Variation Rate Of Field Research Articles

Dynamic remagnetisation of CoFe2O4 nanoparticles: thermal fluctuational thawing of anisotropy

We report a study of the magnetodynamics of cobalt ferrite (CoFe2O4) nanoparticles with an average diameter of ∼6 nm. Hysteresis loops were measured under quasi-static conditions and in pulse fields with amplitudes H0 of up to 130 kOe and for durations τ P of 8 and 16 ms. The growth of coercivity H c observed with an increase in the magnetic field variation rate dH/dt (determined by the values of H 0 and τ P ) and the reduction of H c with temperature is ascribed to the superparamagnetic effect. The proposed theoretical model explains the observed dependences fairly well. Notably, the effective magnetic anisotropy constant obtained exceeds the value for bulk crystals and might be indicative of the contribution of surface magnetic anisotropy.

Perpendicular Magnetic Anisotropy Electric Field Modulation in Magnetron-Sputtered Pt/Co/X/MgO Ultrathin Structures With Chemically Tailored Top Interface

Ab initio calculations of voltage-controlled perpendicular magnetic anisotropy predict that the adjunction of Pt at the top of Co/MgO interface in Pt/Co/MgO ultrathin structures would enhance the perpendicular anisotropy and its electric field ( E-field) variation rate. Following these theoretical expectations, using the magnetron-sputtering technique, we first designed and elaborated magnetic multilayered samples which have subsequently been patterned by UV lithography in magnetotransport devices. The as-deposited samples show perpendicular magnetization configuration whose E-field modulation has been studied by anomalous Hall effect magnetometry experiments under applied E-field in patterned devices. The measured voltage anisotropy modulation effect is found to be asymmetric with respect to the E-field polarity, in qualitative agreement with theoretical predictions.

General Regularities and Differences in the Behavior of the Dynamic Magnetization Switching of Ferrimagnetic (CoFe2O4) and Antiferromagnetic (NiO) Nanoparticles

In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe2O4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force HC at the dynamic magnetization switching is markedly higher than HC at quasi-static conditions. HC increases as the pulse duration τP decreases and the maximum applied field H0 increases. The dependence of HC on field variation rate dH/dt = H0/2τP is a unambiguous function for CoFe2O4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH/dt, and the value of applied field H0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Features of the quasi-static and dynamic magnetization switching in NiO nanoparticles: Manifestation of the interaction between magnetic subsystems in antiferromagnetic nanoparticles

We report on the investigations of a system of 8-nm NiO particles representing antiferromagnetic (AFM) materials, which are weak magnetic in the form of submicron particles, but can be considered to be magnetoactive in the form of nanoparticles due to the formation of the uncompensated magnetic moment in them. The regularities of the behavior of magnetization switching in AFM nanoparticles are established by studying the magnetic hysteresis loops under standard quasi-static conditions and in a quasi-sinusoidal pulsed field of up to 130 kOe with pulse lengths of 4–16 ms. The magnetic hysteresis loops are characterized by the strong fields of the irreversible magnetization behavior, which is especially pronounced upon pulsed field-induced magnetization switching. Under the pulsed field-induced magnetization switching conditions, which are analogous to the dynamic magnetic hysteresis, the coercivity increases with an increase in the maximum applied field H0 and a decrease in the pulse length. This behavior is explained by considering the flipping of magnetic moments of particles in an external ac magnetic field; however, in contrast to the case of single-domain ferro- and ferrimagnetic particles, the external field variation rate dH/dt is not a universal parameter uniquely determining the coercivity. At the dynamic magnetization switching in AFM nanoparticles, the H0 value plays a much more important role. The results obtained are indicative of the complex dynamics of the interaction between magnetic subsystems formed in AFM nanoparticles.

