Frequency Of Ac Field Research Articles

Structural and magnetic studies of the frustrated S = 1 kagome magnet NH4Ni2Mo2O10H3

The strong geometric frustration of the kagome antiferromagnets (KAFMs) can destabilise conventional magnetic order and lead to exotic electronic states, such as the quantum spin-liquid state observed in some S=12 KAFM materials. However, the ground state of S = 1 KAFM systems are less well understood. Spin nematic phases and valence bond solid ground states have been predicted to form but a paucity of experimental realisations restricts understanding. Here, the S = 1 KAFM NH4Ni2Mo2O10H3 is presented, which has the 3-fold symmetry of the kagome lattice but significant site depletion, with ∼64% site occupancy. Frustration and a competition between exchange interactions are evidenced through the suppression of order below the Weiss temperature |θW| and observation of ferromagnetic and antiferromagnetic characteristics in the magnetisation data. A semi spin glass ground state is predicted based on the ac-field frequency dependence of the magnetic transition and ferromagnetic signal.

Frequency-tunable magnetic field sensing using continuous-wave optically detected magnetic resonance with nitrogen-vacancy centers in diamond

The nitrogen-vacancy (NV) center is a promising candidate to realize practical quantum sensors with high sensitivity and high spatial resolution at room temperature and atmospheric pressure. In conventional high-frequency AC magnetometry with NV centers, the setup requires a pulse sequence with an appropriate time synchronization and strong microwave power. To avoid these practical difficulties, AC magnetometry using continuous-wave optically detected magnetic resonance (CW-ODMR) was recently demonstrated. That previous study utilized radio frequency (RF)-dressed states generated by the coherent interaction between the electron spin of the NV center and the RF wave. However, the drawback of this method is that the detectable frequency of the AC magnetic fields is fixed. Here, we propose and demonstrate frequency-tunable magnetic field sensing based on CW-ODMR. In the new sensing scheme, we obtain RF double-dressed states by irradiation with RF fields at two different frequencies. One creates the RF-dressed states and changes the frequency of the target AC field. The other is a target AC field that induces a change in the CW-ODMR spectrum by generating the RF double-dressed states through coherent interaction with the RF-dressed states. The sensitivity of our method is estimated to be comparable to or even higher than that of the conventional method based on the RF field with a single frequency. The estimated bandwidth is 7.5 MHz, higher than that of the conventional method using the RF-dressed states. Our frequency-tunable magnetic field sensor based on CW-ODMR paves the way for new applications in diamond devices.

Magnetic particle spectroscopy and magnetic particle imaging of zinc and cobalt ferrite nanoparticles: Distinct relaxation mechanisms

Magnetic particle imaging (MPI) is an emerging imaging method based on the nonlinear response of superparamagnetic tracers to a sinusoidal AC magnetic field. Higher harmonics obtained by the Fourier transform of acquired signals form a so-called magnetic particle spectrum, whereby straightforward evaluation of different nanoparticles as potential MPI tracers can be carried out. The shape of the spectra is largely dictated by the combined Néel and Brownian relaxation of superspins, whose relative contributions vary significantly depending on the tracer properties. The present study is focused on the comparison of several Zn ferrite and Co ferrite samples, selected for their different magnetic behaviors, together with the commercial tracer Resovist® (SH U 555 A). A new custom-made magnetic particle spectrometer and a commercial MPI system (Bruker) with comparable AC field amplitudes (∼14 mT) and frequencies (∼25 kHz) are used. Magnetic nanoparticles of Zn and Co ferrites have been synthesized by the thermal decomposition method and by the solvothermal/hydrothermal route, achieving four different samples of magnetic cores with the mean crystallite size in the range of 8–16 nm. From all of them, well-comparable silica-coated particles with a shell thickness of 5–6 nm have been prepared. To analyse the effect of the coating layer and hydrodynamic size, three additional samples have been supplemented: solvothermal Zn ferrite nanoparticles stabilized with a citrate monolayer, the same particles coated with 17 nm thick silica shell, and silica-coated Zn ferrite particles from the thermal decomposition with a higher degree of agglomeration. Fundamental characterizations of the nanomaterials by XRD, XRF, TEM, and DLS are followed by detailed investigations of their magnetic properties and structural peculiarities by SQUID magnetometry and 57Fe Mössbauer spectroscopy. Magnetic particle spectroscopy and subsequent MPI study demonstrate: (i) superior properties of Zn ferrite nanoparticles compared to their Co ferrite counterparts, (ii) only negligible to weak effect of the type/thickness of the coating on signal of Zn ferrite nanoparticles, (iii) comparable performance of the solvothermal Zn ferrite particles with a thin silica shell and of the Resovist® tracer, and importantly, (iv) that suitable choice of the magnetic phase enables to separate the contributions of the Néel and Brownian relaxation on the timescale of MPI.

