AC Transverse Magnetic Field Research Articles

Flexible magnetoelectric sensor and nonvolatile memory based on magnetization-graded Ni/FSMA/PMN-PT multiferroic heterostructure

Flexible multiferroic heterostructures are promising to unveil technological developments in wearable magnetic field sensing, nonvolatile memory, soft robotics, and portable energy harvesters. Here, we report an enhanced and a zero-biased magnetoelectric (ME) effect in flexible, cost-effective, and room temperature sensitive Ni/FSMA/PMN-PT magnetization-graded ME heterostructure. Flexible Ni foil with −q (piezomagnetic coefficient) and the ferromagnetic shape memory alloy (FSMA; Ni-Mn-In) layer with +q offers the desired q-grading. The temperature-dependent dielectric behavior shows an anomaly in the martensite transformation regime of the FSMA layer. The Ni/FSMA/PMN-PT ME heterostructure exhibits noteworthy ME output of ∼3.7 V/cm Oe, significantly higher than Ni/PMN-PT (∼1 V/cm Oe). The q-grading-induced bending moment impedes the asymmetry-related flexural strain and strengthens the ME interaction. The zero-bias ME output of ∼0.4 V/cm Oe is ascribed to the interaction between q-grading-induced transverse magnetization and AC magnetic field. Ni/Ni-Mn-In/PMN-PT ME heterostructure displays excellent magnetic field sensing parameters: correlation coefficient, sensitivity, inaccuracy, and hysteresis of 0.99916, ∼0.74 mV/Oe, 1.5% full-scale output (FSO), and 1.8% FSO, respectively. The reversible and repeatable nonvolatile switching of the ME coefficient obtained with positive and negative electric fields is useful for next-generation memory devices. The flexible ME heterostructure shows no degradation in performance up to 1500 bending cycles. Such Ni/FSMA/PMN-PT based ME heterostructures are propitious for multifunctional flexible magnetic field sensors and nonvolatile memory applications.

Magnetization Loss Measurements of Spiral Copper-Plated Multifilament Coated Conductors With Various Filament and Conductor Widths

We can achieve both small magnetization loss and the robustness against local defect or local normal transition by winding copper-plated multifilament coated conductor spirally. The particular focus of this study is the impact of narrowing conductor (tape) width as well as filament width on magnetization losses. We measured the magnetization losses of various spiral copper-plated multifilament coated conductors, separated them into hysteresis losses and coupling losses, and examined the impact of narrowing filament width and conductor width. We demonstrated that the magnetization loss of the spiral 2 mm-wide coated conductor with 0.2 mm-wide filaments and 10 μm-thick copper was one-twentieth of that of a flat standard 4 mm-wide coated conductor in transverse ac magnetic fields of 65.44 Hz.

Dynamics of liquid metal layer in transverse AC magnetic field

Dynamics of the liquid metal free surface is an important aspect in many different metallurgical processes, such as cold crucible melting and continuous casting. An AC magnetic field can be used to change the shape of the liquid metal volume. Here, we study experimentally and numerically a rectangular liquid metal layer exposed to a transverse AC magnetic field. Depending on different parameters, such as the layer thickness and the surface oxidation, different free surface shapes and their dynamics are observed experimentally. We have simulated this fundamentally three-dimensional process using the coupled open-source software. Based on experimental observations, surface oxidation in the numerical model is approximated as a modified contact angle between the liquid metal and the vessel's walls. The numerical results agree well with experiments. Figs 6, Refs 8.

