Quasi-static Hysteresis Research Articles

Magnetization switching assisted by out-of-plane ultrafast pulsed field in high perpendicular anisotropy nanostructures

A microwave test structure has been designed to study the magnetization switching assisted by ultrafast out-of-plane field pulses in perpendicular anisotropy nanostructures. Using extraordinary Hall effect (EHE), we measure quasistatic hysteresis loops while continuously applying field pulses of given amplitude (550Oe), variable duration (0.2–10ns) and repetition frequency (1Hz–100kHz). The maximum measured shift in coercive field exceed 200Oe for 10ns pulse. The shift decreases with decreasing pulse duration and repetition frequency and increases linearly with the pulse amplitude, while switching probability increases with pulse duration.

On the kinetics of slow polarization in the lead magnoniobate ferroelectric relaxor

Polarization and repolarization of a (100)-oriented lead magnoniobate crystal in slowly varying and dc electric fields were measured in a temperature interval including the relaxor and field-induced ferroelectric states. Throughout the interval covered, the polarization was shown to exhibit features characteristic of relaxors, namely, open, nonreproducing polarization trajectories in the first few cycles of quasistatic dielectric hysteresis loops and very long relaxation times. The slow thermally activated relaxation stage follows the universal power law evolution, which permits one to determine possible simple spectra of the relaxation time distribution. Temperature dependences of some relaxation and spectral parameters were derived, and their differences in the relaxor and ferroelectric phases are discussed.

Magnetically soft nanomaterials for high-temperature applications

FeCo-based soft magnetic alloys of compositions Fe/sub 45/Co/sub 43/Cu/sub 1/B/sub 3.6/Zr/sub 7.4-x/AM/sub x/, where x=3.7 at.% and 7.4 at.%, and the alloying metals (AM) are Nb and Hf and were prepared in the form of amorphous ribbons by a melt-spinning technique. A two-phase microstructure, consisting of /spl alpha/-FeCo nanocrystals and amorphous matrix, was created by partial devitrification of the amorphous alloys during annealing for 1 h at temperatures 500/spl deg/C-600/spl deg/C. The crystallization process of the amorphous alloys was studied using differential scanning calorimetry, X-ray diffractometry, and transmission electron microscopy. The quasi-static hysteresis loop was measured using an inductive method. Partial replacement of zirconium by hafnium improved thermal stability of the amorphous structure and the magnetic properties of the alloys studied. The effect of niobium is opposite. The best magnetic properties were found in the alloy Fe/sub 45/Co/sub 43/Cu/sub 1/B/sub 3.6/Zr/sub 3.7/Hf/sub 3.7/. It was found that the nanocrystallization temperature strongly influences the thermal stability of the magnetic properties of the alloys studied.

Step-induced magnetic-hysteresis anisotropy in ferromagnetic thin films

We investigate the quasistatic magnetic hysteresis of ferromagnetic thin films grown on a vicinal substrate, using Monte Carlo simulations within a two-dimensional XY model. Intrinsic in-plane anisotropy is assigned to surface sites according to their local symmetry. The simulated hysteresis loops show a strong anisotropy: the coercive field is the largest when the external field is along the step direction and vanishes when the external field is perpendicular to the step direction. In general, the coercivity increases with increasing step density, but displays a more complex dependence on film thickness. The simulations also suggest that the mechanism for the magnetization reversal is coherent rotation. These results are in good agreement with experiments.

New Possibility of Utilizing Amorphous Ring Cores as Stress Sensor

This paper presents a new method of applying stresses to an amorphous ring core. Due to the special cylindrical backings, compressive stress is applied perpendicular to the direction of flux density. In this method the distribution of stresses in the core is uniform and known. This method was used to investigate magnetoelastic properties of a ring core made of Fe40Ni38Mo4B18 amorphous alloy. The changes of quasi-static hysteresis loops, flux density and permeability for compressive stresses up to 10 MPa are presented. Permeability at Hm = 1.5Hc decreases over 40% for stresses up to 10 MPa. These experimental results confirm that the presented method may be very useful in the construction of stress transducers.

Dynamical losses description in grain-oriented materials

Starting from description of quasi-static hysteresis using Preisach model, a complex quasi-static susceptibility (permeability) can be introduced. Based on this assumption, the correlation between static (frequency independent) Wstat and dynamical (frequency dependent) Wdyn part of total losses via complex permeability is shown. Consequently, non-linear frequency depedeence Wdyn (f) can be explained including a convenient distribution fucction of relaxation parameters into the model.

Losses and permeability improvement by stress relieving fully processed electrical steels with previous small deformations

The strong increase in the 1.5 T total losses of a 2.2% Si electrical steel, when a small plastic deformation is applied, is almost exclusively due to the increase in its quasi-static hysteresis loss. Stress relieving at 700°C promoted no change in the grain size, a significant reduction of 1.5 T total losses and increased permeability, which did not reach, however, the non-deformed values. The anisotropy of total losses of the annealed samples is concentrated in the high-induction component of the hysteresis loss.

