Domain Wall Area Research Articles

Failure analysis on Nd-Fe-B under the multi-physical field coupling of permanent-magnet synchronous machines

In this paper, the Tesla meter and magnetic force microscope were used to study the open-circuit remanence and magnetic domain structure of Nd-Fe-B permanent magnets (PMs) in high-power density permanent magnet synchronous motors (PMSMs). An experimental platform was built to simulate the internal demagnetization environment of PMSM. Nd-Fe-B PMs were subjected to high temperature, DC demagnetizing field and alternating demagnetizing field. The results show that even when the working point of Nd-Fe-B PM is higher than the knee point, demagnetization still occurs in the simulated motor environment. The relationship between macroscopic magnetic properties and magnetic domain structure is investigated. With the decrease of open-circuit remanence, the average width and the magnetization of the magnetic domain in the magnetization direction decrease. Due to the width of the magnetic domain decreasing, the number of magnetic domains increases, and the area of the domain wall increases. As a transition region with weak magnetism, the increase of the domain wall area will lead to a decrease in magnetic properties. This work is helpful to understand the demagnetization mechanism of Nd-Fe-B magnets in the PMSMs.

Dynamics of Single Domain Wall Propagating in Bistable Microwire in Rapidly Changing Magnetic Field

The presented experimental method enables the study of the dynamics of a domain wall (DW) moving in bistable microwire in a region of rapidly changing magnetic field. DW velocity versus DW position dependence is obtained in this experiment. Measurements were carried out on Fe77.5Si15B7.5 microwire with strong unidirectional effect in DW propagation. The propagating DW is forced to change its parameters in a very short time, which results in the velocity increasing above its equilibrium value after the DW passes the region of rapid increase in magnetic field. In order to interpret the observed experimental behavior, a simple theoretical model was proposed. Comparison of experimental and model dependences produced a high value of DW inertial mass, indicating that the DW area was much larger than the cross-sectional area of the microwire. Another interesting conclusion was that a DW with stronger damping (shorter wall) does not have a lower value of inertial mass.

Flexo-phototronic effect in centro-symmetric BiVO4 epitaxial films

With exciting functionality, topological defects in ferroic system have attracted significant attention. Under proper design, the emergence of polar domain walls in non-polar ferroelastics enables a flexo-phototronic effect. In this study, we revealed ferroelastic twin texture with localized flexoelectric effect in centrosymmetric epitaxial BiVO4 film. Supported by photodeposition and localized photocurrent analysis, we found the flexoelectric effect confined at domain wall area facilitates the photocarrier transport and the flexo-phototronic mechanism was further supported by dye-degradation and generation of reactive radicals. This work not only provides new insights into the introduction of flexo-phototronic effects in non-polar materials, but also sheds light on the use of material inhomogeneity for acquiring multifunctionality.

Quantitative analysis of factors increasing coercive field of iron powder cores – Influence of inclusions

The quantitative contribution of inclusions to coercive field in iron powder cores with different amounts of Si oxide inclusions was separated from that of other microstructural factors and analyzed. Coercive field decreased by 2.8 A m−1 with a 0.01 mass% decrease in the amount of Si in oxide. Si oxide inclusions also affected the recovery and recrystallization of the α-Fe phase, and the contribution of crystal grain boundaries and the contribution of dislocations decreased with a decrease in the amount of Si in oxide. The contribution of inclusions was compared with Kersten's calculation model based on changes of the domain wall area and Neel's calculation model based on changes of the number of magnetic poles, and the observed value was closer to the results of Kersten's calculation model. This suggests that the contribution of magnetic poles generated around an inclusion is small and the change of the domain wall area around the inclusion makes a larger contribution to coercive field.