Application of the Lock-In Technique in Magnetoelectric Coupling Measurements of the PZT/Terfenol-D Composite

This paper presents a study of magnetoelectric (ME) properties of the PZT/Terfenol-D composite with a varying number of layers. The composite consists of piezoelectric and magnetostrictive phases that are mechanically coupled. The purpose of this setup is to gain control over the electric polarization of a material via an external magnetic field. Unlike most similar composites, our samples utilize a commercial piezoelectric patch instead of pure PZT. At present, researchers face two main problems regarding magnetoelectric materials: (i) the effect is observed far below room temperature for single-phase materials, and (ii) the ME coupling is too weak to be commercially viable. Our research was carried out via the lock-in technique on two PZT/Terfenol-D samples we synthesized. Relatively strong room-temperature magnetoelectric coupling between piezoelectric and magnetostrictive phases was observed for both samples. Two types of characteristics were investigated: (i) ME voltage versus magnetic AC field frequency, and (ii) ME voltage versus magnetic DC field. We detected multiple, grouped signal peaks ascribed to different resonance modes. Uniquely, the peaks form band-like characteristics which might be an important step in bringing the materials closer to wider commercial use.

Dynamic resistance and voltage response of a REBCO bifilar stack under perpendicular DC-biased AC magnetic fields

The dynamic resistance of REBCO (REBa2Cu3O7-d, RE stands for rare earth), coated conductors (CCs) is a key parameter in many high-temperature superconductor applications where CCs carry DC currents exposed to AC and DC magnetic fields, such as field-triggered persistent current switches, flux pumps, and fault current limiters. In this work, dynamic resistance and dynamic voltage have been studied via experiments and finite element method (FEM) simulations in a REBCO bifilar stack at 77 K, under combined AC and DC magnetic fields with different magnitudes, frequencies, and waveforms. Our results show some distinct features of dynamic resistance and voltage from those under pure AC magnetic fields. With an increasing DC magnetic field, the dynamic resistance exhibits an obvious linearity with the applied AC magnetic field, and becomes less dependent on the AC field frequency. The fundamental frequency of the dynamic voltage under a DC magnetic field becomes the same as that of the applied AC field, which completely differs from the pure AC field case where the fundamental frequency doubles. For the first time, instantaneous threshold field (B th) values are obtained from the dynamic voltage, which are substantially different in the field-increasing and field-decreasing processes. These key differences are attributed to the dominant role of DC magnetic fields in determining the critical current of the superconductor, which significantly dwarfs the influence of AC fields. These new discoveries may help researchers better understand the electromagnetism of superconductors and be useful for relevant applications.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors.

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim-Anderson Arrhenius relation in the framework of the collective creep theory.