Anomalies in the slow polarization kinetics of a ferroelectric relaxor in the temperature region of a diffuse phase transition

It is shown experimentally in the specific example of Cr-doped barium-strontium niobate that the anomalies in the infralow-frequency dielectric properties characteristic of a ferroelectric relaxor persist over the whole temperature region of the diffuse phase transition and decrease gradually with increasing temperature. Experimental data are presented on the anomalous quasi-static dielectric hysteresis loops, the slow polarization kinetics, and the anomalously broad energy-distribution spectra of potential barriers. The anomalies are a signature and a quantitative measure of the structural disorder typical of a relaxor.

Mo-containing Finemet alloys: microstructure and magnetic properties

The influence of the partial substitution of Fe by Mo (up to 5 at.%) on the microstructure and magnetic properties of Finemet-type alloys has been studied. The change with Mo content of the peak temperature of the first exotherm is fit by a linear function, with an increase of about 15 K/at.% Mo. However, for the second exotherm no monotonic function was found to fit the change with Mo content. From the temperature dependence of the low-field (∼20 mT) magnetization, a linear decrease of the Curie temperature of the initial amorphous phase of about 25 K/at.% Mo is found. Room-temperature quasistatic hysteresis loops have been measured for samples annealed at different temperatures, showing that the minimum coercivity is not affected by the Mo content of the precursor alloy.

Modeling of quasistatic magnetic hysteresis with feed-forward neural networks

A modeling technique for rate-independent (quasistatic) scalar magnetic hysteresis is presented, using neural networks. Based on the theory of dynamic systems and the wiping-out and congruency properties of the classical scalar Preisach hysteresis model, the choice of a feed-forward neural network model is motivated. The neural network input parameters at each time step are the corresponding magnetic field strength and memory state, thereby assuring accurate prediction of the change of magnetic induction. For rate-independent hysteresis, the current memory state can be determined by the last extreme magnetic field strength and induction values, kept in memory. The choice of a network training set is motivated and the performance of the network is illustrated for a test set not used during training. Very accurate prediction of both major and minor hysteresis loops is observed, proving that the neural network technique is suitable for hysteresis modeling.

Structural and electrical characteristics of Pb0.90La0.15TiO3 thin films on different bottom electrodes

Pb 0.90 La 0.15 TiO 3 (PLT15) thin films were deposited by the sol-gel method on Pt, Pt/Si, and RuO2 on Si and Pt/Si bottom electrodes. X-ray diffraction, micro-Raman spectroscopy, and atomic force microscopy techniques were used for structural characterization of these films. PLT15 films on different electrodes showed good surface morphology with dense and uniform microstructure. PLT15 films on solution derived RuO2 bottom electrodes show (100) preferred orientation of growth and result in larger crystallites. Films deposited on a Pt bottom electrode show sharp and intense Raman features indicating better crystallinity and insignificant film–electrode interactions. PLT15 film on a Pt bottom electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm and Pr, 68 and 46 μC/cm2, respectively, compared to other electrodes. Films on a RuO2 bottom electrode showed relatively inferior dielectric and ferroelectric properties. The ac field dependence of dielectric permittivity at subswitching fields was fitted using the Rayleigh law. It was found that ∼22% of the total measured permittivity was due to irreversible domain wall displacement for the films on a Pt electrode. The reversible polarization components estimated from the capacitance–voltage (C–V) and quasistatic hysteresis measurements showed that Prev/Psat at Vmax for the case of Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film–electrode interface.

Induced anisotropy and magneto-impedance measurements in Fe 73.5Nb 3Cu 1Si 13.5B 9 nanocrystalline alloys

The effect of induced magnetic anisotropy on GMI has been studied in Fe 73.5Nb 3Cu 1Si 13.5B 9 melt-spun amorphous and nanocrystalline ribbons. The amorphous-to-nanocrystalline transformation has been induced by means of furnace annealing and DC joule heating. The correlation between average magnetic anisotropy calculated from quasi-static hysteresis loops and GMI will be discussed in detail.

Surface magnetic anisotropy in glass-coated amorphous microwires as determined from ferromagnetic resonance measurements

The ferromagnetic resonance frequency of different Co base glass-coated amorphous magnetic microwires about 3.5 μm in diameter with negative, vanishing and positive magnetostriction has been investigated from power absorption measurements in the microwave frequency range. The experimental technique employed here involves the replacement of the dielectric of a coaxial transmission line by the sample to be measured. From the evolution of the resonance frequency with DC applied magnetic field, the surface magnetic anisotropy field of the microwires has been quantitatively obtained and, as expected, found to depend on the sign and strength of the magnetostriction. Similar values for the surface anisotropy are obtained in comparison with bulk anisotropy as determined from quasi-static hysteresis loops measurements.

