Frequency Of Ac Magnetic Field Research Articles

The Hybrid System for the Magnetic Characterization of Superparamagnetic Nanoparticles.

The characterization of nanoparticles is crucial in several medical applications, such as hyperthermic therapy, which heats superparamagnetic nanoparticles with an external electromagnetic field. The knowledge of heating ability (magnetic losses) in AC magnetic field frequency function allows for selecting the optimal excitation. A hybrid system for the characterization of superparamagnetic nanoparticles was designed and tested. The proposed setup consists of an excitation coil and two sensing probes: calorimetric and magnetic. The measurements of the imaginary part of the complex magnetic susceptibility of superparamagnetic nanoparticles are possible in the kilohertz range. The system was verified using a set of nanoparticles with different diameters. The measurement procedure was described and verified. The results confirmed that an elaborated sensor system and measuring procedures could properly characterize the magnetic characteristics of nanoparticles. The main advantage of this system is the ability to compare both characteristics and confirm the selection of optimal excitation parameters.

Experimental study of the thermal-frictional behavior of a horizontal straight tube equipped with a vibrating string turbulators: A novel active method

The active methods are recently considered to increase the heat transfer rate. Compared to the passive methods, the active ones experience less pressure drop. Some inherent disadvantages, such as inefficiency in terms of energy consumption or the necessity of specific properties for the working fluid can limit using active methods. In this study, a novel idea is experimentally studied to explore the changes in the rate of heat carried by the fluid passing through a heated tube. To this end, a stretched string is implemented coaxially inside the tube and excited to vibrate at its natural frequency in the presence of an adjustable frequency AC magnetic field. This new approach to vibrating a starched oscillator remotely is called electro-magnetic vibration method (EMV). The vibration of a string inside the tube is predicted to cause severe turbulence and strong radial flow. The impacts of the changing diameter of the vibrating string turbulator (VST) and the position of the exciter on the heat transfer rate are studied at different mass flow rates. The thermal enhancement factor (TEF) is calculated to choose the best diameter and excitation location. The results showed that in the best scenario, TEF could reach up to 1.47, which indicates that the EMV method is an effective way to generate harsh radial flow to increase heat exchanger's thermal performance.

Dynamic Resistance of Series-Connected HTS Stacks Considering Electromagnetic and Thermal Coupling

When a high temperature superconductor (HTS) coated conductor carries a DC transport current within the background of an oscillating magnetic field, the effect of dynamic resistance takes place. Under the circumstance when the dynamic resistance effect is significant, heat dissipation will be generated to change the temperature of HTS tapes. The amplitude of the instantaneous dynamic resistance increases with time due to the rising temperature. The phenomenon is more obvious when it comes to the HTS stacks where superconducting tapes are assembled so close. This paper proposes a numerical model with electromagnetic and thermal coupling to study the dynamic resistance of the series-connected HTS stack. Experimental results have shown that the proposed numerical model can describe dynamic resistance more accurately than conventional models. Besides, investigations are carried out on how the insulation thickness in the stack and the frequency of AC magnetic fields impact the dynamic resistance of HTS stacks under the influence of Joule heat. This can provide a useful reference for the design of superconducting devices in which dynamic resistance occurs.

Promotion of ferrofluid microchannel flows by gradient magnetic fields

• Ferrofluid Couette-Poiseuille flows subject to gradient magnetic field are analyzed. • Steady gradient magnetic fields promote the ferrofluid Couette-Poiseuille flows. • The backflow is completely suppressed and the flow separation disappears. • The gradient magnetic field enhances the effect of magnetization relaxation. • The gradient magnetic field increases magnetic stress and reduces shear stress. Inspired from the wide engineering applications of ferrofluids, we investigated the ferrofluid microchannel flows subject to a gradient magnetic field applied perpendicular to the channel walls. The ferrohydrodynamic problem encompassing the Fokker-Planck magnetization equation in a Couette-Poiseuille configuration is solved numerically. The velocity and spin profiles, vorticity, magnetization, flow rate, and stresses acting on the moving plate are examined for favorable and unfavorable field gradients, as well as the adding of different pressure gradients. We show that the steady gradient magnetic fields promote the flow of both pure Couette and Couette-Poiseuille flows in the microchannel geometry. The increment of the velocity attains 35% and the resulting volumetric flow rate increases by about 24% at the most. This phenomenon was only observed in ferrofluid flows subject to high frequency AC magnetic fields or temperature gradient field in the past. The backflow occurs in normal Couette-Poiseuille flows can be completely suppressed by the gradient fields and the flow separation disappears correspondingly. The gradient magnetic fields are able to enhance the effect of magnetization relaxation and dramatically increase the magnetic stress and reduce the shear stress by comparison with the cases subject to uniform magnetic fields, yet the tradeoff between them leads to a rise in the total tangential stress acting on the moving plate.