Charge-Plasma-Based Negative Capacitance Ring-FET: Design, Investigation and Reliability Analysis

In this work, an architecture of charge-plasma-based negative capacitance ring-field-effect transistor (ring-NCFET) is proposed and described. Negative capacitance phenomenon is applied to the proposed structure and various parameters such as device characteristics, analog parameters and linearity parameters are analyzed. Negative capacitance helps in scaling down the operating voltage of the proposed device by enhancing the total capacitance and the subthreshold slope. The Landau-Khalatnikov equation was used to attain the effective gate bias across the gate electrode and ferroelectric material. Various calculated device parameters are energy bandgap, electron concentration, hole concentration, electric field variation, potential variation and recombination rate of the dopingless ring-NCFET. The subthreshold slope reduction of 20 mV/decade and approximately twofold increase in drain current at lower gate bias as compared to conventional dopingless ring-FET shows the effectiveness of implementing a negative capacitance technique. Single-gate (SG) dopingless ring-FET (DRing-FET) is compared with SG-DRing-NCFET and double-gate (DG)-DRing-NCFET. The variation of ferroelectric material thickness and interface trap charges (ITCs) is analyzed for device performance optimization and reliability. Various analog/linearity parameters are obtained in the presence of ITCs.

Features of the Pulsed Magnetization Switching in a High-Coercivity Material Based on ε-Fe2O3 Nanoparticles

The magnetic structure of the e-Fe2O3 iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to ~150 K. As the temperature decreases, e-Fe2O3 undergoes a magnetic transition accompanied by a significant decrease in the coercivity Hc and, in the low-temperature range, the compound has a complex incommensurate magnetic structure. We experimentally investigated the dynamic magnetization switching of the e-Fe2O3 nanoparticles with an average size of 8 nm in the temperature range of 80–300 K, which covers different types of the magnetic structure of this iron oxide. A bulk material consisting of xerogel SiO2 with the e-Fe2O3 nanoparticles embedded in its pores was examined. The magnetic hysteresis loops under dynamic magnetization switching were measured using pulsed magnetic fields Hmax of up to 130 kOe by discharging a capacitor bank through a solenoid. The coercivity Нс upon the dynamic magnetization switching noticeably exceeds the Нс value under the quasi-static conditions. This is caused by the superparamagnetic relaxation of magnetic moments of particles upon the pulsed magnetization switching. In the range from room temperature to ~ 150 K, the external field variation rate dH/dt is the main parameter that determines the behavior of the coercivity under the dynamic magnetization switching. It is the behavior that is expected for a system of single-domain ferro- and ferrimagnetic particles. Under external conditions (at a temperature of 80 K) when the e-Fe2O3 magnetic structure is incommensurate, the coercivity during the pulsed magnetization switching depends already on the parameter dH/dt and is determined, to a great extent, by the maximum applied field Hmax. Such a behavior atypical of systems of ferrimagnetic particles is caused already by the dynamic spin processes inside the e-Fe2O3 particles during fast magnetization switching.

Общие закономерности и различия в поведении динамического перемагничивания ферримагнитных (CoFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=-) и антиферромагнитных (NiO) наночастиц

Abstract In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe_2O_4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force H _C at the dynamic magnetization switching is markedly higher than H _C at quasi-static conditions. H _C increases as the pulse duration τ_ P decreases and the maximum applied field H _0 increases. The dependence of H _C on field variation rate dH / dt = H _0/2τ_ P is a unambiguous function for CoFe_2O_4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH / dt , and the value of applied field H _0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Internal Charging Characteristics in Typical Navigation Satellite Orbits