Nanocrystalline solid slabs of pure and CdS added KCl–KBr for energy storage application: Preparation and properties

The present study illustrates, for the first time, the possibility of preparing nanocrystalline solid slabs of pure and CdS (5 wt%) added KCl1−xBrx(with x = 0.0, 0.25, 0.5, 0.75 and 1.0) with good quality by using a melt quenching process. The ten samples prepared have been found to exhibit the halite crystal structure with reduced crystal grain size (average crystallite sizes observed in the range 43 to 97 nm), homogeneous morphology (cuboid shape) and phase purity. Optical measurements (absorption/reflectance and photoluminescence) show a significant difference between the melt quenched samples, precursors and ball milled samples (prepared for comparison); CdS addition leads to an extra photoluminescent peak at around 280 nm. The melt quenched samples exhibit much higher dielectric constants (indicating that they can be used as efficient energy storage materials) than the bulk (single/ poly) crystalline KCl–KBr (simple and mixed) systems, which could be due to their lower crystal grain/ crystallite sizes; for example, the ɛr value observed for the CdS added KCl0.5Br0.5 (with an applied AC field frequency of 1 kHz) increases from 2075 at 30 °C to 6188 at 190 °C, which is very interesting.

Controlling the electrostatic Faraday instability using superposed electric fields

We present an experimental study of the electrostatic Faraday instability at the interface between a dielectric and a conducting liquid. We study the response of the interface to an ac electric field, which is superposed by either a second ac field of different frequency, or by a dc field. An important control parameter is the mixing ratio, which denotes the relative amplitudes of the different components of the driving signal. For ac/ac driving, gradual variations of the mixing ratio can induce a jump of the pattern wavelength, and for ac/dc driving, the response wavelength can be tuned continuously by adjusting the mixing ratio.

Influence of weak ac electric field on the temperature of dielectric maximum of (1-x)PbMg1/3Nb2/3O3-xPbTiO3 relaxor ferroelectric single crystals based on acoustic emission data studies

(1-x)PbMg1/3Nb2/3O3-xPbTiO3, [100]-oriented relaxor ferroelectric crystals were studied by means of dielectric and acoustic emission methods in dependence on temperature and under ac measuring electric field with several values and frequencies. The dependence of temperature of dielectric constant maximum on and without ac field value and frequency was plotted. Also a scatter of the temperature of dielectric constant maximum was calculated in dependence on ac field frequency. Such the phenomenon was discussed from a point of view of growth of the polar nanoregions on cooling.

Enhanced luminescence, electric-field and actinic-light modulation of emission in nematic-CdSeS gradient nanocrystal composites by polymer confinement

We demonstrate that dispersing a small amount of CdSeS gradient quantum dots (QDs) in a nematic liquid crystal and geometrically confining the nanocomposite in a polymer network leads to their ordered linear assembly, as observed through electron microscopy. Such an orientational order not only enhances the base photoluminescence (PL), but brings in significant anisotropy in the emission magnitude, a feature barely seen for the system without the polymer network. A high frequency (∼kHz) AC field adds the attractive dimension of on-demand and spatially addressable PL modulation. Using a photoisomerizable azobenzene derivative and a carefully selected excitation wavelength, it is shown that an actinic-light driven modulation of PL is also possible, which is furtherenhanced by the presence of the polymer network.

Dielectric dispersion of Ag/PAN nanocomposites

The dependence of the electric modulus of silver/polyacrylonitrile nanocomposites on the ac field frequency has been studied at different temperatures and AgNO3 content in the base mixture. The observed relaxational maxima on the frequency dependences of the electric modulus’ imaginary part are connected with the interfacial polarization. It was shown that the frequency electric modulus’ experimental points are well-described by the Cole-Davidson model, correspondingly. The values of the relaxation times and the activation energies of these structures have been estimated by this model.

Microfluidic Concentration Enhancement of Bio-Analyte by Temperature Gradient Focusing via Joule Heating by DC Plus AC Field: A Numerical Approach

Abstract We present a detailed numerical analysis of electrophoresis induced concentration of a bio-analyte facilitated by temperature gradient focusing (TGF) in a phosphate buffer solution via Joule heating inside a converging–diverging microchannel. The purpose is to study the effects of frequency of AC field and channel width variation on the concentration of target analyte. We tune the buffer viscosity, conductivity, and electrophoretic mobility of the analyte such that the electrophoretic velocity of the analyte locally balances the electroosmotic flow (EOF) of the buffer, resulting in a local build-up of the analyte concentration in a target region. An AC field is superimposed on the applied DC field within the microchannel in such a way that the back pressure effect is minimized, resulting in minimum dispersion and high concentration of the target analyte. Axial transport of fluorescein-Na in the phosphate buffer solution is controlled by inducing temperature gradient through Joule heating. The technique leverages the fact that the buffer’s ionic strength and viscosity depend on temperature, which in turn guides the analyte transport. A numerical model is proposed and a finite element-based solution of the coupled electric field, mass, momentum, energy, and species transport equations are carried out. Simulation predict peak of 670-fold concentration of fluorescein-Na is achieved. The peak concentration is found to increase sharply as the channel throat width decreases, while the axial spread of concentrated analyte increases at lower frequency of AC field. The results of the work may improve the design of micro concentrator.

