Dependence Of Coercive Field Research Articles

Simulation of Thickness Dependent Coercive Field in PZT Thin Films

The thickness-dependent coercive field in perovskite-type ferroelectric thin films has been simulated using the four-state Potts model. In this model, four mutually perpendicular dipole orientations result in four different kinds of domains in a film of tetragonal phase. In order to simulate the switching due to the domain wall movement, only those dipoles in the domain boundaries are allowed to rotate. Each rotation is restricted only to 90°, either clockwise or counter-clockwise. The influence of space charge is also included. For large thickness, the simulation result agrees with the trend observed in most experiments that the coercive field decreases with thickness. On the other hand, when the thickness is smaller than a critical value, the coercive field increases with thickness. The intrinsic influence governed by the domain-wall movement and the extrinsic effect induced by oxygen vacancies are discussed. This result not only reflects the two different regimes for polarization switching but also imposed design implication on the thin film ferroelectric memory in small thickness.

Temperature dependence of hysteresis loops and AC-susceptibility of as-cast and annealed Nd 60Fe 30Al 10 hard magnetic alloy

Magnetic properties of melt-spun Nd 60Fe 30Al 10 alloy were studied between 2 and 350 K. The behaviour of the temperature dependence of coercive field is explained by the presence of magnetic multi-phases, which were determined by means of transmission electron microscopy and X-ray diffraction. Low coercivity at room temperature is attributed to the low Curie temperature of the amorphous phase.

On Magnetization Reversal in Hard Magnetic Sm-Fe-N Permanent Magnets

The investigations were carried out on Sm-Fe-N permanent magnets produced by reactive diffusion method. The magnets consist of hard magnetic phases: Sm 2 Fe 1 7 N 0 . 8 6 and SmFe 5 and soft magnetic α-Fe phase. From the comparison of experimentally determined angular dependence of coercive field with appropriate theoretical predictions and from the dependence of coercive field on the external magnetic field determined from the minor hysteresis loops, it was stated that magnetization reversal process in Sm-Fe-N magnets is controlled by the nucleation of reversed domains process.

Role of particle size distribution on the temperature dependence of coercive field in sputtered Co/Cu granular films

The temperature dependence of coercive field and of the ratio between the remanent and saturation magnetization of granular Co/Cu films grown by sequential sputtering has been studied with magneto-optic Kerr effect measurements in the temperature range 8.5–60 K. The observed temperature dependence of coercive field does not fit any of the plausible Tx laws commonly used to describe systems of single-domain ferromagnetic particles. We worked out a generalized model, which also includes temperature effects related to nonuniform single-domain particle size distribution. The model predictions account well for the observed temperature behavior of both coercive field and ratio between the remanent and saturation magnetization.

Domain reversal and nonstoichiometry in lithium tantalate

Recent studies have shown that lithium nonstoichiometry has a tremendous influence on domain reversal characteristics in ferroelectric lithium tantalate. This work presents a systematic study of the domain reversal characteristics such as threshold coercive fields for domain reversal, domain stabilization times, “backswitching” phenomena, domain switching and wall pinning times, and sideways wall mobility in near-stoichiometric LiTaO3 with Li/(Li+Ta)∼0.498. These properties are contrasted with those of congruent LiTaO3 [Li/(Li+Ta)∼0.485]. A qualitative model is proposed based on nonstoichiometric dipolar defects to explain the dependence of threshold coercive field on defect density, and on repeated field cycling, the origin of domain backswitching, and domain stabilization times.

Oriented, single domain Fe nanoparticle layers in single crystal yttria-stabilized zirconia

To create an ensemble of oriented, single crystal particles of iron, Fe ions were implanted into the near-surface region of single crystal yttria-stabilized zirconia (YSZ). With thermal processing, the implanted species precipitated to form faceted, predominantly single domain ferromagnetic nanoparticles. The YSZ substrate isolates, protects, and crystallographically textures the nanoparticles, creating a magnetically anisotropic layer. Experimental studies of the magnetization M(H,T) at various orientations of the applied field H show a major distinction only between H /spl par/ n/spl circ/ (hard direction) and H /spl perp/ n/spl circ/ (easy direction) where n/spl circ/ is the surface normal; there is little angular variation within the plane. This feature and the behavior of the orientationally dependent coercive field H/sub c/ and magnetic remanence M/sub r/ are attributed to magnetostatic interactions between the particles. Non-interacting single-domain particles should show a Stoner-Wohlfarth-like behavior that is poorly approximated in this far-from-close-packed system.