Circumferential magnetoelectric effect in cylindrical piezoelectric-magnetostrictive composites with transverse bias magnetic field

Strong circumferential magnetoelectric (ME) effect has been observed in cylindrical PZT/Terfenol-D composite with transverse bias magnetic field. With dc bias magnetic field along axial direction and ac magnetic field along circumferential direction, strong ME voltage is obtained as 0.93 V·cm-1·Oe-1 at off-resonance frequency 1 kHz and as high as 32 V·cm-1·Oe-1 at resonance frequency f_r=46.0 kHz, which is much higher than that observed in planar ring-type ME laminate. The enhanced circumferential ME effect is attributed to improved interface mechanical coupling via normal stress. At room temperature, induced voltage from this cylindrical ME composite exhibits a near-linear response to applied ac vortex magnetic field amplitude over a wide magnetic field range of 10-8<Hac<10-5 T both at low frequency and resonance frequency. Moreover, induced voltage has an excellent flat response to applied ac magnetic field frequency between Hz and kHz. The strong and stable circumferential ME effect in cylindrical ME laminate offers potential in vortex magnetic field sensor applications. Key words: ME effect, piezoelectric, frequency response, magnetic sensor.

Кольцевой магнитоэлектрический эффект в цилиндрических структурах пьезоэлектромагнитострикционных композитов при поперечном намагничивающем поле

Strong circumferential magnetoelectric (ME) effect has been observed in cylindrical PZT/Terfenol-D composite with transverse bias magnetic field. With dc bias magnetic field along axial direction and ac magnetic field along circumferential direction, strong ME voltage is obtained as 0.93 Vcm-1Oe-1 at off-resonance frequency 1 kHz and as high as 32 Vcm-1Oe-1 at resonance frequency fr=46.0 kHz, which is much higher than that observed in planar ring-type ME laminate. The enhanced circumferential ME effect is attributed to improved interface mechanical coupling via normal stress. At room temperature, induced voltage from this cylindrical ME composite exhibits a near-linear response to applied ac vortex magnetic field amplitude over a wide magnetic field range of 10-8<Hac<10-5 T both at low frequency and resonance frequency. Moreover, induced voltage has an excellent flat response to applied ac magnetic field frequency between Hz and kHz. The strong and stable circumferential ME effect in cylindrical ME laminate offers potential in vortex magnetic field sensor applications.

Physical model for effects of microwaves on nucleoids in living cells: role of carrier frequency, modulation and DC and AC magnetic field

The effect of static and alternating magnetic fields on the conformation of nucleoids in cells of different types is considered. The model of slow and nonuniform rotation of the charged DNA domain is used. An equation is obtained for the resonance frequencies of the alternating magnetic field.

Breathing mode of relativistic twisted electron beams under periodic magnetic field

Transmission electron microscopy typically works with relativistic electron beams of energy 200–300 keV whose wavelength is on the picometer scale. Here, we study theoretically the relativistic dynamics of twisted Dirac-electron beams driven by a time-oscillating magnetic field. The spatial density distribution and the spinor wave-functions of twisted electron beams at each instant time t are obtained. It is demonstrated that the spatial density distribution of electrons can be changed periodically by the external field, whose change is very similar to the “Breathing”. Correspondingly, the azimuthal component of the current changes periodically with the magnetic field. The central “spot size” of spatial distribution of is directly related to the magnitude of the magnetic field. Our results suggest that twisted electron beams are promising for testing the magnitude and frequency of ac magnetic fields created by microsources.

Specific absorption rate of magnetic nanoparticles: Nonlinear AC susceptibility

In the context of magnetic hyperthermia, several physical parameters are used to optimize the heat generation, and these include the nanoparticles concentration and the magnitude and frequency of the external AC magnetic field. Here, we extend our previous work by computing nonlinear contributions to the specific absorption rate, while taking into account (weak) inter-particle dipolar interactions and a DC magnetic field. In the previous work, the latter were shown to enhance the specific absorption rate in some specific geometries and setup. We find that the cubic correction to the AC susceptibility does not modify the qualitative behavior observed earlier but does bring a non-negligible quantitative change of specific absorption rate, especially at relatively high AC field intensities. Incidentally, within our approach based on the AC susceptibility, we revisit the physiological empirical criterion on the upper limit of the product of the AC magnetic field intensity H0 and its frequency f and provide a physicist’s rationale for it.