Critical magnetic behavior in [Ag8/Co0.5]x64, [Ag8/Co1]x32 and [Ag16/Co1]x32 epitaxial multilayers

We investigate the low temperature magnetic behavior of three epitaxial Co/Ag multilayers, grown onto MgO (001) substrates, with a nominal content per period of either half a monolayer or one monolayer of Co, and either 8 or 16 Ag monolayers. The samples were studied by X-ray reflectivity and diffraction, transmission electron microscopy, magnetometry and ac susceptometry. The results indicated a well defined stacking sequence in the growth direction, the number of periods and of Ag monolayers per period being coincident with the nominal values for each sample. The Co layers were found to be discontinuous and corresponded to a quasi-monodisperse in-plane distribution of Co nanoparticles embedded in a Ag(001) matrix. The zero-field cooled and field cooled temperature variations of the low field magnetization indicated the presence of irreversibilities at temperatures below 20 K. The ac field frequency (f) and temperature (T) dependencies of the real part of the susceptibility (χ′) corresponded to a Vogel–Fulcher behavior in the three samples, and indicated a frequency shift parameter (Γ) of the order of 4 x 10-2. For each sample, the experimental data corresponding to the variations of the imaginary part of the ac susceptibility (χ″) with f and T were found to collapse into a single curve according to the dynamic scaling law. Taken together, these results allow us to conclude that the three multilayers experience a phase transition of the paramagnetic to superspin glass type, driven by the dipolar interactions between the Co nanoparticles. Regarding the influence of the multilayer features, we found a clear dependence of the order parameter of the transition on the nominal number of Co monolayers per period.

Microwave spectroscopy of spin–orbit coupled states: Energy detuning vs interdot coupling modulation

We study the AC field induced current peaks of a spin-blockaded double quantum dot with spin–orbit interaction. The AC field modulates either the interdot tunnel coupling or the energy detuning, and we choose the AC field frequency range to induce two singlet–triplet transitions giving rise to two current peaks. We show that for a large detuning, the two current peaks can be significantly stronger when the AC field modulates the tunnel coupling, thus making the detection of the spin–orbit gap more efficient. We also demonstrate the importance of the time dependence of the spin–orbit interaction.

Mixed state properties analysis in AC magnetic field of strong pinning Fe(Se,Te) single crystal

The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with the aim of analyzing its superconducting properties that can be also suitable for power applications such as the critical current density and the pinning energy in AC regime. From the analysis of the fundamental Cole-Cole plots a finite influence of flux creep phenomena has been detected. Its study, by means of the Arrhenius plots, evidences thermal activation energy U values as high as 103 K, whose dependence on the DC and AC fields, and of the temperature, has been also obtained. The magnetic field dependence shows the existence of the strong pinning regime characterized by plastic deformations of the vortex lattice, whereas the temperature dependence of the critical current density Jc(T) is consistent with the δl pinning model. Finally, the irreversibility field values Hirr(T) have been extracted from the Jc(T) at the different AC frequencies showing very high values if compared with the literature.

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4 is slightly smaller than that in Ba0.6K0.4Fe2As2, which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of Jc0(0) ∼ 1.0 × 108 and 2.2 × 108 A cm−2 for CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors, respectively, which suggest potential applications.

