External Dc Magnetic Field Research Articles

Optical Negative Refraction in Ferrofluids with Magnetocontrollability

We numerically demonstrate optical negative refraction in ferrofluids containing isotropic Fe3O4 nanoparticles, each having an isotropic Ag shell, in the presence of an external dc magnetic field H. The all-angle broadband optical negative refraction with magnetocontrollability arises from H-induced chains or columns. They result in hyperbolic equifrequency contour for transverse magnetic waves propagating in the system. The finite element simulations verify the analyses using the effective medium approximation. Experimental demonstration and potential applications are suggested and discussed.

Manipulating the transparency and other optical properties of metamaterials by applying a magnetic field

The light transmission through metallic films with different types of nano-structures was studied both theoretically and experimentally. It is shown that the positions of the surface plasmon resonances depend on nano-structural details. Those can be changed from sample to sample or in given sample by applying an external dc electric or magnetic field. The dependence of transmission spectrum on the shape of holes (inclusions) and external fields can be used for manipulation of the light transmission, as well as the polarization of the transmitted light and other optical properties, by external field. Two complementary situations are considered: a metal film with dielectric holes and a dielectric film with metallic islands. A new analytical asymptotic approach for calculation of the optical properties of such plasmonic systems is developed.

Coil-less fluxgate effect in amorphous wires

In this study, the coil-less fluxgate properties of as-received amorphous microwires with the compositions (Co 0.94Fe 0.06) 72.5Si 12.5B 15, Fe 77.5Si 7.5B 15 and Co 72.5Si 12.5B 15 have been investigated. As its name implies, a coil-less fluxgate is a new type of magnetic field sensor without a coil. When the (Co 0.94Fe 0.06) 72.5Si 12.5B 15 wire is magnetically saturated in the circumferential direction with a 30 kHz and 48 mA driving current and under the effect of 33 π rad/m torsional strain, there is a linear variation in the second harmonic of the voltage from wire ends, U wire, as a function of the applied external dc magnetic field, H z, along the wire length. The slope of the second harmonic versus H z curve changes in opposite directions as the direction of torsional strain changes from the positive to the negative direction. Coil-less fluxgate effects in as-received Fe 77.5Si 7.5B 15 and Co 72.5Si 12.5B 15 wires have also been investigated. A very small change in the second harmonic of voltage U wire is observed in Fe 77.5Si 7.5B 15 wire, with some small hysteresis.

Giant ac electrical response of La0.7Sr0.3MnO3in sub-kilogauss magnetic fields

AbstractWe report ac electrical transport in the metallic ferromagnet La0.7Sr0.3MnO3. Both ac resistance (R) and reactance (X) were measured as a function of temperature (T= 400-100 K), frequency of the ac current (f= 100 kHz – 20 MHZ) and external dc magnetic field (H= 0-100 mT) applied parallel to the current direction. It is shown that, whileR(H= 0 T) decreases smoothly around the Curie temperature (TC) forf= 100 kHz, an abrupt increase followed by a peak close toTCoccurs forf≥ 500 kHz. The peak decreases in magnitude, broadens and shifts down in temperature with increasing values ofH. The peak inRis completely suppressed underH= 100 mT resulting in a huge low-field ac magnetoresistance (R/R= -53 % forf= 2MHz) whereas the dc magnetoresistance only -31 % even at H = 7 T. While the reactanceX(H= 0 T) also shows an abrupt increase atTCforf< 10 MHz, it decreases abruptly atTCforf≥ 12 MHz. The magnetoreactance is largest (X/X= -47 %) atf= 100 kHz and it changes sign from negative to positive with increasing frequency. It is suggested that the observed huge ac magnetoresistance arises from decrease of magnetic permeability which enhances skin depth under a magnetic field. Our results indicate that the extraordinary sensitivity of the ac magnetoresistance to low dc magnetic fields can be exploited for device applications.

