Number Of Domain Walls Research Articles

Loss improvement in Finemet by surface modification

Finemet is a nanocrystalline material with excellent soft magnetic properties. Though the losses are very low compared to other crystalline metallic soft magnets the fraction of the frequency dependent dynamic losses in Finemet is high and therefore offers a large potential for further improvement. Different surface defects like scratches and micro hardness indentations were produced on the surface of amorphous Fe 73.5Cu 1Nb 3Si 13.5B 9 ribbons. Afterwards these and also untreated ribbons were nanocrystallised. It was found that the surface treatments can substantially reduce the power losses. This effect is already known for commercial grain oriented silicon steels, where the decrease of the power losses is attributed to domain refinement and the activation behaviour of 180 ° domain walls. The frequency dependence of the relative loss change indicates that domain refinement also occurs in surface treated Finemet. The treatment increases the power loss at very low frequencies. This can be attributed to the defects acting as obstacles for domain wall movement under nearly static conditions. For higher frequencies the dynamic loss becomes dominant and decreases with increasing number of movable domain walls.

DC-magnetoresistance in surface crystallized FeSiB amorphous wire

Measurements of DC-magnetoresistance (MR), axial and circular magnetization versus axial and circular field in surface crystallized Fe77.5Si7.5B15 amorphous wire are presented. Negative longitudinal MR immediately under saturatin has been observed. It has been found that a relatively strong transverse domain-wall contribution is present in these samples. On the basis of this fact, supposing some distribution of easy axes relative to the circular direction, an interpretation of the negative MR has been proposed. Changes in the number of transverse domain walls during magnetic reversal are supposed in the corresponding model.

Studies of ferroelastic domain structures in (NH 4) 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SeO 4) 2 crystals

The domain structures of (NH 4) 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SeO 4) 2 crystals grown from the aqueous solutions were studied by using an optical polarizing microscope. Three orientational domain states were observed in the sample crystals in their ferroelastic phase. The spontaneous strain tensors are used to describe the characteristic property of ferroelasticity. The observed number of domain states and domain walls can be well derived by the symmetry reduction, and by the compatibility condition of the spontaneous strain tensor on the boundary through the structural phase transition between the paraelastic and ferroelastic states. The twin operations found in these crystals are mirror plane, two-fold and three-fold rotations which are the missing symmetries during the structural phase transition.

Structural study by NMR in Co/Cu multilayers at second antiferromagnetic maximum

A 59Co NMR study has been carried out in a series of Co/Cu multilayers with t Cu = 20 Å and 2 Å < t Co < 30 Å. The Co layer is found to have a strained fcc structure with hcp stacking faults above 15 Å Co. The lateral size of columnar grains is estimated to be 45 Å. A transformation from continuous to granular Co layer is evidenced below 10 Å Co. Below 3 Å all deposited Co exists in form of a diffused CoCu alloy and Co clusters disappear. These structural changes, together with the decrease of a number of domain walls inferred from the rapid drop of NMR enhancement factor are to be related with the previously observed disappearance of magnetoresistive hysteresis.

POSSIBLE ELECTRONIC STRUCTURE OF DOMAIN WALLS IN MOTT INSULATORS

We discuss the quantum numbers of domain walls of minimal length induced by doping Mott insulators, carefully distinguishing between holon and hole walls. We define a minimal wall hypothesis that uniquely correlates the observed spatial structure with the doping level for the low-temperature commensurate insulating state of La 2−x Ba x CuO 4 and related materials at x = ⅛. We remark that interesting walls can be supported not only by conventional antiferromagnetic but also by orbital antiferromagnetic (staggered flux phase, d-density) bulk order. We speculate on the validity of the minimal wall hypothesis more generally, and argue that it plausibly explains several of the most striking anomalous features of the cuprate high-temperature superconductors.