Internal charging is an important hazard for navigation satellites, which operate in outer electron radiation belt and experience a very variable radiation environment because of the inclination of orbits. A rapid analysis method of internal charging is introduced, including physical model, conductivity model, shielding and electron transportation algorithm, and charging calculation method. Using this method, we calculate the charging processes of dielectric in medium earth orbit (MEO), geostationary earth orbit (GEO), and inclined geosynchronous orbit (IGSO). The results show that charging field fluctuates a lot in orbit and variation rate of field is proportional to electron flux. The saturated charging field in MEO is higher than that in GEO, and the field in GEO is higher than that in IGSO. So, the internal electrostatic discharging risk in MEO, GEO, and IGSO decreases in turn. If conductivity of dielectric is smaller, saturated field is larger and the difference of field in MEO, GEO, and IGSO is smaller. The dark and radiation-induced conductivity (RIC) of dielectric are important parameters in simulation, so we study the impact of conductivity in charging processes. In IGSO and MEO, when dark conductivity-induced time constant of charging is greater than orbital period and RIC is not large, the deposited electrons between different orbits accumulate and the charging field increase constantly. In this situation, the dark conductivity has a big influence on internal charging and it is important to measure it accurately.

Effect of the variation rate of an external magnetic field on the high-frequency response of surface superconductivity in a critical state

The effect of the magnetic field variation rate on the high-frequency absorption of surface superconductivity of cylindrical samples has been studied. It has been shown that the magnitude of additional highfrequency absorption attributed to Kulik vortices does not demonstrate saturation and other critical changes up to a magnetic field variation rate of 350 kOe/s, which indicates that the velocity of motion of Kulik vortices is higher than that of Abrikosov vortices. The effect of the normal core of the sample on the dynamics of a magnetic flux has been discussed.

Suitable Excitation Condition in Overshooting Process for Suppressing Magnetic Field Attenuation in Y-Based Coated Conductor Coils

In Y-based coated conductor coils, a shielding current is induced by the magnetic field component perpendicular to the broad face of the coated conductor. Since the magnetic field induced by the shielding current gradually attenuates with time, the temporally stable magnetic field required for high-resolution imaging in magnetic resonance imaging (MRI) applications is difficult to obtain. We investigated the suitable excitation conditions in an overshooting process required for suppressing the magnetic field attenuation. We fabricated a double-pancake coil composed of GdBCO tapes and measured the shielding magnetic flux density and its variation rate using Hall probes attached to the outermost layer of the coil. The variation rate of magnetic flux density decreased with the maximum coil current. The overshooting current required for zero variation rate of magnetic flux density increased with target coil current in the small coil current region, and decreased with target coil current in the large coil current region. The suitable powering conditions for the overshooting process could be obtained by considering the relationship between the final operating current of the coil, the shielding magnetic flux density, and the required overshooting current for zero field variation rates at the end of the current ramping.

Ramping Operation of the Conduction-Cooled High-Temperature Superconducting Magnet for an Active Magnetic Regenerator System

An active magnetic regenerator (AMR) system requires fast ramp rate of magnetic field variation. The magnetic field change enables a magnetic refrigerant to create magnetocaloric effect for magnetic refrigeration. In this paper, a conduction-cooled high-temperature superconducting (HTS) magnet has been thermally analyzed and tested for an AMR system. A GdBCO conductor insulated with polyimide tape was wound by dry winding and standard double-pancake coil method. Stycast 2850FT was applied to the edge of the tape conductor so that the cooling plates, which were located at the top and bottom of the coil, should effectively cool the magnet. The whole thermal bridging between the magnet and a two-stage GM cryocooler is analyzed by the numerical simulation and a Joule heating experiment of the HTS magnet at normal state. The inductance of the HTS magnet was measured as 350 mH. The HTS magnet is intended to be operated with maximum 1 T/s at 20 K, cooled by the cryocooler. A solenoid switch and an external dump resistor were employed in order to discharge the HTS magnet. A continuous ramping operation test was conducted with a 60-A peak current and a maximum 20-A/s ramp rate by 3-kW dc power supply. This paper describes the experimental results of the ramping operation of the HTS magnet for an AMR system, and the technical issues on the results are discussed.