Effective reduction of magnetisation losses in copper-plated multifilament coated conductors using spiral geometry

We wound copper-plated multifilament coated conductors spirally on a round core to decouple filaments electromagnetically under ac transverse magnetic fields and measured their magnetisation losses. Although the coated conductors were plated with copper, which connects all filaments electrically and allows current sharing among them, the spiral geometry decoupled filaments similar to the twist geometry, and the magnetisation loss was reduced effectively by the multifilament structure. The measured magnetisation loss of a 4 mm wide, 10-filament coated conductor with a 20 μm thick copper wound spirally on a 3 mm core was only 7% of that of the same 10-filament coated conductor with a straight shape under an ac transverse magnetic field with an amplitude and frequency of 100 mT and 65.44 Hz, respectively. We separated the measured magnetisation losses into hysteresis and coupling losses and discussed the influence of filament width, copper thickness, and core diameter on both losses. We compared the hysteresis losses with the analytical values given by Brandt and Indenbom and compared the coupling losses with the values calculated using a general expression of coupling loss with the coupling time constants and geometry factors.

Coupling time constants and ac loss characteristics of spiral copper-plated striated coated-conductor cables (SCSC cables)

We conducted electromagnetic field analyses of spiral copper-plated striated coated-conductor cable (SCSC cable), where the striated (multifilament) and copper-plated coated conductors were twisted spirally around a round core in order to decouple the filaments. We used a numerical model which was formulated with current vector potential and in which thin-strip approximation was applied to the superconductor layer of the coated conductors. The nonlinear conductivity obtained from measured electric field–current density characteristics was used to represent the superconducting properties. To confirm the effect of the spiral twist in the SCSC cables, we determined their coupling time constants from the frequency dependences of their calculated magnetisation losses, under an ac transverse magnetic field with a small amplitude. Moreover, we calculated the ac losses in the SCSC cables under simultaneous application of ac transverse magnetic fields and ac transport currents, which represent practical operating conditions for electrical power devices and magnets.

Electromagnetic Response of DC Type-II SC Wires Under Oscillating Magnetic Excitations

Type-II superconductors are expected to be extensively used in the designing of dc power grids due to their reduced use of space, high transport current capability, and nearly zero resistive losses. Nevertheless, these systems will have to share the right of way of the currently installed ac network, reducing the costs of development associated with the superconducting (SC) cables without the need of increasing the right of way. Under the theoretical framework of the critical state model and the numerical solution of Maxwell equations in the magneto quasi-steady approach, in this paper, we present a comprehensive study of the effects of applying an ac transverse magnetic field to a type-II SC wire of rounded cross section it utilized for direct current power transmission. Our numerical results can be used as a practical benchmark for determining the minimal losses and magnetic impact of dc SC lines subjected to external oscillating magnetic fields. The local dynamics of the flux front profile of current density, the resulting density of magnetic flux, the total magnetic moment of the wire, and the curve of ac losses for different conditions of dc current and transverse AC magnetic field are presented. Our results are compared with simplified analytical approaches, demonstrating the importance of considering the concomitant action of dc current and ac magnetic field by the use of numerical methods, mainly at low values of applied magnetic field relative to the intensity of transport current and the full penetration profile of current density.

Magnetic moment and local magnetic induction of superconducting/ferromagnetic structures subjected to crossed fields: experiments on GdBCO and modelling

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed.

Detection of topological states in two-dimensional Dirac systems by the dynamic spin susceptibility

We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac electrons with spin-orbit interactions to characterize topological insulators. The imaginary part of the DSS appears as an absorption rate in response to a transverse ac magnetic field, just as in an electron spin resonance experiment for localized spin systems. We found that when the system is in a static magnetic field, the topological state can be identified by an anomalous resonant peak of the imaginary part of the DSS as a function of the frequency of the transverse magnetic field $\omega$. In the absence of a static magnetic field, the imaginary part of the DSS becomes a continuous function of $\omega$ with a threshold frequency $\omega_{\rm c}$. In this case, the topological and the trivial phases can also be distinguished by the values of $\omega_{\rm c}$ and by the line shapes. Thus the DSS is an experimentally observable physical quantity to characterize a topological insulator directly from bulk properties, without observing a topological transition.