Ferromagnetic resonance force microscopy of individual domain wall

We report on ferromagnetic resonance force microscopy (FMRFM) based investigations of the ferromagnetic resonance of a single domain wall (DW) in a V-shaped planar permalloy nanowire (NW) which is bent by 60°. A pronounced resonance associated with the DW is observed at 1.6 GHz. FMRFM imaging at the resonance frequency confirms the localization of the resonant mode in the DW area. The measured spectra and spatial distribution of the resonant signal are in good agreement with the results of micromagnetic modeling.

Structural and magnetic properties of a core-shell type L1 FePt/Fe exchange coupled nanocomposite with tilted easy axis

Structural and magnetic properties of core-shell type L10 FePt/Fe exchange coupled nanocomposites are studied systematically. Core-shell nanocomposites with FePt core and Fe shell are obtained by depositing Fe cap layers on granular L10 FePt films. Epitaxial growth is disclosed by x-ray diffraction. Coercivity decreases drastically for FePt/Fe with the thickness increase of Fe cap layers. The coercivity reduction is due to the much increased domain wall area pinned and compressed at the soft-hard interface, and the tilted effective easy axis because of the presence of demagnetized energy. L10 FePt/Fe with a 3 nm Fe layer has high thermal stability and gain factor for media applications.

Effect of domain variations on damping capacity of Fe–16Cr–2.5Mo alloy solution annealed at 1373 K and 1473 K

The damping capacity of Fe–16Cr–2.5Mo alloy heat-treated at different temperatures was investigated. A water-based magnetic fluid was used to analyze domain morphologies. The experimental results show that there is a maximum value of damping capacity when the solution annealing temperature of the material is 1373K. When the annealing temperature is higher, the damping capacity of the alloy drops quickly. The change in damping capacity with the solution annealing temperature is believed to be due to different domain morphologies. The domains are larger and the domain-wall area is smaller in the alloy annealed at a higher temperature. The wedge-shaped domains acted as obstacles for pinning the domain-wall movement, even though movement of the 90° domains is easy. As a result, the damping capacity of the alloys drops when the annealing temperature is very high.

Vibration damping of high-chromium ferromagnetic steel and its dependence on magnetic domain structure

The aim of the present work is to study the vibration damping capacity of annealed high-chromium (16%) ferromagnetic steel. The alloys were prepared from raw materials of 99.9% purity melted in a high frequency induction furnace under high vacuum. The samples were heat-treated in vacuum at various temperatures (800–1200 °C) for 1 h followed by slow cooling (120 °C/h). The duration of 1 h considered was the same for all the specimens and no other duration was considered for evaluation. The inverted torsional pendulum method was used to evaluate the vibration damping capacity. The results indicated that the vibration damping capacity of the alloys is influenced by annealing and there exists a critical annealing temperature after 1000 °C. When the alloy is annealed over the critical temperature, the size of the magnetic domains becomes larger so that the domain wall area in metal decreases, making the damping capacity decrease as well.

Effect of annealing treatment on damping capacity of Fe–7Al–0.5Ti alloy

The damping capacity of the Fe–7Al–0.5Ti alloy in different annealing conditions was measured using reversal torsion technique. A water-based magneto-fluid was used to analyze magnetic domain morphology of the alloy using optical microscopy. The experimental results show that there is a damping peak after annealed at 1000 °C × 1 h. The damping variation of this alloy depends on the domain wall mobility and its size that cause a critical temperature. Below the critical annealing temperature, the decrease of local internal stress and structure defects causes magnetic domains more easily to move. As a result, the damping capacity increases quickly. However, when this alloy is annealed over the critical temperature, the size of magnetic domains becomes so large that the domain wall area in metal decreases, making the damping capacity decreases as well.

