Coercive Field Measurements Research Articles

Exploring the Impact of Different Milling Parameters of Fe/SiO2 Composites on Their Structural and Magnetic Properties.

This research focuses on the production process of soft magnetic composites in the form of 3D bulk compacts made from insulated powder particles using different milling parameters, aiming to enhance their magnetic properties and to study an innovative method of the powder surface "smoothing" technique. A structure analysis using scanning electron microscopy (SEM), EDS, and optical microscopy is also included. We found out that the samples made by the innovative method have lower density values. This can be caused by a more consistent SiO2 insulation layer on highly pure iron powder particles. A correlation between the mechanical smoothing method and better insulation of powder particles can help to provide eco-friendlier solutions for the preparation of soft magnetic composites, such as less usage of reagents and more consistent coverage of powder particles with lower final insulation thickness. The magnetic properties of these compacts are evaluated by coercive field, permeability, and loss measurements. The particle-level smoothing technique in some cases can reduce the value of coercivity up to 20%. For some samples, the ball-to-powder ratio has a bigger impact on magnetic properties than surface treatment, which can be caused by an increased amount of insulation in the SMC compacts.

Angular dependence of the magnetization relaxation in Co/Pt multilayers

We study the influence of defects in Co/Pt multilayers on the room-temperature magnetization reversal and relaxation mechanisms via angle-dependent magnetic viscosity and coercive field measurements. The data reveal a transition from pinning-dominated domain wall propagation to a sequence of pinning-dominated and uniform switching, with increasing tilt away from the normal direction. The leading role of the dendritic domain wall propagation in the nanogranular exchange-coupled films is corroborated by the scaling of relaxation times, the angular dependence of the coercive field, and Kerr microscopy.

Magnetic transitions and coercive field variations in La1−xSrxMnO3 thin films (0.04 ≤ x ≤ 0.50)

We have investigated the electronic and magnetic ground state of La1−xSrxMnO3 thin films, especially around doping x = 0.50 in comparison to the well studied system of x = 0.33 and the low doped system of x = 0.04. The films discussed here were grown by molecular-beam epitaxy, epitaxially strained to (001) oriented strontium titanate substrates. Apart from resistivity and magnetization measurements probing the electronic and magnetic ground state, we also study the temperature dependence of the coercive field of these samples. Our measurements reveal that the coercive field increases as the doping is changed from x = 0.33. The coercive field at 5 K for the x = 0.50 sample is Hc = 0.0450 T compared to Hc = 0.0080 T for the x = 0.33 sample. The temperature dependent coercive field measurements on the x = 0.50 sample show a dramatic cusp around 100 K that is coincident with more subtle features observed in both magnetization and resistivity data at this temperature.

Novel Approaches of Nanoceria with Magnetic, Photoluminescent, and Gas-Sensing Properties.

The modification of CeO2 with rare-earth elements opens up a wide range of applications as biomedical devices using infrared emission as well as magnetic and gas-sensing devices, once the structural, morphological, photoluminescent, magnetic, electric, and gas-sensing properties of these systems are strongly correlated to quantum electronic transitions between rare-earth f-states among defective species. Quantitative phase analysis revealed that the nanopowders are free from secondary phases and crystallize in the fluorite-type cubic structure. Magnetic coercive field measurements on the powders indicate that the substitution of cerium with lanthanum (8 wt %), in a fluorite-type cubic structure, created oxygen vacancies and led to a decrease in the fraction of Ce species in the 3+ state, resulting in a stronger room-temperature ferromagnetic response along with high coercivity (160 Oe). In addition to the magnetic and photoluminescent behavior, a fast response time (5.5 s) was observed after CO exposure, indicating that the defective structure of ceria-based materials corresponds to the key of success in terms of applications using photoluminescent, magnetic, or electrical behaviors.

Verwey transition temperature distribution in magnetic nanocomposites containing polydisperse magnetite nanoparticles