Spin glass transition of single-crystalline TmFe2O4−δ

TmFe2O4 is one kind of multiferroic material in which equivalent amounts of Fe2+ and Fe3+ occupy a two-dimensional triangular lattice, leading to charge and spin frustrations. The spin frustration is expected to be increased as the fraction of Fe2+ (Fe3+) becomes larger than that of Fe3+ (Fe2+). We have grown single-crystalline TmFe2O4−δ with oxygen vacancies by using floating zone melting method and examined its magnetic properties. On cooling the compound, a long-range magnetic ordering develops around ∼240 K. With further cooling, a maximum of zero-field-cooled (ZFC) magnetization is observed at 186.2 K. The ac magnetic susceptibility obtained by ZFC process also manifests a maximum in its temperature dependence, and the variation of spin-freezing temperature with frequency of ac magnetic field is explainable in terms of the dynamic scaling law with the critical component of 8.68(8). This value suggests that the spin glass transition occurs at 186.2 K. The effect of external dc magnetic field on the irreversible transition temperature is coincident with the de Almeida–Thouless (AT) line. Aging-memory and rejuvenation effect is also observed below the spin-freezing temperature. These facts support the idea that TmFe2O4−δ undergoes spin glass transition below the ferrimagnetic transition temperature. In other words, TmFe2O4−δ can be regarded as a reentrance spin glass. It is thought that the oxygen vacancies bring about unequal number of Fe2+ and Fe3+ ions and thereby strengthen the magnetic frustration among the iron ions coupled with antiferromagnetic interactions, leading to the spin glass behavior.

Magnetic shielding of long paraboloid structures in the inhomogeneous magnetic field

Shielding efficacy of the high-temperature superconducting (HTS) magnetic shields depends on the superconductor properties and on the orientation of the external magnetic field. For precise magnetic field measurements in areas with changing direction of magnetic noise it is important to reduce both the parallel and perpendicular components of the magnetic field. We have designed and fabricated magnetic shields of 25 cm long paraboloid shape with closed sides from second-generation HTS tapes. We have characterized HTS shields in DC and variable frequency AC magnetic fields at 77 K above a copper electromagnet acting as the source of inhomogeneous magnetic noise. The HTS magnetic shields reduce the magnetic field noise penetration and enhance the sensitivity of magnetic field sensors. The measurements were performed with the magnetic shield placed between the noise source and the sensor. 2D finite element analysis using Comsol model was generated and the results were compared with the experimental data of magnetic field dependences of the shielding factor (SF).

Empirical expression for harmonics of AC magnetization of magnetic nanoparticles with core size distribution

It is well known that the core size of practical magnetic nanoparticle (MNP) samples is widely distributed. This study investigated an empirical expression for harmonics of AC magnetization of MNPs for magnetic nanoparticle thermometer (MNPT) and used two main core diameters to approximate the effect of the core size distribution. When the frequency of AC magnetic field was 200 Hz, the harmonics calculated from the Langevin function with two-core diameters (two-core-diameters model) agreed well with the experimental data. When the frequency was 3 kHz, where the effect of the Brownian relaxation could not be ignored, the harmonics could also be empirically expressed by the two-core-diameters model by adequately choosing the parameters of the model. The simple two-core-diameters model is useful when MNPs sample with core size distribution is used for MNPT.

Tunable specific-loss power of magnetic nano-spheres in vortex state for high-efficiency hyperthermia bio-applications: A theoretical and simulation study

We explored the reliable control of high-efficiency specific-loss power (SLP) using magnetic nanoparticles in the vortex state, where the value of power is one order of magnitude higher than those of the conventional mechanisms of SLP reported thus far. The underlying mechanism is based on the precession of a single vortex core and subsequent dissipation due to the intrinsic damping when the vortex-state spheres are resonantly excited. Owing to the dynamic characteristics of vortex-state nanoparticles, the resonant excitation of vortex-core precession is variable with particle size as well as tunable by the size-specific resonant frequency and strength of ac magnetic fields applied to the particles. The ac magnetic-field energy absorbed by the particles can be converted very efficiently to other energy forms such as heat. We derived, semi-analytically and by micromagnetic simulations, the quantitative relationships of the SLP quantity with the particle size and intrinsic damping constant of nanoparticles, and with externally controllable parameters including the frequency and strength of ac magnetic fields and dc magnetic-field strength. This work provides a reliable means of control as well as an optimal design of high-value SLPs for high-efficiency hyperthermia bio-applications.