Aging and domain growth inK1−xLixTaO3(x<~0.05)

We present experimental results on the dielectric constant in orientational glasses ${\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{TaO}}_{3}$ (KLT) with $x<~0.05,$ together with the detailed (analytic and numerical) study of a model which attributes the observed aging to the motion of the walls of polarization domains. We show that the dielectric constant after a positive temperature jump goes through a maximum as a function of the subsequent time. This observation and those previously reported (aging, cooling rate dependence, etc.) are compared with the predictions of the model, in which the variations of the dielectric constant are attributed to the change of polarization domain wall area. The total area decreases by domain growth and increases by nucleation of new small domains inside the larger ones. These two opposite variations are both hindered by static random fields (equivalent to energy barriers) due to the frozen dipoles borne by the off-center ${\mathrm{Li}}^{+}$ ions. Many results are well explained by the model with a single energy barrier. However, some effects can only be understood if a broad distribution of energy barriers is assumed. We use the experimental data to determine this distribution and find it to be unimodal with a width comparable to its most probable value.

Investigation of the magnetic field and stress dependence of 180° domain wall motion in pipeline steel using magnetic Barkhausen noise

Magnetic Barkhausen noise (MBN) measurements were performed on 2% Mn pipeline steel samples as a function of magnetic field ( H) and as a function of orientation of an applied sweep field for various tensile and compressive stress values. For the magnetic field dependent measurements, the H field and stress were applied along the same axis, which was aligned within 10° of the sample's magnetic easy axis. Tensile stresses between 210 MPa and 0 and compressive stresses between 0 and -200 MPa were applied. The MBN signal was observed to initially increase with H followed by a decrease and eventual suppression of the MBN signal at higher H values. The application of tensile stress increased the total MBN signal and increased the field at which suppression occurred, while the application of compressive stress decreased the total signal with a corresponding decrease in the suppression field value. The stress and low-field behaviour of the MBN signal was correlated with the irreversible differential permeability ( μ IDP ) measured in a similar 2% Mn pipeline steel sample. The behaviour of the MBN signal as a function of H and stress was attributed to the effect of these parameters on the relative population and/or size of 180° domain walls, that is, the total 180° domain wall area, within the sample.

Application of electron holography to ferroelectric study

With the development of FE TEM, electron holography becomes a reality to materials scientists, which opens a new window for materials study. Weak phase objects, such as a thin transparent specimen or an electric or a magnetic field, which have little or no effect on the intensity of the transmitted wave, can readily be observed via holography because of the phase shift that they produce. Application of the electron holographic method has been extended to the study of ferroelectric domain wall structures. This work presents the most recent results in this area.Polarization gradients within domain walls are extremely important for the understanding of the extrinsic elastio-dielectric properties of ferroelectrics. Electron holographic studies of the local domain wall profiles provide essential input parameters for phenomenological theories of domain structure and of the macroscopic properties derived from the theories. Figure 1(a) is an electron hologram of the ferroelectric (BaTiO3) 90° domain wall area.

Domain wall pinning transition in an iron-rich FeCu alloy

Specimens of α-phase solid solution of iron-rich FeCu(1.07 at. % Cu) were thermally aged at 700 °C for different time durations. Precipitation and growth of εCu took place during the heat treatments. The specimens were polished and were examined with a transmission electron microscope. The size, density, and shape of the precipitates were determined for different aging conditions. These parameters were fit into the strong and weak domain wall pinning models, allowing values of the coercive field Hc to be predicted. Comparison between the predicted and measured coercive fields shows that weak pinning theory agrees with the observed Hc in low age specimens. For specimens aged over 1700 min, strong pinning theory can provide acceptable calculated Hc values. The pinning index β′ related to the ratio of the domain wall area per pin in the static and dynamic states is also calculated for each specimens. β′ values in low age specimens are found to be <1, which indicates weak pinning. For prolonged age specimens, β′ values are close to or larger than 1, and strong pinning is indicated.

Initial dielectric properties and domain walls area in KDP crystals at low temperatures

Measurements of ∊'c and ∊''c are performed in KDP crystals without d.c. applied electric field at low temperatures, and with a low measuring field amplitude. Employing thermal and electrical cycles, different domain textures are obtained for a given temperature: the ∊'c and ∊''c values display a linear variation with the total domain wall area S. It has been possible to calculate the lateral wall displacement. Such types of measurements are promising to a better understanding of the microscopic processes at the domain wall level.