A GHz Spintronic-Based RF Oscillator

A nano-sized oscillator for RF applications is presented which is based on two spintronic effects, the tunneling magnetoresistance (TMR) and the spin momentum transfer torque. The oscillation frequency is several GHz and can be tuned by both a DC bias current and an external DC magnetic field. High compactness, high tunability and full compatibility with standard CMOS process make this spin torque nano-oscillator (STNO) a promising candidate for future RF transceivers. The main issues to be addressed are spectral purity and output power. First measurements on a hybrid built connecting the STNO to a dedicated wideband amplifier show that today's performance in terms of power is close to but not yet compatible with telecommunication standard requirements. Using time domain analysis we show that frequency fluctuations are an issue for spectral purity. Frequency synthesis concepts based on STNOs are also discussed.

Excitation of surface plasma waves across the layers of intrinsic Josephson junctions

We analytically study the excitation of surface Josephson plasma waves (SJPWs) propagating across the junctions in layered superconductors in the presence of external dc magnetic field. Both the attenuated total reflection and the modulation of the superconducting parameters methods of the SJPWs excitation are considered. We show that the reflection of the incident electromagnetic wave can be substantially decreased due to the resonance excitation of SJPWs, for certain angles and frequencies of the incident wave when changing the magnetic field. Moreover, we find physical conditions guaranteeing the total suppression of the specular reflectivity. The analytical results are supported by the numerical simulations.

Inverse shift of absorption peak versus applied dc magnetic field in interlayer exchange‐coupled trilayer ferromagnetic films with inplane uniaxial anisotropy

AbstractIn this paper, we theoretically demonstrate the somewhat novel properties of normally incident microwaves in interlayer exchange‐coupled trilayer ferromagnetic films induced by external dc magnetic fields. It is found that, under suitable conditions, one absorption peak associated with an optic mode may shift inversely versus the applied dc magnetic field in the magnetizations abnormally, which is also attributed to the coexistence of gyrotropy of the permeability tensor, interlayer exchange coupling (IEC), and inhomogeneous exciting fields along the film thickness. This work proposes a feasible way to tailor microwave properties of the multilayer ferromagnetic films by combining the effects of the multiple factors mentioned above.

Critical Current in YBCO Coated Conductors in the Presence of a Macroscopic Defect

We have studied the effects of localized defects in the YBCO coated conductors on the critical current. The artificial defects were introduced into 4, 10 and 12 mm wide tapes as cuts of various lengths made either by laser ablation or mechanical means. Transport measurements were carried out in an external variable magnetic field to obtain the I-V characteristics of the damaged areas. The distribution of the magnetic field in the vicinity of the defects has been mapped as well. The reduction of the critical current by the defects, with and without an external DC magnetic field are discussed and compared with existing theories. A criterion for determining the critical current in the area containing a defect is suggested.

Low-field magnetoimpedance in La 0.7Sr 0.3MO 3 (M=Mn,Co)

We compare the ac magnetotransport in two ferromagnetic metallic oxides, La 0.7Sr 0.3MO 3 (with M=Co and Mn) which show similar dc electrical properties. The ac resistance ( R ) and inductive reactance ( X = ω L ) of the complex impedance ( Z = R + j X ) are studied as a function of temperature at a few fixed frequencies ( f = 10 kHz to 7 MHz) of the ac current excitation in zero and a low external dc magnetic field ( μ 0 H d c = 60 mT ) . It is shown that as the frequency ( f ) increases, R in zero field shows a transition from a smooth decrease to an abrupt increase and a peak close to the ferromagnetic Curie temperature ( T C ) . A rapid increase in X at T C was also found for frequencies above 10 kHz. We report a large low-field ac magnetoresistance of 33% (7%) for Mn (Co) in μ 0 H d c = 60 mT at f = 2 MHz , comparable to the high field ( μ 0 H d c = 7 T ) dc magnetoresistance. It is suggested that the low-field ac magnetoresistance in these compounds arises from the change in the magnetic penetration depth and suppression of magnetic fluctuations close to T C .

Possible negative refraction in two dimensional ferromagnetic wire lattice

In this paper, we present theoretical and experimental results for the indication of negative refraction in ferromagnetic metallic wire lattice. We have studied microwave transmission through a two dimensional wire lattice made of ferromagnetic metallic wires under the applied static magnetic field. We have found that, the microwave transmission were significantly changed at ferromagnetic resonance frequency region. Thus the magnetic permeability can be tuned by external dc magnetic field. Since the dielectric permittivity of metallic wire lattice is negative and can take a value close to unity then the crystal exhibits negative index of refraction at microwave region under the external magnetic field.