Weakly conditionally stable finite-difference time-domain method

A novel weakly conditionally stable finite-difference time-domain (FDTD) method, which is extremely useful for problems with very fine structures over a large computational domain in one or two directions, is presented. The time step in this method is only determined by one space discretisation and is presented analytically. Compared with the alternating-direction implicit (ADI) FDTD method, this new method has higher accuracy, especially for larger field variation rate and/or time-step size. By defining the field components at only two time steps, this new method requires the solution of four tridiagonal matrices and four explicit updates at each time step, which is more computationally efficient than the ADI-FDTD scheme. While maintaining the same size of time step, the CPU time for this method can be reduced to about 2/3 of that for the ADI-FDTD scheme. The numerical performance of the proposed method over the ADI-FDTD method is demonstrated through numerical examples.

V: A new index to represent the variation rate of geomagnetic activity

By calculating the hourly standard deviation of the first-order differences of the horizontal geomagnetic component minute data, a new index Vr to represent the variation rate of the geomagnetic field was introduced. Vr-indices show similar trends in the temporal change at different observatories and have simultaneous peak values at the observatories covering a large span geographically, which reveals that the source of geomagnetic disturbances represented by Vr is in the magnetosphere. Based on the comparison among Vr, Kp and ap, it is found that generally Vr changes linearly with Kp and ap, which means that the rapid changes of magnetic field usually exist together with magnetic disturbances. But there are exceptions. As Vr can be easily produced by individual observatory in quasi real time and is more sensitive to the variation rate of geomagnetic field rather than the field itself, it can be expected to serve for monitoring or predicting the geomagnetic-induced event in a quick and intuitive way.

Effect of cooling rate on electrical properties, impulse surge and dc-accelerated aging behaviors of ZPCCD-based varistors

The effect of cooling rate on nonlinear electrical properties, and stability against impulse surge and dc-accelerated aging stress of ZPCCD-based varistors was investigated. The cooling rate relatively slightly affects the microstructure, whereas it did not significantly modify the densification process. The nonlinear coefficient increased with the increase of cooling rate up to 240 °C/h, whereas the further fast cooling rate caused it to decrease. The samples cooled at 240 °C/h exhibited the highest nonlinearity, in which the nonlinear coefficient is 61.6 and the leakage current density is 6.8 μA/cm2. The aging was not significantly affected by the specified impulse surge stress, whereas it was greatly affected by dc-accelerated aging stress. The samples cooled at 120 °C/h exhibited the highest stability, in which the variation rate of breakdown field is −5.5% and the variation rate of nonlinear coefficient is −29.1% under continuous stress composed of the specified impulse surge and dc-accelerated aging stress of 0.95 E B/150 °C/24 h.

Response of the magnetic field in the geosynchronous orbit to solar wind dynamic pressure pulses

We do a statistical survey of solar wind dynamic pressure (Pd) pulses and geosynchronous magnetic fields observed between 1998 and 2005. In geomagnetic quiet times with Dst > −50 nT, we find 111 solar wind dynamic pressure pulses which produce geosynchronous magnetic field responses. These responses are often observed by two or three GOES spacecraft at different local times in geosynchronous orbit. The magnitudes of the geosynchronous magnetic field changes (dBz) have a peak near the noon meridian, similar to the results obtained in the study of the response of the geosynchronous field to the large and sharp solar wind dynamic pressure variations. However, the relative change of the geosynchronous magnetic field dBz/AV‐Bz (where AV‐Bz is the average of the geosynchronous magnetic field Bz observed during the response to the pressure pulse) depends weakly on the local time; thus the change of Bz(dBz) is proportional to the average field (AV‐Bz). As the magnitude of the relative change of solar wind dynamic pressure (dPd/Pd) increases, the rate of geosynchronous magnetic field variation increases correspondingly. These results imply that the magnitude of the geosynchronous magnetic field response could be determined by AV‐Bz. In addition, the interplanetary field orientation does not affect the response significantly. Using an MHD code which models the global behavior of the solar wind‐magnetosphere‐ionosphere system, we reproduce the main characteristics of the observations.