Anomalous magnetic properties of CaRuO3 probed by AC and DC magnetic measurements and by low Ti impurity doping

Calcium ruthenate ($CaRuO_3$) is widely believed to be located close to a quantum critical point due to the strong non-Fermi-liquid behavior expressed in the temperature dependence of electronic transport, specific heat, optical conductivity, etc. However, the corresponding anomalous behavior, marking crossover temperature regimes in the magnetic response of $CaRuO_3$, is still lacking. Here we report detailed AC and DC magnetic susceptibility measurements of $CaRuO_3$ and $CaRu_{0.97}Ti_{0.03}O_3$. The AC magnetic susceptibility measurements of the $CaRuO_3$ show a slight dependence on the frequency of AC magnetic field below ~ 40 K, and an additional subtle change of curvature around 12 K. We interpret these results as a critical slowing down of spin fluctuations towards T=0 K. We confirm these observations by magnetic measurements of $CaRu_{0.97}Ti_{0.03}O_3$, which show a pronounced magnetic response corresponding to the above temperatures.

Local magnetostriction measurement in a cobalt thin film using scanning probe microscopy

The local magnetostriction measurement has become an emerging issue because strain-mediated nanocomposites have received considerable attention due to their potential applications for high sensitivity sensors and high density energy harvesters. Compared to many instruments capable of measuring nanometric magnetic domains, techniques for measuring local magnetostriction have been rarely reported. Here, we introduce a local magnetostriction measurement method by modifying an atomic force microscope (AFM) by combing a solenoid to apply an external ac magnetic field and induce magnetostrictive excitation. The distribution of magnetostrictive response is mapped by contact mode AFM and lock-in detection techniques with a few nanometer lateral resolution. We have found that a few micrometer-size domains of a cobalt film showing different amplitude and phase signals of the magnetostrictive response are observed by the second-harmonic response of the ac magnetic field frequency, which is not detected in a linear response signal. We suggest that the phase and amplitude signals observed in each domain are related to magnetic-easy-axis directions. Our findings provide a unique pathway to understand the local magnetostrictive response based on scanning probe microscopy.

Growth and characterization of Bi3.15Nd0.85Ti2.95Hf0.05O12/La0.67Sr0.33MnO3 composite film with strong magnetoelectric effect by chemical solution deposition under moderate crystallization temperature

Using chemical solution deposition method, La0.67Sr0.33MnO3 (LSMO) and Bi3.15Nd0.85Ti2.95Hf0.05O12 (BNTH) layers were successively fabricated onto (001) LaAlO3 (LAO) substrates, and then BNTH/LSMO ferromagnetic-ferroelectric composite films with a 2-2 layered type structure were obtained. The LSMO ferromagnetic layer was epitaxially grown on the LAO substrate and showed the low resistivity of 5.0 × 10−3 Ω cm. It was not only used as a seed layer to induce the oriented growth of BNTH ferroelectric layers, but also as a bottom electrode to measure ferroelectric/magnetoelectric (ME) properties. The effects of crystallization temperature on the orientation degree, the surface morphology, and the dielectric and ferroelectric performances of the composite films were investigated. The composite film crystallized at 730 °C has the largest remnant polarization (Pr) of 35.0 μC/cm2, the lowest leakage current density of 1.35 × 10−7 A/cm2 and the high saturated magnetization (Ms) of 370 emu/cm3. The composite film also exhibits a strong ME response in the absence of DC bias magnetic field. Its ME voltage coefficient, αE, gradually increases from zero with the increasing of the AC magnetic field frequency, and eventually reaches 35.8 V/cm·Oe at 100 kHz.

Magnetoelectric behaviors in BaTiO3/CoFe2O4/BaTiO3 laminated ceramic composites prepared by spark plasma sintering

The BaTiO3 (BTO) / CoFe2O4 (CFO) / BaTiO3 (BTO) laminated ceramic composites were prepared via spark plasma sintering (SPS). The ceramic composites were highly dense with clear boundaries between the three layers. The short time and low temperature required for densification during the SPS process diminish the possibility for unwanted reactions to occur. The BTO/CFO/BTO laminated ceramic composites exhibit excellent ME properties including: (1) maximal ME voltage coefficient as high as ~ 1.0 V/Oe cm; (2) obvious AC magnetic field frequency response and sensitivity from 102 to 107 Hz; (3) perfect ME anisotropy matches well with the theoretical fitting curve; (4) giant transverse ME voltage coefficient as large as ~ 4.1 V/Oe cm at resonance frequency of ~ 105 Hz. It is possible to fabricate several kinds of ME function devices with the BTO/CFO/BTO laminated ceramics prepared in this work.