Polymeric nanocomposites containing Fe3O4 nanoparticles were prepared through a chemical route under different precursor-to-solvent ratios and were submitted to structural and morphologic characterization. The embedded nanoparticles, containing pure magnetite and characterized by considerable polydispersity, are rather homogeneously dispersed in the matrix. The magnetic properties of two representative samples were analyzed in detail between T = 5 K and room temperature. Magnetic effects clearly associated with the Verwey monoclinic to cubic transition with transition temperatures distributed in the interval 95–120 K were put in evidence. On heating through this region, the coercive field and the maximum susceptibility of hysteresis loops display marked downward/upward steps, respectively, while the high-field magnetization is not affected at all; a comparable upward step is measured in the FC/ZFC curves. Reporting the maximum susceptibility as a function of the reciprocal of the coercive field in the interval from T = 95 to T = 120 K, and using the predictions for single-domain nanoparticles with randomly distributed axes of uniaxial and cubic anisotropy (the former/latter case being applicable below/above the Verwey transition, respectively), the evolution of the transformed cubic-anisotropy fraction upon heating has been studied, and the distribution of Verwey transition temperatures related to the sample polydispersity has been accurately determined. The low-temperature value of the uniaxial anisotropy constant is obtained from coercive field measurements and found to be comparable to, albeit slightly higher than the corresponding quantity measured in bulk crystalline magnetite.

Structure and size dependence of the magnetic properties of Ni@C nanocomposites

Carbon-coated nickel (Ni) nanoparticles, Ni@C nanocomposites, have been synthesized using solid-state pyrolysis of nickel phthalocyanine and metal-free phthalocyanine (NiPc)x· (H2Pc)1−x solid solutions, 0⩽x⩽1. The Ni concentrations in carbon matrix (cNi) of the prepared samples continuously varied in the range of 0–3at.% (0–12wt.%). The average nanoparticle size varied within 4–40 nm range. All samples containing single domain Ni nanoparticles exhibit both ferromagnetic and superparamagnetic properties because of the wide range of size distribution. An abrupt drop of saturation magnetization has been observed with decrease in size of Ni nanoparticles from 40 nm to 12 nm. Nearly linear dependence of saturation magnetization on the nanoparticle surface/volume ratio can be interpreted as a result of contact interaction between Ni nanoparticles and the carbon matrix which provides an electron transfer from carbon matrix to nickel. However, further reductions in nanoparticle size increase magnetization growth of which can apparently contribute to the emergence of the giant paramagnetism due to large orbital moments of conductive electrons. The size effects and surface magnetic anisotropy in Ni@C nanocomposites are revealed in the measurements of coercive field, zero-field cooling (ZFC) susceptibility, blocking temperatures and ferromagnetic resonance spectra. Concentration dependencies of ferromagnetic and electron paramagnetic resonance parameters in Ni@C nanocomposites have also been investigated and their peculiarities highlighted. A correlation between concentration dependencies of FMR and SQUID magnetometry parameters, namely between the g-factor curves – geff, the resonance linewidth – ΔHFMR and coercive field – Hc, have been observed.

Assessing the recovery and recrystallization kinetics of cold rolled microalloyed steel through coercive field measurements

Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name “back annealing” or “recovery annealing”. In the present work, various annealing treatments were applied to a Nb-microalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350–450°C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550°C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.

Magnetocoupling and domain structure of BiFeO3/ LaFeO3 heterostructures deposited on LaSrCoO3/Pt/TiO2/SiO2/Si (100) substrates by the soft chemical method

BiFeO3 (BFO) and LaFeO3 (LFO) heterostructures were obtained at room temperature on Pt/TiO2/SiO2/Si (100) substrates by chemical solution deposition at a temperature of 500 °C for 2 h. For comparison, the films were also deposited on La0.5Sr0.5CoO3 coated Pt/TiO2/SiO2/Si (100) substrates. The magnetoelectric coefficient measurement was performed to show magnetoelectric coupling behavior of such heterostructures. La0.5Sr0.5CoO3 (LSCO) bottom electrode strongly promotes the formation of high intensity (104) texture of BFO-LFO heterostructures resulting in huge crystal size. The dielectric constants of the films increased from 307 to 400 at 1 MHz with the bottom electrode while the leakage current behavior at room temperature of the films decreased from 10 to 10–10−7 A/cm2 at a voltage of 5 V. Improvement of the P–E hysteresis loop was observed for the heterostructure due the decrease of leakage current caused by the LFO sublayer. Room temperature magnetic coercive field measurements indicate that the magnetic behavior is influenced by the nature of bottom electrode.

Magnetoelectric coupling of LaFeO3/BiFeO3 heterostructures

BiFeO3 (BFO) and LaFeO3 (LFO) heterostructures were obtained at room temperature on Pt/TiO2/SiO2/Si (100) substrates by chemical solution deposition. The films were coherently grown at a temperature of 500°C for 2h. The magnetoelectric coefficient measurement was performed to show magnetoelectric coupling behavior of such heterostructures. The bottom BFO layer in the heterostructure was able to promote the grain growth of the LFO during the annealing process resulting in huge crystal size. Dielectric permittivity and dielectric loss measurements demonstrated only slight dispersion with frequency due to the lower two-dimensional stress in the plane of the film. Improvement of the P–E hysteresis loop was observed for the heterostructure due the decrease of leakage current caused by the LFO sublayer. Room temperature magnetic coercive field measurements indicate that the LFO and LFO/BFO present different magnetic behavior. The magnetic behavior of the heterostructure is influenced by the crystallite size.