Quantum dynamic simulations for single molecular magnets using anisotropic spin models

We performed quantum spin dynamics in anisotropic electron-spin systems to clarify the mechanism in single molecular magnets (SMM) theoretically. Usually, SMMs have large negative anisotropic D value and the large total spin angular moment ( S total). The splitting ground spin states in one SMM molecule are usually caused by the anisotropic terms ( D and E), which become more important as well as isotropic term ( J). In order to examine the effects of such anisotropic magnetic terms, model systems with large spin multiplicity of the ground state ( S total = 5 and 2) were treated. First, energy diagram in external magnetic field is shown, since negative D terms are responsible to the zero-field magnetic moment. Next, the quantum Liouvile approach is used for our spin dynamics simulation involving the relaxation processes of the system. Our simulations in S total = 5 system successfully reproduced magnetic quantum tunneling (MQT) behavior in DC-external magnetic field. Moreover, in S total = 2 system under AC-external magnetic field, frequency dependence of real ( χ′) and imaginary ( χ″) parts of χ can be shown with Fourier transformation. Our theoretically drawn half circles in Cole-Cole plot ( χ′ versus χ″) will give information to many experimental studies.

SO-FDTD analysis of EM scattering of magnetized ferrite

The shift operator （SO） method is extended to the case of magnetized ferrite subject to an external dc magnetic field of arbitrary direction. The FDTD with shift operator （SO-FDTD） formulations are derived based on the relative permeability tensor in magnetized ferrite and Maxwell curl equations of anisotropic media. The electromagnetic （EM） scattering by magnetized ferrite layer, sphere and Von Karman radome with magnetized ferrite coating is computed by the proposed method respectively. The computed results indicate the correctness and feasibility of our method.

Origin of the plateau in the temperature dependence of the normalized magnetization relaxation rate in disordered high-temperature superconductors

The temperature $T$ dependence of the normalized magnetization relaxation rate $S$ in optimally doped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films with the external dc magnetic field $H$ oriented along the $c$ axis exhibits the well-known plateau in the intermediate $T$ range, associated with the presence of elastic (collective) vortex creep. The disappearance of the $S(T)$ plateau in the high-$H$ domain $(H\ensuremath{\ge}20\text{ }\text{kOe})$ is not completely understood. We show that in the case of high-temperature superconductors with significant quenched disorder the $S(T)$ plateau is directly related to a crossover in the vortex-creep process generated by the macroscopic currents induced in the sample. In dc magnetization measurements the creep-crossover temperature decreases rapidly with increasing $H$, reaching the low-$T$ region where the magnetization decay is dominated by micro flux jumps. Consequently, at high $H$ no well-defined elastic-creep domain is present and the $S(T)$ plateau disappears.

Magnetic-field-induced optical anisotropy in ferrofluids: A time-dependent light-scattering investigation

We report an experimental investigation of time dependent anisotropic light scattering by an aqueous suspension of tetramethyl ammonium hydroxide coated Fe3O4 nanoparticles (approximately 6 nm) under the ON-OFF transient of an external dc magnetic field. The study employs the synchronized recording and measurement of the two magnetic-field-induced light-scattering patterns produced by two identical orthogonal He-Ne laser beams passing through the ferrofluid sample and propagating parallel and perpendicular to the applied field, respectively. From these patterns, we extract the time dependence of the induced optical anisotropy, which provides a measure of the characteristic time scale and kinematic response for field-induced structure formation in the sample. We propose that the time evolution of the scattering patterns, which is very fast at short times and significantly slower at long times, can be explained using a model based on a two-stage chain formation and coarsening processes.