Introduction of columnar defects in colossal magnetoresistive manganites by heavy ions irradiation

The effects of swift heavy Pb (6 GeV) ions irradiation have been studied in the colossal magnetoresistive (CMR) Pr 0.7Sr 0.3MnO 3 compound. High-resolution electron microscopy (HREM) has shown that the induced defects are nearly continuous amorphous columns. The resistivity is slightly increased by the irradiation, due to the presence of the defects. Furthermore, the coupling between the columnar defects and the strong Jahn–Teller distortion existing in the related compound Pr 0.7Sr 0.1Ca 0.2MnO 3 also decreases the Curie temperature. The magnetic hysteresis in the ferromagnetic state is strongly increased in the irradiated samples, reflecting the hindrance of the magnetic domain walls (DWs) by the induced defects. The strong pinning regime of the DW by the defects is shown to be relevant to our experiment. A model based on this strong pinning effect is used to qualitatively explain the coercive field dependence on the density of defects.

Co/Cu spin valves electrodeposited on Si

Co/Cu spin-valve structures have been electrodeposited from a single bath directly onto n-type (1 0 0) Si substrates. The structures were based on the fact that Co layers on Si show a dependence of coercive field on layer thickness. By sandwiching a stack of 3, 5 or 8 hard antiferromagnetic-coupled Co/Cu multilayers between two soft Co layers, it was possible to obtain low-field-sensitive magnetoresistive structures, showing MR ratios ranging from 5.9% to 8.6%, as well as field sensitivities in the range of 0.10% Oe at 15–40 Oe. Samples with 8.6% MR ratio were obtained by stacking up to 10 magnetic layers.

Preparation and Electrical Properties of Rhombohedral Pb(ZrxTi1-x)O3 Thin Films by RF Magnetron Sputtering Method

Rhombohedral lead zirconate titanate (PZT) thin films with a thickness of 0.3–1.2 µm were successfully grown on (111)Pb(La,Ti)O3/Pt/Ti/SiO2/Si and (111)Ir/SiO2/Si substrates by an RF magnetron sputtering method using a multi-target consisting of calcinated PbO and metal titanium pellets on a zirconium metal plate. The composition of the films could be controlled by adjusting the area ratio of PbO/Zr/Ti. The crystal structures of the films were sensitive to the area of PbO and the substrate temperature. The pyroelectric current, relative dielectric constant, Curie temperature, remanent polarization (Pr) and coercive field dependence on the Zr content of the films are described. The pyroelectric coefficient of the as-prepared PZT films showed a peak from 94°C [for Pb(Zr0.91Ti0.09)O3] to 36°C [for Pb(Zr0.97Ti0.03)O3], which corresponded to the phase transition from the low-temperature to high-temperature rhombohedral ferroelectric phase. The fatigue characteristic was also measured using a double bipolar pulse. The PZT films preserved an initial switching charge value over a switching cycle of 1012.

Temperature dependence of magnetic properties and phase transitions in a soft magnetic Co-based nanostructured alloy

We report the temperature dependence of magnetic properties and phase transitions in a soft-magnetic Co-based nanostructured alloy obtained from amorphous Vitrovac 6025. Homogeneous nanostructured samples are achieved by suitable thermal treatments below the conventional crystallization temperature and have ultrafine grain structures with average grain size of about 10 nm and volume fraction 0.6. The grains are embedded in a residual ferromagnetic amorphous matrix and this leads to superior soft-magnetic properties which, in our view, can be justified on the basis of the random anisotropy model. The temperature dependence of initial reversible permeability and coercive field emphasizes a different behaviour in the magnetic properties of nanostructured and amorphous samples. The sharpness in the Hopkinson peaks of the initial permeability is the best tool to show the first magnetic transition in both specimens: the results are compared with those detected by thermogravimetric curves and by magnetization intensity measurements. Differential thermal analysis is used to evaluate the transformation enthalpy of the amorphous-to-microcrystalline and the nano-to-microcrystalline processes, which both occur at about 820 K where another ferromagnetic phase appears.