Differences in Dynamic Performance of Molded Polyurethane Foam as a Function of Pad Thickness and Supported Load

Comfort and durability have evolved into essential performance requirements for the polyurethane foam used in automotive seating. A great variety of tests have been developed in an attempt to measure and predict these properties, both for the foam and the assembled seat. However, the results from these various tests can be quite different depending on the test conditions employed. Furthermore, most tests measure comfort properties at some standard deflection, or pressure, which is nominally designed to represent the average population that will be riding on the seat. Missing from these tests is a recognition that foam comfort properties are not the same at all deflections, and thus performance and perceived comfort will change as a result of the size and shape of the person riding the seat. This research presents information demonstrating the dependence of comfort properties, such as vibrational transmissivity and quasi-static hysteresis, on foam thickness and foam chemistry. In addition, the effect of pressure is demonstrated using various masses for the transmissivity testing. This work will be analyzed and discussed from the perspective that comfort properties may need to be defined relative to the sitting pressure of the person riding on the vehicle seat.

Irreversible polarization in donor doped Pb(Zr, Ti)O3

In this paper, the reversible and irreversible polarization contributions in donor doped PbZr0.45Ti0.55O3 are studied. Small signal capacitance measurements under dc-bias are used to measure the reversible contributions. Quasi-static hysteresis measurements are used to obtain the total polarization, i.e., the sum of the reversible and irreversible contributions. The combination of both measurements allows the separation of the reversible and irreversible contributions. For the sub-coercive regime measurements of the Rayleigh coefficient were performed that allow a comparison to theory.

Effect of bottom electrodes on the structural and electrical characteristics of lanthanum doped lead titanate thin films

Ferroelectric Pb0.90La0.15TiO3 (PLT15) thin films were deposited by sol-gel method on Pt, Pt/Si, ruthenium oxide (RuO2)/Si and RuO2/Pt/Si bottom electrodes. X-ray diffraction, and micro-Raman spectroscopy techniques were used for structural characterization of these films. PLT15 films deposited on RuO2 electrodes show (100) preferred orientation of growth and result in larger crystallites. Films on Pt electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm, Pr values, 68 and 46 μC/cm2 respectively. AC field dependence of dielectric permittivity at sub-switching fields was fitted using the Rayleigh law. The reversible polarization components estimated from the CV and quasi-static hysteresis measurements for films on Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film-electrode interface.

Influence of eddy currents on magnetic hysteresis loops in soft magnetic materials

In this paper an attempt has been made to extend the Jiles and Atherton (J–A) quasi-static hysteresis model to describe magnetisation of a material with an alternating magnetic field. In low – industrial – and medium frequency of magnetic field it is possible to ignore the magnetic relaxation and resonance. The field penetration is assumed to be uniform through the material. The influence of eddy currents on the hysteresis loop could be considered and calculated using the method of successive reactions of eddy currents, where a reaction is an additional magnetic field, called reaction H d, induced in the material by the eddy currents according to rot J= γ∂ B/∂ t where γ is the electrical conductivity. The reaction field H d was added to the basic field H 0∝ Iz 1, where I is the current intensity in the magnetising coil of z 1 number of windings. By solving the J–A equation for the magnetic field H w= H 0+ H d it has achieved an extension of the hysteresis loop at an increased frequency of the current, caused by increased losses of the eddy currents. At the frequency f→0 Hz , the hysteresis loop approaches the shape of the quasi-static one.

Unstable states of antiferroelectric liquid crystal devices

The unstable states of an antiferroelectric liquid crystal cell have been investigated, mapping out the unstable branch of the hysteresis loop. It was found that the unstable branch extended over a much greater voltage range than the quasistatic hysteresis loop of the cell. The narrow hysteresis loop is attributed to the influence of defects within the cell, which can seed switching in the presence of a small energy barrier between initial and final states. The measurements indicate that the ferroelectric state may be fundamentally stable at 0 V.

Giant magnetoimpedance and induced anisotropy in joule-heated and conventionally annealed Co-based amorphous materials

The effect of induced magnetic anisotropy on GMI has been studied in Co 71Fe 4B 15Si 10 melt spun amorphous ribbons, using two different techniques of thermomagnetic annealing. Quasi-static hysteresis loops were measured at controlled induction. From the quasi-static hysteresis loop evolution, the value of the macroscopic anisotropy K u induced by the field annealing treatment was determined. The evolution of the domain structure was observed using a Kerr-effect apparatus.

Lifetime estimation due to imprint failure in ferroelectric SrBi2Ta2O9 thin films

Two different failure modes for a ferroelectric memory cell caused by imprint, the read failure due to the loss of polarization, and the write failure due to the shift of the hysteresis loop are investigated. The quasistatic hysteresis loop allows us to distinguish which failure mode is dominating in a ferroelectric random access memory application and, hence, it can also be used as a powerful tool for lifetime estimation of ferroelectric thin films limited by imprint failure under operating conditions. The experimental results show that the write failure is only decisive for very low voltage operation (Vp<1.25 V), whereas for the Pt/SrBi2Ta2O9/Pt under investigation the read failure is the dominant failure mode for operating voltages exceeding 1.25 V.