Magnetic properties and dynamic domain behavior in grain-oriented 3% Si-Fe

The magnetic properties of soft magnetic materials are based on dynamic behavior of magnetic domain walls. Many domain structure studies have contributed to the progress of magnetic materials. In this paper, the present state of new development in advanced grain-oriented silicon steel is introduced through the aid of dynamic domain observation of real materials with forsterite film. The basic magnetic phenomena of material, such as the dynamic behavior of surface closure domains, domain wall pinning, and domain structure under rotating magnetization, are explained mainly through the observation of static and dynamic domain wall movements under a high-voltage scanning electron microscope. A few techniques for manufacturing the latest grain-oriented silicon steel, such as improvement in alignment with (110) [001] orientation, increase in the number of mobile domain walls, and relaxation of domain wall pinning, are also described through the observation of dynamic domain patterns. Examples of core losses are given for ideal grain-oriented silicon steel as industrial material, and future developments are predicted.

Composition dependence of the magnetomechanical effect and magnetostriction

Magnetomechanical and magnetostriction measurements were performed on a series of plain carbon steels, with carbon contents ranging from 0.1 to 0.9 wt.%. The microstructures in all specimens were similar, being a mixture of ferrite and pearlite, with the pearlite volume fraction increasing with increasing carbon. The magnetomechanical effect was found to vary with carbon content. Specifically, the change in magnetization with stress was proportional to the remanent magnetization for a given carbon level. Also, the sign of the magnetization change upon removal of compressive stress reversed at 0.6 wt.% carbon. The results were explained by the increased number of domain wall pinning sites produced by the increase in carbon content.

Large scale structure from biased non-equilibrium phase transitions - percolation theory picture

We give an analytical description of the spatial distribution of domain walls produced during a biased nonequilibrium phase transition in the vacuum state of a light scalar field. We discuss in detail the spectrum of the associated cosmological energy density perturbations. It is shown that the contribution coming from domain walls can enhance the standard cold dark matter spectrum in such a way as to account for the whole range of IRAS data and for the COBE measurement of the microwave background anisotropy. We also demonstrate that in case of a biased phase transition which allows a percolative description, the number of large size domain walls is strongly suppressed. This offers a way of avoiding excessive microwave background distortions due to the gravitational field of domain walls present after decoupling.

Domain Wall Motion and Eddy Current Losses in Very Thin 3% Si-Fe Core

The number of domain walls and the domain wall velocity of very thin (110) [001] 3% Si-Fe strip-wound cores were observed by SEM at excitation frequencies up to 5 kHz. Eddy current losses were calculated on the basis of the dynamic domain behavior using two models. a modified Pry and Bean model, and a model which considers the difference between each domain wall velocity. Both models assume sinusoidal domain wall motion. The measured eddy current losses were compared with calculated values. However, the measured losses disagreed with estimations in the frequency range from 50 to 500 Hz. The domain wall motion in each layer was estimated from the flux voltage waveform of a movable pickup coil inserted into a layer of a 3% Si-Fe strip. We found that the voltage waveform was not sinusoidal even if the secondary voltage waveform of the core was sinusoidal, and that the magnitude of the voltage in each layer was not uniform. The results mean that the domain wall motion is not uniform even in the 50 to 500 Hz range. The nonuniformity of the domain wall motion makes the eddy current losses larger than the results estimated on the basis of the two models.

Eddy current losses and number of domain walls with respect to total number of layers for very thin 3% Si-Fe strip-wound cores

The relation between the domain wall motion and the eddy current losses of very thin (110)[001] 3% Si-Fe strip-wound cores were investigated. The measured eddy current losses were larger than those estimated using a modified Pry and Bean model in the frequency region 50–500 Hz. It is known that the domain walls of thick (110)[001] 3% Si-Fe strips move nonuniformly in the low-frequency range (0.1–100 Hz), but this frequency range is lower than that usually used for very thin strips. By measuring the flux voltage in every layer, we found that the voltage waveforms were not sinusoidal, even if the secondary voltage waveform of the core was sinusoidal, and the magnitude of the voltage in each layer was not uniform. These results indicate that the domain wall motion of very thin strips is not uniform, even at 50–500 Hz. The nonuniformity of the domain wall motion makes the eddy current losses larger than those estimated by means of the modified Pry and Bean model.