The aging behaviour of the complex material parameters ε, d and s in ferroelectric PZT ceramics

The complex dielectric constant e as well as the complex piezoelectric constant d and the complex elastic constant s of iron doped PZT ceramics were measured after poling as a function of time. It was found that the aging behaviour of the material parameters is similar. The time dependence of the losses shows that aging effects due to 180°-domain wall processes are, if present, small. Aging is caused by 90°-domain walls, either by a reduction of the domain wall area or by a reduction of their mobility. A procedure for the separation of the volume and domain wall contribution to the real parts is presented.

Aging of fe-doped pzt ceramics and the domain wall contribution to the dielectric constant

Abstract The time dependence of the complex dielectric constant e of ferroelectric ceramics is well known as aging which is ascribed by most authors to a modification of the 90°-domain walls. Measurement of aging of Fe-doped PZT ceramic (rhombohedral) and Cr-doped BT ceramic (tetragonal), show that aging in these ceramics can be understood as a reduction of vibrating 90°-domain wall areas. This reduction of the domain wall contribution to the dielectric constant can be due either by pinning of parts of 90°-domain walls having high activation energies and/or by an increase in domain sue in time which reduces the total domain wall area. The increase of the domain sue during aging is ascribed to a thermally activated relief of internal stresses just by rearrangement of the domain configuration. The stress relaxes in a logarithmic time law which results from uniformly distributed activation energies between an upper and a lower limit. The measurements allow to distinguish clearly the bulk contribution and the...

Magnetic properties of Co-Cr/mumetal double layer films

A dc magnetron sputtered multilayer (Co-Cr mumetal) film is examined for magnetic coupling of the sublayers. No evidence for physical coupling is observed. The multilayer coercivity near the perpendicular direction has a strong dependence on angle due to the planar mumetal component. The coercivity of the mumetal exhibits Kondorsky-type angular behavior. Coercivities of the Co-Cr film measured parallel and perpendicular to the field direction both decrease linearly from 100 K to 340 K, with a room temperature coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\beta = (1/H_{c})(\DeltaH_{c}/\DeltaT) = -0.2</tex> percent/K. The temperature dependence of coercivity can be modeled on a domain wall pinning theory, based on exchange fluctuations over a small domain wall area. It is shown that high Curie temperatures are needed to minimize thermal variation in recording output signal. X-ray transmission attenuation is shown to be a useful tool for determining the Co-Cr thickness in a multilayer film.

Investigation of dielectric after-effects in poly- and single crystalline BaTiO3

Dielectric relaxation occurring in single- and polycrystalline BaTiO3, following different polarization treatments of the specimens has been investigated in the temperature range 10 K<T<450 K. The observed relaxation phenomena are discussed in terms of polarization induced growth of extra-domains which, via corresponding enhancement of the total domain wall area, may account for the time dependence of the dielectric constant. On the basis of this relaxation model the activation parameters of the different processes are determined and discussed in terms of variations of the wall mobility within the different crystallographic phases occurring in BaTiO3.

Specific bloch wall area of tape wound cores of amorphous and crystalline material

At low magnetizing frequencies not the total available domain wall area is instantaneously in motion. A method is presented to separate the simultaneously irreversibly moved domain wall area (specific area S irr) from that wall area which is elastically attached at pinning centres. For tape wound cores of iron silicon and of metallic glass S irr was determined as function of the magnetizing frequency by measuring the complex core impedance and applying a simple domain model. At very low magnetizing frequencies f m<10 -2s -1 the quantity S irr is found to be approximately proportional to f 0.8 m.

Magnetization reversal and domain boundary configurations in amorphous alloys

The flux reversal at pulsed field was investigated for Fe—Co- and Fe—Ni-based amorphous alloys. For Fe—Ni-based alloys the domain wall area n is essentially constant at low fields, while in all other cases variations of n appear during the reversal.