Magnetic nanoparticle thermometry independent of Brownian relaxation

An improved method of magnetic nanoparticle (MNP) thermometry is proposed. The phase lag ϕ of the fundamental f0 harmonic is measured to eliminate the influence of Brownian relaxation on the ratio of 3f0 to f0 harmonic amplitudes applying a phenomenological model, thus allowing measurements in high-frequency ac magnetic fields. The model is verified by simulations of the Fokker–Planck equation. An MNP spectrometer is calibrated for the measurements of the phase lag ϕ and the amplitudes of 3f0 and f0 harmonics. Calibration curves of the harmonic ratio and tanϕ are measured by varying the frequency (from 10 Hz to 1840 Hz) of ac magnetic fields with different amplitudes (from 3.60 mT to 4.00 mT) at a known temperature. A phenomenological model is employed to fit the calibration curves. Afterwards, the improved method is proposed to iteratively compensate the measured harmonic ratio with tanϕ, and consequently calculate temperature applying the static Langevin function. Experimental results on SHP-25 MNPs show that the proposed method significantly improves the systematic error to 2 K at maximum with a relative accuracy of about 0.63%. This demonstrates the feasibility of the proposed method for MNP thermometry with SHP-25 MNPs even if the MNP signal is affected by Brownian relaxation.

Self-biased magnetoelectric charge coupling in transducer of SmFe2, Pb(Zr,Ti)O3 stack, and stepped horn substrate with multi-frequency effect

This paper presents a broadband, self-biased magnetoelectric (ME) charge coupling in a transducer comprising of a negative magnetostrictive SmFe2 plate, a piezoelectric Pb(Zr,Ti)O3 (PZT) stack, and a stepped horn substrate. By using the SmFe2 plate with a large anisotropic field, an outstanding self-biased piezomagnetic effect is realized. The horn serves as a waveguide with multiple resonances and converges vibrating energy excited by the SmFe2 plate from the wide side to the narrow side, which results in a higher vibrating magnification at the position of the PZT-stack. Then, a strong mechanical-electric coupling is realized by the use of the PZT-stack with high capacitance. Consequently, several large peaks of ME charge response with magnitudes of 1.02–18.99 nC/Oe in the 0.1–50 kHz range are observed at zero-biased magnetic field. This demonstrates that the proposed broadband self-biased structure may be useful for multifunctional devices such as low frequency AC magnetic field sensors or multi-frequency energy harvesters.

Integration of c-axis oriented Bi3.15Nd0.85Ti2.95Hf0.05O12/La0.67Sr0.33MnO3 ferromagnetic-ferroelectric composite film on Si substrate

A La0.67Sr0.33MnO3 (LSMO) ferromagnetic layer and a Nd3+/Hf4+ co-substituted Bi4Ti3O12 (Bi3.15Nd0.85Ti3-xHfxO12 (BNTHx, x = 0, 0.025, 0.05, 0.1 and 0.15)) ferroelectric layer were successively deposited onto the (00 l)-oriented LaNiO3 (LNO) layer buffered (001) Si substrate via all chemical solution deposition (CSD) method. As a result, the BNTHx/LSMO ferromagnetic-ferroelectric composite films integrated on Si substrate exhibit high c-axis orientation. The Nd3+/Hf4+ co-substituted BNTHx films have the lower leakage current and the better ferroelectric properties than the mono-substituted Bi4Ti3O12 (Bi3.15Nd0.85Ti3O12 and Bi4Ti2.95Hf0.05O12) films. In particular, the BNTH0.05/LSMO/LNO film has the lowest leakage current density of 2.5 × 10−7 A/cm2 at 200 kV/cm, and the highest remnant polarization (Pr) of 27.3 μC/cm2. The BNTH0.05/LSMO/LNO composite film also exhibits the soft ferromagnetism characteristics with a high saturated magnetization of 258 emu/cm3 at 300 K, and the excellent magnetoelectric (ME) effect. The variations of ME voltage coefficient αE values with DC bias magnetic field Hbias shows that the BNTH0.05/LSMO/LNO film has the high αE value at near zero Hbias. Moreover, at Hbias = 0 Oe, the αE value gradually increases from zero with the increasing of the AC magnetic field frequency, and eventually reaches about 18.9 V/cm·Oe at 100 kHz, suggesting the existence of self-biased ME effect.