Impact of Spin Torque Oscillator Frequency in Microwave-Assisted Magnetic Recording

In this article, we present a systematic modeling investigation for some important physical insights in microwave-assisted magnetic recording (MAMR). Specifically, the characteristics of the effective write field in the presence of the ac field generated by the embedded spin torque oscillator (STO) is analyzed in terms of field strength and field-gradient gain for recording, as well as the enhancement of reverse erasure field. It is found that the rotating frequency of the field generating layer (FGL) of the STO plays a very important role. The effective write field enhancement for both recording and erasure due to the rotating ac field increase with increasing the ac field frequency with a rate close to 1 T per 28 GHz. Either the recording field gain or erasure field enhancement rolls off at high frequencies. The roll-off frequency for the recording field gain is significantly higher, approximately 20 GHz, than that for erasure field enhancement, leaving a broader frequency range for the ac field over which substantial gains for the recording field strength and gradients can be obtained while avoiding any erasure field enhancement. The understanding also provides guidelines for both head and media designing.

Dielectric and electrical characterizations of transition metal ions-doped nanocrystalline nickel ferrites

This paper describes the room temperature dielectric and electrical response of several transition metal ions (Cr3+, Mn2+, Zn2+ and Cd2+)-doped nickel ferrite nanoparticles. The formation of pure cubic spinel phase, microstructural and magnetic properties of these three series of nanocrystalline-doped nickel ferrite samples was reported in previous papers. It is observed that hopping of electrons is the possible charge conduction mechanism for all doped nickel ferrite samples at room temperature. All the doped ferrite samples exhibited increment in ac conductivity with increasing ac field frequency. The hopping length is found to decrease with increasing dopant concentration in Zn–Cd–Ni ferrite and Cr–Ni ferrite. For Mn-doped nickel ferrite, a slight increase in hopping length is observed. All the doped nanoferrites showed good dielectric behavior below 100 Hz along with a rapid decline above 100 Hz and finally attained negligible value. The existence of two semicircles in Nyquist plots indicates that grains and grains boundaries contribute more in dielectric responses.

Influence of alkali concentration, electric waveform, and frequency on the critical electric field strength of droplet–interface partial coalescence

During the process of electro-dehydration, the electric field strength should be set below a critical value (Ecrit) to inhibit droplet–interface partial coalescence. The alkali present in alkaline–surfactant–polymer flooding may affect Ecrit because they change the electrical properties of produced liquids. In view of this, this study investigated the effects of Na2CO3 solution concentration, electric field waveform, and frequency on the root mean square (RMS) values of Ecrit. The results show that elevated Na2CO3 concentrations increase Ecrit, owing to the enhanced solution conductivity which may affect the charge transfer during coalescence. The convection of ions is associated with penetration of fluid into the bulk phase and an increase in Na2CO3 concentration decreases the penetration velocity. Moreover, increasing the AC or pulsed electric field frequency can increase Ecrit, and the optimal waveform (i.e., with the largest Ecrit) varies with frequency. These findings will be useful for optimizing electro-coalescence systems.

Harmonic AC magnetic susceptibility analysis of FeSe crystals with composite morphology

The AC magnetic response of multi-domain FeSe crystals has been explored by means of fundamental and third harmonic AC magnetic susceptibility (ACMS) analysis. Our previous studies have revealed a complex morphology which especially modifies the mixed state properties. The effects were expressed by an additional ‘pseudo’ peak feature of the magnetic hysteresis and in the present study we analyze its vortex dynamics nature and irreversibility behavior in the context of ACMS. We find that the effect is detectable mainly with the sensitive third harmonic component and especially at high AC field frequencies. From the analysis, the temperature dependence of the ‘pseudo’ peak effect at different DC fields ranges was determined, confirming the previously established superconductor–normal metal–superconductor type links between the domains in the crystal. The irreversibility line, the most important parameter for high power applications, shows a typical glass/liquid transition in the vortex matter. In addition we have observed surface barrier effects and vortex avalanche activity, the behavior of which appears to be influenced by the superconducting and magnetic nature coexistence and morphology. The presented results show the effective application of the harmonic AC magnetic susceptibility technique for a versatile analysis of complex nonlinear phenomena in materials with a sophisticated AC magnetic response.