Capture of nonmagnetic particles and living cells using a microelectromagnetic system

We develop a microelectromagnetic system to trap nonmagnetic materials such as micropolystyrene particles and yeast cells in particular areas. We fabricate gold films, the width of the central narrow part is 22 μm, and flow an electric current through the films. We then apply an external uniform dc magnetic field to weaken the local magnetic field at the narrow part so that a nonuniform magnetic field is produced. We demonstrate that the particles, which are dispersed in magnetic fluid, are successfully trapped at the narrow part of the film. We evaluate the driving force acting on a microparticle in the nonuniform magnetic field and carry out a Stokesian dynamics simulation of the motion of the particles. We show that yeast cells are also trapped at the narrow part of the film. Finally, we fabricate multichannel microelectromagnets so that yeast cells are trapped at multiple points in the microelectromagnetic system. The present system may be applied to cell transfection on a cell microarray and, therefore, eventually contribute to progress in the identification and determination technologies of different drug targets and the functions of genes and proteins.

Feeding position dependent effects of Ag thick film EMC patch antenna due to Mn substituted Mg ferrite loading

The characteristics of Ag thick film electromagnetically coupled patch antenna due to Mn substituted Mg ferrite loading is reported. The effect of thickness of ferrite and feed position is also studied. Ferrite loading in the absence of external dc magnetic field provides additional resonance frequencies of the same antenna. The intrinsic magnetisation of the ferrite also seems to be contributing to the observed effects. The off resonance radiated output increases due to the ferrite overlay. The non-symmetries in the E plane pattern are enhanced due to the ferrite loading.

Broadband ferromagnetic resonance measurements of a micromagnetic disk array using a meander-line technique

A simple technique involving transmission through a copper-wire meander line is developed to study ferromagnetic resonance (FMR) in magnetic arrays. Using this technique, we carried out FMR measurements on a square array of submicrometer-size Permalloy magnetic disks for an external dc magnetic field parallel to the plane of the array, and found several magnetic excitation modes. The dominant mode obeys the Kittel formula for a plain film. The technique is suitable for studying the collective excitation modes in the magnetic nanoarrays over a broad range of frequencies.

Powerful THz sources based on laser wakefields

MW-order coherent THz pulses with a broad spectrum can be produced from a laser-driven wakefield through linear mode conversion in inhomogeneous magnetized plasmas. The external dc magnetic field is a few tesla. By changing the strength and direction of the magnetic field, one can enhance or suppress the THz emission. The maximum energy conversion efficiency in the magnetized plasmas can be double that in the unmagnetized plasmas. We also propose that single-cycle THz pulse can be directly emitted from a transient net current driven by laser ponderomotive force in a THz-wavelengh-scale plasma slab. Theory and simulations show that this THz source is capable of providing power of MW-GW, field strength of MV/cm-GV/cm, and tunability range of 0.1-10 THz, which would open up new horizons for nonlinear THz science and applications.

Hybrid magnetoresistive∕microelectromechanical devices for static field modulation and sensor 1∕f noise cancellation

Low frequency 1∕f noise in magnetoresistive (MR) spin valve sensors was suppressed by modulating an external dc magnetic field at high frequency microelectromechanical system using a (MEMS) microcantilever structure with an integrated magnetic flux guide. With this hybrid MR∕MEMS device, direct detection of dc magnetic fields in the sensor high frequency thermal noise regime was achieved. The microcantilever was actuated using a gate electrode by applying an ac voltage with frequency f, causing it to oscillate at 2f. Measurements show detection of a dc magnetic field at 2f frequency (400kHz), where sensor 1∕f noise is two orders of magnitude lower than dc.

Stable and unstable thermo -current states of high temperature superconductors

Formation peculiarities of the stable and unstable states of high-Tc superconductors are discussed. To understand the basic physical trends, which are characteristic for the current penetration mechanism in high temperature superconductors, the operating states of Bi2212 slab without stabilizing matrix placed in DC external magnetic fields at low coolant temperature are theoretically investigated. It is proved that the temperature of a high-Tc superconductor is not equals to the coolant temperature before instability onset. Therefore, the voltage-current characteristic of a high-Tc superconductor has only a positive slope during continuous current charging. As a result, it does not allow one to find the boundary between stable and unstable thermo - current states. This peculiarity has to be considered during experiments at which the critical current of high-Tc superconductors is defined.