Effect of tempering on the microstructure, electrical, and magnetic properties of Eurofer-97 steel

Reduced-activation ferritic–martensitic Eurofer-97 steel is a potential candidate for structural application in future nuclear fusion reactors. Samples of Eurofer-97 steel were cold rolled to 80% reduction in thickness, austenitized at 1050 and 1150°C for 30min and tempered at several temperatures up to 800°C for 2h each. The microstructural characterization of the samples was performed using Vickers microhardness testing and electron backscatter diffraction (EBSD). Electrical resistivity and coercive field measurements were also performed to follow microstructural changes during isothermal tempering. Results were discussed with focus on the precipitation of MX and M23C6 carbides and related changes in these properties.

Long-term microstructural stability of oxide-dispersion strengthened Eurofer steel annealed at 800 °C

Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled samples (80% reduction in thickness) from 1h up to 4320h (6months) at 800°C, i.e. the maximum temperature in the ferritic phase field, to follow its softening behavior. The microstructural stability of this steel was mapped using several characterization techniques including scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, Vickers microhardness testing, X-ray diffraction texture measurements, low-temperature electrical resistivity, and magnetic coercive field measurements. ODS-Eurofer steel displays good microstructural stability. Discontinuous recrystallization occurs at the early stages of annealing resulting in a low volume fraction of recrystallized grains. Extended recovery is the predominant softening mechanism at this temperature for longer times.

Structure of Fe-Based Metallic Glass after Crystallization Process

The work presents a crystallization process of Fe-based amorphous alloy by characterization of the influence of annealing temperature on structural changes and magnetic properties of Fe72B20Si4Nb4 metallic glass. The studies were performed on the samples in the form of ribbons and rods. Crystallization behaviour of the studied alloy was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The studies of soft magnetic properties of tested material involved magnetic permeability, saturation induction, coercive field and magnetic after-effects measurements.

Low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics

We report on the structural, thermal, and magnetic properties of polycrystalline BiFeO3 synthesized by sol-gel route. Powder x-ray diffraction data of the BiFeO3 sample was refined with rhombohedral structure with space group R3c. Magnetization and coercive field measurements showed weak ferromagnetic nature below 10 K. We also investigated the low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics. The anomalies observed in the magnetic entropy change of the BiFeO3 ceramics were close to that of low temperature phase transitions. The combined relative errors in the magnetic entropy change are found to vary from 4% to 15% with increase in temperature.

Giant magnetoresistance in electrodeposited Co-Cu/Cu multilayers: Origin of the absence of oscillatory behavior

A detailed study of the evolution of the magnetoresistance was performed on electrodeposited Co/Cu multilayers with Cu layer thicknesses ranging from 0.5 nm to 4.5 nm. For thin Cu layers (up to 1.5 nm), anisotropic magnetoresistance (AMR) was observed whereas multilayers with thicker Cu layers exhibited clear giant magnetoresistance (GMR) behaviour. The GMR magnitude increased up to about 3.5 to 4 nm Cu layer thickness and slightly decreased afterwards. According to magnetic measurements, all samples exhibited ferromagnetic (FM) behaviour. The relative remanence turned out to be about 0.75 for both AMR and GMR type multilayers. This clearly indicates the absence of an antiferromagnetic (AF) coupling between adjacent magnetic layers for Cu layers even above 1.5 nm where the GMR effect occurs. The AMR behaviour at low spacer thicknesses indicates the presence of strong FM coupling (due to, e.g., pin-holes in the spacer and/or areas of the Cu layer where the layer thickness is very small). With increasing spacer thickness, the pin-hole density reduces and/or the layer thickness uniformity improves which both lead to a weakening of the FM coupling. This improvement in multilayer structure quality results in a better separation of magnetic layers and the weaker coupling (or complete absence of interlayer coupling) enables a more random magnetization orientation of adjacent layers, all this leading to an increase of the GMR. Coercive field and zero-field resistivity measurements as well as the results of a structural study reported earlier on the same multilayers provide independent evidence for the microstructural features established here. The large GMR reported previously on such Co/Cu multilayers at Cu layer thicknesses around 1 nm can be attributed to the presence of a fairly large superparamagnetic (SPM) fraction rather than being due to a strong AF coupling.