Crossed-magnetic-field experiments on stacked second generation superconducting tapes: Reduction of the demagnetization effects

The crossed-magnetic-field effect on the demagnetization factor of stacked second generation (2G) high temperature superconducting tapes is presented. The superconducting sample was initially magnetized along the c-axis by the field cooling magnetization method and after achieving the magnetic relaxation of the sample, an extensive set of experimental measurements for different amplitudes of an applied ac magnetic field parallel to the ab-plane was performed. On the one hand, a striking reduction of the demagnetization factor compared with the reported values for superconducting bulks is reported. On the other hand, the demagnetization factor increases linearly with the amplitude of the ac transverse magnetic field confirming the universal linear behavior for the magnetic susceptibility predicted by Brandt [Phys. Rev. B 54, 4246 (1996)]. The study has been also pursued at different frequencies of the ac transverse magnetic field in order to determine the influence of this parameter on the demagnetization factor measurements. We report an even lower demagnetization factor as long as the frequency of the transverse magnetic field increases. Thus, the significant reduction on the demagnetization factor that we have found by using stacked 2G-superconducting tapes, with higher mechanical strength compared with the one of superconducting bulks, makes to this configuration a highly attractive candidate for the future development of more efficient high-power density rotating machines and strong magnet applications.

Construction of orthogonal synchronized bi-directional field to enhance heating efficiency of magnetic nanoparticles

Magnetic hyperthermia using magnetic nanoparticles (MNPs) has attracted considerable attention as one of the promising tumor therapy. The study has been developed under single magnetic field. Recently, we found that the immobile MNP may generate more heat under two synchronous ac magnetic fields than traditional single and circular polarized fields based on model simulation result. According to this finding we constructed an orthogonal synchronized bi-directional field (OSB field). The system contained two LC resonant inverters (L: inductor, C: capacitor) and both vertical and transverse ac magnetic fields were generated by two Helmholtz coils. To reduce the interference, the axis directional of two coils were arranged orthogonally. The experiments showed that the heating ability of aggregated MNPs is greatly enhanced under this newly designed OSB field without increasing the strength of magnetic field. The OSB field system provides a promising way for future clinical hyperthermia.

Improved Poynting's Vector Method: AC Loss Measurement of HTS Tapes Formed into a Short Straight or a Solenoidal Coil

Abstract Improved Poynting's vector method without movements was proposed to measure ac losses of HTS tapes, formed into a short straight or a solenoidal coil, which were carrying ac transport currents under transverse ac magnetic fields. In the previous Poynting's vector methods, for a short straight case two sets of both potential lead pairs and pickup coils were rotated around tapes in order to measure local energy flow densities, i.e., the distribution of Poynting's vector; for a solenoidal coil case two sets of both potential lead coils and spiral pickup coils were shifted to the coil axis direction by one winding pitch of the solenoidal coil. During moving these sets at low temperature, the data acquisition system was working and the value of ac losses was finally obtained. These previous methods, therefore, have inconveniences such as much time to get the value, much space needed for moving sets, or frequent troubles of movements due to low temperature deformation. The present method based on the following idea can resolve them: as for pillar parts of potential lead lines which are standing on the tape surface, only one plays a substitute role for all as long as all lines are connected together. As a result, immovable arrays of sets can be rigidly arranged, so that the signal measured at each position can be simultaneously. Moreover, information enough to clarify the electromagnetic behavior inside the tape can be obtained, because the sets can put more closely to the tape surface. The validity of the present method was confirmed by preliminary experiments using Bi-2223 multifilamentary tapes in liquid nitrogen.

The influence of nanoparticle size and external AC magnetic field on heating ability

The effect of a two-way field containing a vertical and transverse AC magnetic field on heating abilities of Fe 3O 4 is investigated for magnetic nanoparticles with average sizes of 8.8 nm and 13.5 nm. The measurement shows that the heating abilities of our samples are size-dependent and field-dependent. We find that applying a two-way field can strongly increase the heating ability of larger-sized magnetic nanoparticles. That heating ability is different from the one under a single magnetic field. These large-sized Fe 3O 4 MNPs are thus suitable candidates for hyperthermia under a suitable two-way field.