Domain structures and magnetic properties of advanced grain-oriented silicon steel

The dynamic behavior of surface closure domains governing the soft magnetic properties of grain-oriented 3% silicon-iron and the basic magnetic phenomena, such as domain wall pinning, are explained mainly through the observation of static and dynamic domain wall movements under a high-voltage scanning electron microscope. Techniques for manufacturing the latest grain-oriented silicon-iron, such as improvements in the alignment of the (110)[001] orientation, increases in the number of mobile domain walls and relaxation of domain wall pinning, are also described through the observation of domain patterns.

Direct observation of ferroelastic phase transition in Er-doped lanthanum pentaphosphate with synchrotron topography

In situ observation of the ferroelastic phase transition in Er-doped lanthanum pentaphosphate has been carried out by real time white-beam synchrotron radiation x-ray topography. The ferroelastic domain walls are found to vanish and reappear with characteristics of a classical second-order transition on topographs on cycling the sample between room temperature and above the transition temperature. However, fluctuations of the number of domain walls with temperature approaching the Curie point are revealed for the first time and shown to be reversible. Discussions on these interesting phenomena are presented.

極薄珪素鋼板磁心における磁壁移動の均一性とうず電流損失

The number of domain walls and the domain wall velocity of very thin (110)[001] 3%Si-Fe strip-wound core were observed up to 5 kHz by using an SEM. On the basis of the dynamic domain behavior, the eddy current losses were calculated by means of two models: A modified Pry and Bean model, and a model that considers the difference between each domain wall velocity. Both models assume sinusoidal domain wall motion. The measured eddy current losses were compared with the calculated ones. However, the measured eddy current losses disagreed with the estimated ones in the frequency region of 50∼500 Hz. The domain wall motion of each layer was estimated from the flux voltage waveform of a movable pick-up coil inserted into a layer of a 3%Si-Fe strip. We found that the voltage waveform was not sinusoidal even if the secondary voltage waveform of the core was sinusoidal, and that the magnitude of the voltage in each layer was not uniform. The results mean that the domain wall motion is not uniform even in the 50∼500 Hz range. The nonuni-formity of the domain wall motion made the eddy current losses larger than those estimated by means of the two models.

Exact solution of a phase-separation model with conserved-order-parameter dynamics and arbitrary initial concentration.

We obtain an exact time-dependent solution of the zero-temperature Kawasaki-type dynamics of a phase-separation model of a binary mixture of A and B on a linear lattice, starting from a random arbitrary initial concentration p of one of the components. This generalizes a recent result by Privman [Phys. Rev. Lett. 69, 3686 (1992)] in which only the symmetrical case of equal initial concentrations of A and B was studied. In contrast to the case of two-species diffusion-limited annihilation, the symmetrical case can be obtained from the asymmetrical one by letting p tend to 1/2 continuously. At any concentration p, the number of domain walls between A and B decays exponentially to its p-dependent quenched steady value. A Monte Carlo simulation shows excellent agreement between the numerical and the analytical solutions.

Dynamic Properties of Very Thin 3% Si-Fe Cores in Relation to the Number of Layers

We previously reported that the estimated eddy current losses of very thin 3% Si-Fe strip-wound cores in the 500 Hz to 5 kHz frequency range agreed comparatively well with measured values. The eddy current losses were estimated by employing the number of domain walls in a modified Pry and Bean model. We observed the number of domain walis on outermost surfaces by SEM. In this study, in order to investigate the difference between the measured eddy current losses and the estimated losses below 500 Hz, we removed one by one the layers of a 3% Si-Fe strip-wound core, and examined the relation between the number of domain walls and the eddy current losses. In these experiments, the cross-sectional area of a strip-wound core had to be large enough to maintain a sinusoidal flux waveform during core loss measurements after several layers were removed. We also examined the use of continuous annealing to fabricate a long and very thin grain-oriented 3% Si-Fe strip.