Temperature dependence of low field switching and coercive field in ferroelectric TGS

The temperature dependence of the maximum switching current density j m=j(tm) has been investigated for ferroelectric triglycine sulfate (TGS) at low fields (1kV/cm<E m<2kV/cm) under almost linearly rising pulses, being E m = E(tm) the field value at which the maximum switching current occurs. Increasing temperature above room temperature shows an intermediate zone between the two different switching behaviors already reported, which is intimately connected with the presence of a small bias in the sample. We discuss the meaning of coercive field in a ferroelectric in connection with the observed switching behavior.

Magnetic propertiesof nanostructured CuFe2O4

The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mössbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.

Low-Voltage Pb(Zr,Ti)O3 Film Capacitors: Control of Charge Relaxation at the Interfaces

AbstractIt is suggested that the processes of charge injection and entrapment at the interfacial layer of Pb(Zr,Ti)O3 (PZT) film capacitors are responsible for both polarization fatigue and size effects on ferroelectric switching. The study of the charge injection by analyzing the dependence of coercive field on maximum polarization shows that there is a direct relationship between the charge injection properties and fatigue performance. Based on our results, we conclude that enhancement of charge relaxation at the interfaces of PZT capacitors results in two positive effects: improvement of polarization fatigue performance and suppression of size effects on ferroelectric switching, which are detrimental for low-voltage PZT film capacitors.This idea has been implemented experimentally by introducing a thin RuO2 layer into the top-electrode interface of the Pt/PZT/Pt capacitor. PZT film capacitors of 100–170 nm thickness prepared in this way exhibited substantially improved fatigue in combination with weak size effects, which allows reduction of the operation voltage down to 0.8 V without degradation of the hysteresis properties. Our results show that the control of charge relaxation at the interface is a key issue for development of the low-voltage ferroelectric capacitors.

Compositional dependence of electrical characteristics of SrBi2(Ta1−xNbx)2O9 thin-film capacitors

Ferroelectric capacitors using SrBi2(Ta1−xNbx)2O9 (SBTN) were compositionally altered varying Nb concentration from 0 to 1, the corresponding I–V and P–E electrical characteristics evaluated from room temperature to 145 °C. These temperature evaluations reveal that the leakage current will increase with larger Nb concentration and the dominant conduction mechanism changes from Schottky to Frenkel–Poole emission. The ferroelectric hysteresis curve shifts in the direction of negative polarization as the temperature or the concentration of Nb increases. Concentration increases in Nb reduces the temperature dependence of remnant polarization and coercive field. Film resistance to imprint and degradation from elevated temperature improves. Substituting b-site Nb for Ta allows imprinted capacitors to recover by application of either bipolar fatigue pulses at room temperature (RT) or, cycling P–E measurement pulses at elevated temperature. Due to this asymmetrical tradeoff of film ferroelectric properties, there is an optimum identifiable range of Nb concentrations (0.25–0.5) capable of achieving memory performance. Optimized SBTN ferroelectric films will suitably perform in integrated circuit memory function applications provided that the leakage current incurred at higher Nb concentrations can be reduced.

Dielectric anomaly in strontium bismuth tantalate thin films

The authors have succeeded in observing the dielectric anomaly in ferroelectric strontium bismuth tantalate (SBT) thin films, and studied how it varies with film composition. For a stoichiometric SBT film, the dielectric anomaly was diffused, and the Curie temperature (Tc) was approximately 260 °C, which is lower than that for the bulk ceramic. A SBT film with excess Bi composition showed Tc of approximately 300 °C, and Tc for Sr deficient SBT films was higher than 340 °C. The composition dependence of remanent polarization and coercive field near room temperature for the SBT films reflects the composition dependence of the Tc values.