AC Magnetization Loss of a YBCO Coated Conductor Measured Using Three Different Techniques

In-field ac losses were measured on a 2 G YBCO tape by three different methods; these consisted of an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ) loop (inductive) based method as well as two different calibration-free approaches. For the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ) loop-based method the sample was placed inside a pick-up coil connected in series with, and in opposition to, a compensation coil. The voltage developed in series across these two coils was directly proportional to the magnetic moment of the sample. AC loss was determined from the area of the hysteresis loop generated under an externally applied cyclic field. The two separate calibration-free methods used a lock-in amplifier-based technique. The first of these used a pick-up coil wound in parallel with the ac magnet winding. Two identical systems were used, each containing an ac magnet equipped with a measuring coil. One of the magnets contained the sample and the other one was left empty. The second calibration-free method measured components of the sample's magnetic moment in a transverse ac magnetic field. Here the sample was placed inside an ac magnet; two pick-up coils were placed outside the magnet. The ac magnet, which is rectangular in shape, has an aspect ratio such that a homogeneous field is produced in the sample space and almost no stray fields are present in the pick-up coils. To measure ac loss, a z-component of the magnetic moment of the tape is determined using a dipole approximation. The AC losses for a coated conductor in fields of amplitude up to 0.14 T applied perpendicular to the broad face of the tape were measured from 36 Hz-75 Hz in a liquid nitrogen bath using all three methods. These results were compared, and a satisfactory agreement between all three methods was obtained.

Superconducting tubular wires in transverse magnetic fields

The electromagnetic response of a round tubular wire of superconducting film to a transverse magnetic field is investigated theoretically. For a superconducting tubular wire (STW) in which the thickness $d$ of the superconducting layer is much smaller than the radius $R$ of the wire, analytical expressions for the magnetic-field and current distributions are obtained on the basis of the critical state model with constant critical current density ${j}_{c}$. When an applied transverse magnetic field ${H}_{a}$ increases monotonically from zero, the penetration of the magnetic field into an STW occurs in two stages: for $0l{H}_{a}\ensuremath{\leqslant}{j}_{c}d/2$ the magnetic field in the interior of an STW is shielded, whereas for ${H}_{a}g{j}_{c}d/2$ the magnetic field extends into the interior. Analytical expressions of the hysteretic ac loss ${Q}_{\mathrm{tube}}$ of an STW in a transverse ac magnetic field of amplitude ${H}_{0}$ are also obtained, and ${Q}_{\mathrm{tube}}({H}_{0})$ is found to have an abrupt change at ${H}_{0}\ensuremath{\simeq}{j}_{c}d/2$.

Fabrication and characterization of Bi2223 tapes with Al 2O 3 barriers for reduction of AC losses

We fabricated and characterized the Bi2223 tape with Al 2O 3 as interfilamentary resistive barriers for the filament decoupling and also for the reduction of AC losses under AC external magnetic field. The tapes with thin Al 2O 3 barrier layers around the Bi2223 filaments were prepared by using a standard powder-in-tube (PIT) method. Al 2O 3 powders with mean grain size below 1 μm were used and they are coated with the slurry on the outside surface of hexagonal monocore rod before stacking. The phase formation inside the filament was evaluated by XRD measurement. The uniformity of transport properties ( J c) for Al 2O 3 barrier tapes was measured on the order of several metre lengths and compared with data for the tapes without barriers and with Ca 2CuO 3 + Bi2212 barriers. To evaluate the effect of introducing Al 2O 3 barriers on the filament decoupling, frequency dependence of AC losses under AC transverse magnetic field for barrier tapes were investigated.

Suppression of magnetic levitation force in melt-textured YBa2Cu3O7-x superconductors by a transverse AC magnetic field

We have studied experimentally the influence of transverse ac magnetic fields on the levitation force arising between a permanent NdFeB magnet and a bulk melt-textured HTSC YBCO superconducting sample. The axes of superconducting disc and cylindrical magnet were coinciding while the transverse ac magnetic field generated by resistive coil was directed parallel to surface of a disc i.e., perpendicular to the disc axis. We found that application of both impulse and alternative transverse magnetic fields results in suppression of the value of levitation force and its relaxation rate. Namely, the variable magnetic field with amplitude 12 mT, that approximately in 20 times is less than field of a constant magnet, causes suppression of force more than twice. Monotonous behavior of value of levitation force reduction with the increase in transverse magnetic field amplitude was observed. The possible origin of observed phenomenon is discussed.