Quantitative evaluation of extrinsic contribution to piezoelectric coefficient d33 in ferroelectric PZT ceramics

The contribution of 90° domain wall motion to the piezoelectric coefficient d 33 in ferroelectric PZT ceramics has been evaluated quantitatively by a simple model. The microstructures of PZT ceramics have also been considered through the evaluation process. In order to avoid some formidable and somewhat ambiguous calculations involved and also in order to offer a clear physical picture of the domain contribution to the piezoelectric coefficient d 33 in ferroelectric ceramics, we have introduced a new concept of the effective number of domain walls in the quantitative evaluation process which reflects the magnitude of domain wall density in ceramics. The obtained calculation result, Δd 33 = 219.4 × 10 −12 m/V, supports the measured data and also is inherently plausible in terms of the contribution of domain wall motion to the electromechanical properties of ferroelectric ceramics.

Metastable states of the ring domain structure

The structure of concentric ring domains in a cylindrical specimen and its change with magnetic field H are studied by computation, and the results are compared with those of analogous investigations of plane domain structures in rectangular and ellipsoidal specimens. Common to all three cases is the existence of metastable states caused by magnetostatic barriers between states with different numbers of domain walls N; the essential influence of these barriers on the character of the dependence of N or H; the possibility of asymmetry of the magnetization curve and of the existence of remanent magnetization in an equilibrium state. Common to the ring structure and the plane domain structure in a rectangular specimen is the existence of an intermediate phase transition corresponding to changing N in fields less than the saturation field. Common to the ring structure and the plane domain structure in an ellipsoidal specimen are jump-like phase transitions corresponding to the changing N in the magnetization process and the possibility of the existence of metastable states of a domain structure at H ≠ 0 for specimens where only a single-domain state is possible at H = 0.

Processes of magnetization of a ferromagnet with a finite number of domain walls

The process of magnetization of the system of a finite number N of plane domain walls in a right-angled specimen is investigated by computing. The topology of the system allows only three types of behaviour of the domain structure depending on the parity of N and on the direction of the magnetization in the domains located at the edges of a specimen. The first type is accompanied only by one structural phase transition N → 0 at the field of saturation H s, the other types have one more intermediate transition ( N → N - 2 or N → N - 1) at H < H s. The universal graph of the succession of the phase transitions is drawn which is independent of the material constants and of the ratio of the specimen size. Energy barriers between metastable states, in which the system is in the intervals between the phase transition, are calculated.

Magnetic structures with a finite number of domain walls

Abstract The equilibrium configurations and the process of magnetization of structures with a finite number N of plane domain walls are considered. In comparison with models of a periodic domain structure new effects are found: a difference from zero of the average magnetization at H = 0 and asymmetry of a hysteresis loop for even N , as well as the possibility of the existence of metastable states. The effect of changing the domain structure period in the process of magnetization known for the periodic models is manifested in the structures with a finite N as a sequence of 1st kind phase transitions corresponding to the change in the number of domain walls.

Thermal variations of domain wall thickness and number of domains in magnetic rectangular grains

Equilibrium domain wall thickness and number of domains in rectangular magnetic grains are determined by using a modified Amar model. It is shown that domain structure, particularly domain wall thickness, in a magnetized grain depends strongly on grain shape and orientation. These dependencies are attributed to the existence of two competing self‐magnetostatic interactions, one from the ends of the grain and the other from the sides. One of the consequences of this is that the thermal variation of domain wall thickness in an elongated grain is greater (smaller) than predicted by classical theory when the grain is magnetized along the shortest (longest) dimension. For magnetite, classical theory provides a good approximation in predicting both domain wall thickness and number of domains in equal‐dimensional grains larger than about 4 μm.