Design and Test of a Compound Persistent-Pulsed Magnet for Fast Field Cycling NMR

In order to investigate the possibility of using superconducting magnets for fast field cycling (FFC) NMR relaxometry, a NbTi magnet able to achieve high field variation rates in a constant background field has been designed and tested. The design layout consists in an outer superconducting magnet working in persistent mode and an inner superconducting magnet working in pulsed mode. The layout of the inner magnet has been optimized taking into account field strength, field homogeneity, stray field and inductive coupling with outer magnet. A demonstration pulsed magnet has been tested in a background field, achieving the main desired working features.

Comparison between HIE‐FDTD method and ADI‐FDTD method

AbstractThis letter gives the comparison of the 3D hybrid implicit–explicit finite‐difference time‐domain (HIE‐FDTD) method with the ADI‐FDTD method through numerical examples. It shows that the HIE‐FDTD method has higher accuracy than the ADI‐FDTD method, especially for larger rate of field variation; the time step size, which has a detrimental effect on the accuracy of ADI‐FDTD, does not affect the accuracy of HIE‐FDTD method. The computation efficiency of the HIE‐FDTD method is higher than ADI‐FDTD method. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1001–1005, 2007; Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.22340

A New Approach for Finite-Element Modeling of Hysteresis and Dynamic Effects

Macroscopic behavior of ferromagnetic materials can be considered as the resultant of three phenomena: hysteresis, eddy current, and excess loss. Hysteresis is the behavior of the material under quasi-static variation of magnetic field. Eddy-current and excess losses are dependent on the rate of field variation and are evident in the fast variation of the magnetic field, so they are called dynamic effects. This paper presents a simple and practical technique in field analysis of electromagnetic systems having hysteresis and dynamic effects. Based on the Preisach model for hysteresis and existing formulations for eddy currents and excess loss, an equivalent expression for field intensity has been introduced. A new technique has been presented in order to include this expression in the finite-element code. A typical system has been modeled by this code. Effects of relaxation and time step were examined on the stability and the convergence rate of the method. The validity of the proposed model has been checked by comparing its results with experimental measurements

Error between unconditionally stable FDTD methods and conventional FDTD method

Both the ADI-FDTD method and the CN-FDTD method can be viewed as approximate factorisation of the conventional FDTD scheme, so the error between these methods and the conventional FDTD method is obtained. It is shown that the error is associated with the time step size, the permittivity and the permeability of the medium, and the spatial variation rate of field. The space discretisation has no relation with the error, which is demonstrated by numerical examples.

Residual magnetic field sensing for stress measurement

The evaluation of both applied and residual stresses in engineering structures to provide early indications of stress status and eventual failure is a fast growing area in non-destructive testing. Much work has been done in recent years in the development of magnetic stress measurement techniques for ferromagnetic materials using applied magnetic fields to monitor changes in the magnetic properties of materials, such as variations in the hysteresis curve or Barkhausen emission. But the area of passive field measurement is relatively unexplored. When magnetic metals are strained, they irreversibly transformed from a non-magnetic state to a magnetic state, this is referred to as metal magnetic memory (MMM) or the residual magnetic field (RMF). This paper investigates the phenomena under different circumstances and applies the residual magnetic field technique to stress measurement. A three axis magneto-resistive magnetic field sensor is used to measure the residual magnetic fields parallel to the applied stress and the material surface (Bx) and perpendicular to the material surface (Bz), generated by the magneto-mechanical effect without the application of an external field, using steel samples exposed to stresses. The test results show that without using an applied field, the stresses in a sample can be measured using magnetic field sensing, with Bx showing particularly good correlation. The work concludes that the novel passive field technique including analysis of the magnetic field pattern and magnetic field variation rate, would prove advantageous in certain circumstances, for example in-service inspection of structures with complex geometries. Further research directions are also highlighted.