Simple calibration free method to measure ac magnetic moment and losses

A calibration free method to measure components of a vector of ac magnetic moment in a transverse ac magnetic field have been developed. This method is oriented to study the features of the ac vector magnetization and losses in HTS tapes, wires and long slabs at low temperatures. The method is suitable for measurements in the case of a transport current carrying sample. The advantages of the method, besides its vector character, are an extremely compact ac field excitation coil and the pick-up coil positions outside with respect to the excitation one. Finite dimensions of the sample and pick-up coils are taken into account. To illustrate the potential of the method, the measurement of Bi2223/Ag tapes at 77 K were performed. The ac field amplitude 2–100 mT, the frequencies 20–230 Hz and various orientations of the transverse ac field to the sample plane were used. Besides, the dc magnetization and hysteresis losses of the samples were measured. The observed dependence of the ac magnetic moment on the angle between the ac field direction and the superconductor plane is consistent with the conception of the ‘Geometrical locking of the irreversible magnetic moment to the normal of a thin-plate superconductor’ (Zhukov A A et al. 1997 Phys. Rev. B 56 2809–19).

Fabrication and characterization of Bi2223/Ag tapes with interfilamentary oxide barriers for reducing AC losses

We prepared and characterized Bi2223/Ag tapes with interfilamentary oxide barriers for AC loss reduction in external AC magnetic field. Ca2CuO3, Al2O3, and SrZrO3 were selected as barrier materials. 20-30wt% Bi2212 were added to Ca2CuO3 and SrZrO3 powders to improve their deformation properties for cold working. The multifilamentary tapes with interfilamentary oxide barriers were fabricated with a conventional powder-in-tube method. The influence of introducing different oxide barriers on the final tape properties such as the phase formation in filaments, critical current density Jc and its uniformity along a tape length, was evaluated. Based on the results of the preliminary studies, we determined the oxide materials for fabrication of barrier tapes with twisted filaments. AC losses in barrier tapes with twisted filaments were measured at 77 K in transverse AC magnetic field. The AC loss characteristic of the tape with barrier was compared to the one of the tape without barrier.

Total AC Loss in Twisted Multifilamentary Coated Conductors Carrying AC Transport Current in AC Transverse Magnetic Field

Numerical electromagnetic field analysis was made to study the total AC loss characteristics of twisted multifilamentary coated conductors carrying AC transport current in an AC transverse magnetic field. The superconducting property was represented with the power law E – J(electric field—current density) characteristic, and the magnetic field component tangential to the wide face of a conductor was neglected to use a thin strip approximation with which current and magnetic flux distributions on a wide face of a coated conductor can be calculated. The transverse resistance between filaments, the amplitude of the transport current, and that of the transverse magnetic field were varied to study magnetic flux penetration processes and total AC loss characteristics.

Critical current density distribution and magnetization loss in YBCO coated conductors

Lateral distributions of the critical current density (Jc) in YBCO coated conductors were measured by the magnetic knife method. It was observed that Jc decreased near the edges of the coated superconductors. By considering the measured lateral distributions and the magnetic field dependence of Jc, electromagnetic field analyses were performed to calculate the magnetization losses in the coated superconductors subjected to an AC transverse magnetic field. The calculated and measured magnetization losses were in good agreement. Deviations of the measured magnetization losses from the analytical values based on a uniform Jc distribution can be explained by non-uniform Jc distributions in actual coated superconductors. Larger loss generation mechanism in YBCO coated conductors with non-uniform Jc distributions at small magnetic field was revealed by temporal evolutions of current density, magnetic flux density, and electric field in the numerical analyses.