Characterization of microstructural defects from temperature dependence of domain wall coercive field

The steep temperature dependence of the domain wall coercive field, H CW (T), of highly anisotropic rare earth magnetic garnet films, recently described as a piece-wise exponential curve, was analysed. The quantitative analysis revealed the H CW (T) shape to be caused by the exponential temperature dependence of the anisotropy field. modified by functions describing the efficiency of the wall-defects interaction, at least for two different sets of defects. Characteristic periods of the two sets of defects were found and the defects were identified in one case with point defects caused by local fluctuation of the anisotropy constant at sites of individual rare earth ions in the crystal lattice and in the other case with fluctuation of local stresses due to local fluctuation of the material composition.

Temperature changes of coercivity in garnets

The temperature dependence of domain wall coercive field, H CW , of an epitaxial magnetic rare earth garnet film was modified by covering the garnet layer with sputtered SiO 2 . The result supports a recent model of domain wall coercivity. According to this model the typical shape of the H CW (T) curves originate in the temperature dependence of the anisotropy field, modified by several efficiency functions of the wall-defect interaction. The efficiency functions reflect predominance of different sets of sample defects within different temperature regions. Introduction of some extra stress, due to the sputtered SiO 2 , changed the wall-defect interaction in a certain range of temperatures and altered there the shape of the H CW (T) curve. Characteristics of the original system of defects and estimates of those for the modified sample are presented.

Investigation of the limits of statistical power loss model for 3.2% SiFe electrical steels. II. Role of crystallographic texture

In many experimental works it has been proved that the results obtained by the statistical power loss model of Bertotti (SPLM) [1] on different qualities of SiFe electrical steels are markedly influenced by the nonmagnetic properties of the material. This work intends to give a systematic contribution to the problem of texture effects on the polarisation dependence of internal coercive field V o and hysteresis loss P hys , in the case of 0.35mm thick 3.2wt% SiFe non-oriented electrical steel. Six materials have been produced from the same composition to assure a wide variety of the average grain size, without significant changes in the crystallographic texture. Single sheet samples have been cut at different angles with respect to the rolling direction (RD), from 0° to 90°, with a step of 15°. Power loss has been measured between 10Hz and 400Hz, at densely distributed polarisation levels. Then, the SPLM was applied to separate the different loss contributions and to determine the V o and P hys values as a function of polarisation. Based on the previously revealed polarisation dependence of these parameters [3,4], two single coefficients of power function representation, the P o (hys) coefficient and n(1) exponent have been found to represent the hysteresis loss, whereas the V o field data have been synthesised by a simple average value V (avg) o , over the 0.3T-1.2T polarisation range. The grain size effect has been successfully separated from the effect of texture in both P o (hys) hysteresis and V (avg) o field. Similar dependence on texture has been found for both quasi-static and dynamic responses in function of the magnetisation direction. These extracted textural effects have been compared with the power loss description proposed by Bunge [5,6] and with a texture-based minimum force model developed in this work. Both theoretical approaches, based on measured ODF of materials, resulted not fully satisfactory, probably due to their simple basic assumptions with respect to the real magnetisation mechanisms.

A domain wall model for relaxor ferroelectrics

A domain wall model is proposed for relaxor ferroelectrics typified by Pb(Mg1/3Nb2/3. Under a large applied field or a large internal field, domain walls move over a long distance by overcoming localized barriers. This movement is responsible for the hysteresis loop and depolarization at temperatures below T max. The activation volumes obtained by analyzing the coercive field dependence on temperature and frequency suggest that the barriers encountered by the domain walls are of the size of 1000 unit cells, and they increase as the relaxor becomes more like a normal ferroelectric. Since polarization is usually incomplete in relaxors even under a large polarizing field, depolarization readily occurs without the need of nucleating new domains. Domain walls are also responsible for permittivity peaks at high temperature under a weak field. However, without a strong field, domain walls are partially pinned by barriers and can only oscillate within the unpolarized regions between barriers. The mobility of domain walls can be modified by alloying to vary the spacing between ferroelectrically active ions and the internal field driving the walls. Furthermore, below a certain temperature, polarization domains become increasingly sharp and domain wall oscillation becomes increasingly difficult. This leads to the exhaustion of oscillation at an apparent freezing temperature below T max. Overall, the domain wall model provides a simple framework that can be used to quantitatively analyze relaxor behavior over a broad range of compositions. Its theoretical basis is further justified by analogy to dislocation theory and interface mechanics in phase transformations.

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.