Adsorbate influence on the coercive field of ultrathin Co/Cu{1 1 0}

We report on experimental coercive field measurements for ultrathin films of Co on Cu{1 1 0}, revealing a complex dependence upon the surface coverage of oxygen and hydrogen adatoms. With reference to first-principles calculations, we rationalise this behaviour in terms of initial reaction with surface contaminants and/or adsorption at defects, followed by (in the case of oxygen) subsequent surface ordering and eventual formation of surface oxide.

Effect of stored energy and recovery on the overall recrystallization kinetics of a cold rolled low carbon steel

The evolution of recrystallization kinetics of a cold rolled low carbon steel with an initial coarse grain size and heterogeneous microstructure is studied by means of nucleation and interface-averaged grain boundary migration rate. Recrystallization is governed by two annealing time ranges, characterized by two Avrami exponents. Range 1 is associated to the recrystallization of high stored energy regions mostly belonging to the γ-fiber and Range 2 to the progression of recrystallization mainly through low stored energy α-fiber grains. The transition from Range 1 to Range 2 is consistent with the decrease of high stored energy regions. Afterwards, recrystallization is very sluggish due to the low stored energy regions remaining in the material and leads to incomplete recrystallization for annealing times of practical significance. Additionally, coercive field measurements are used to infer the evolution of concurrent recovery with recrystallization. Recovery only accounts for a small part of the decrease of the total stored energy, the rest being due to the heterogeneous distribution of the stored energy.

Sensitivity of Conventional and Non-destructive Characterization Techniques to Recovery and Recrystallization

The sensitivity and limitations of magnetic coercive field, hardness, metallographic measurements and electron back scattered diffraction analysis are studied and discussed in terms of monitoring recovery and the onset and evolution of recrystallization in a cold rolled low carbon steel. The magnetic coercive field shows much higher resolution than hardness measurements when measuring recovery. Hardness is almost insensitive to the microstructural changes taking place in the material during annealing treatments performed at low temperatures. The conditions under which the magnetic coercive field measurements can also be used to monitor recrystallized fraction are also discussed.

Influence of top electrode design on pMUTs performance

Piezoelectric micromachined ultrasonic transducers (pMUTs) were designed, fabricated and tested to measure resonance and anti-resonance frequencies, and quality factor. Film quality was monitored via saturation polarization, remnant polarization and coercive electric field measurements in different films. Effective coupling coefficient and device efficiency are calculated based on those measured values. Single and split top electrode pMUTs designs, X and Y, respectively, are evaluated to understand the influence of top electrode design on device performance. It was found that resonance frequency decreases with an increase in membrane width. Top electrodes, smaller than the membrane width by 10μm, show a positive effect on the effective coupling coefficient of the pMUTs. It was found that both quality factor and device efficiency are maximized when top electrode width is the same as that of membrane width. An increase in top electrode dimension beyond the membrane decreases device performance.

Dielectric Permittivity Enhancement in PZT by Light Doping with Gd

ABSTRACTPbZr0.53Ti0.4703 doped with 0.6% at. of Gd2O3 has been produced following the traditional solid state reaction technique. The ferro-paraelectric transition temperature is reduced from 386 °C to 352 °C. Remnant polarization and coercive field measurements were made from the hysteresis loops obtained at 23 °C and at different applied electric fields up to a maximum of 17 kV/cm. Piezoelectric performance was studied in the 4 Hz to 4 MHz frequency range at 25 °C and the radial electromechanical coupling factor was determined. A detailed electrical conductivity study in the 30-450 °C temperature range is performed in a wide frequency interval. The different participating transport mechanisms are elucidated and the corresponding activation energies were determined by fitting the experimental data. In particular, in the 30 to 300 °C temperature interval, the ac conductivity follows Jonscher universal relaxation law.

Application of Coercive Field Measurements to the Evaluation of Recovery and Recrystallization in Cold Rolled Interstitial Free (IF) Steel

The recovery and recrystallization kinetics of a cold rolled interstitial free (IF) steel were studied during isothermal annealing. Magnetic methods based on coercive field measurements, hardness tests and metallography were applied so as to follow the kinetics experimentally. The coercive field measurement technique reveals a higher degree of resolution for monitoring recovery than conventional hardness determination and also allows the recrystallization progress to be monitored. The results obtained are compared to those previously determined for a non-stabilized extra low carbon (ELC) steel. The observed differences are discussed in terms of the presence of microalloying elements, Ti and Nb, which slow down recovery and delay recrystallization.