nanocomposites scaffolds can offer great potential in magnetic cell guiding applications.

Influence of varying magnetic order in an external magnetic field on the electronic structure and Fermi surface within the t–J model

We investigate the electronic structure of the two-dimensional t–J model in a transverse external static magnetic field with canted long-range magnetic order using cluster perturbation theory. The distribution of the spectral weight in the whole range of fields from zero to ferromagnetic saturation is explored. We demonstrate the possibility of a sharp change in a distribution of spectral weight at the Fermi level associated with the magnetic correlations when varying magnetic field.

A Simulation Study on the Specific Loss Power in Magnetic Hyperthermia in the Presence of a Static Magnetic Field

Our purpose in this study was to present a method for estimating the specific loss power (SLP) in magnetic hyperthermia in the presence of an external static magnetic field (SMF) and to investigate the SLP values estimated by this method under various diameters (D) of magnetic nanoparticles (MNPs) and amplitudes (H0) and frequencies (f) of an alternating magnetic field (AMF). In our method, the SLP was calculated by solving the magnetization relaxation equation of Shliomis numerically, in which the magnetic field strength at time t (H(t)) was assumed to be given by , with Hs being the strength of the SMF. We also investigated the SLP values in the case when the SMF with a field-free point (FFP) generated by two solenoid coils was used. The SLP value in the quasi steady state (SLPqss) decreased with increasing Hs. The plot of the SLPqss values against the position from the FFP became narrow as the gradient strength of the SMF (Gs) increased. Conversely, it became broad as Gs decreased. These results suggest that the temperature rise and the area of local heating in magnetic hyperthermia can be controlled by varying the Hs and Gs values, respectively. In conclusion, our method will be useful for estimating the SLP in the presence of both the AMF and SMF and for designing an effective local heating system for magnetic hyperthermia in order to reduce the risk of overheating surrounding healthy tissues.

Tunable surface plasmon-polaritons in a gyroelectric slab sandwiched between two graphene layers

We study numerically the properties of surface plasmon-polaritons (SPPs) in a gyroelectric slab sandwiched between two graphene layers, where the external static magnetic field is applied in the Voigt geometry. It is shown that the dispersion characteristics and propagation lenghts of the SPPs for both the optical and the acoustic branches can be tuned flexibly by the external magnetic field and graphene's chemical potential, and that the nonreciprocal properties of the SPPs caused by the external magnetic field are rather obvious. The results provide a method for adjusting and improving the dispersion and propagation properties of the SPPs, which might be helpful for the design of the related plasmonic devices.

The effect of current frequency and magnetic field direction in alternative current-field annealing on the GMI and magnetic properties of Co-based wires

The effects of applied current annealing frequency, amplitude and magnetic field direction during alternative current-field annealing on giant magnetoimpedance (GMI) and magnetic properties of Co68.15Fe4.35B12.5Si15 wire have been investigated. This annealing process leads to a complex domain structure because of the existence of the variable internal circumferential field and external static magnetic field. We have demonstrated that the applied current annealing with different frequencies, amplitudes and magnetic field directions can improve the magnetic properties and the GMI response. In the as-quenched wire, GMImax and saturation magnetization (Ms) are equal to 220% and 78.5 emu/g respectively. They grow up to 400% and 87.2 emu/g, respectively, while the amplitude of the 50 Hz annealing current is 400 mA and the applied magnetic field, perpendicular to the wire, cancels out the earth's magnetic field. In contrast, the samples annealed by 50 Hz (500 mA) and 100 kHz (400 mA) alternative currents show GMImax = 50% and 140%, and also Ms = 41.8 and 40.7 emu/g, respectively. Altogether, our results show that alternative current-field annealing at appropriate conditions improves the GMI response and makes the samples more suitable for magnetic sensing.