Type Of Domain Structure Research Articles

Effect of polarization on tunnelling electroresistance in ferroelectric tunnel junctions

High-quality epitaxial BiFeO3 (BFO) films were grown on (001)-, (110)- and (111)-oriented Nb:SrTiO3 (NSTO) substrates by pulsed laser deposition. The type of domain structure can be modulated using BFO ferroelectric films with different crystalline orientations. The ON/OFF ratios obtained in (001)-, (110)- and (111)-oriented Au/BFO/NSTO ferroelectric tunnel junctions (FTJs) are 6 × 103, 3 × 104 and 2 × 105, respectively. Analysis of the I–V curves of tunnelling current and average BFO ferroelectric barrier height proves that the polarization intensity of the BFO films modulates both the ferroelectric barrier and the Schottky barrier profile, which further influences the electronic tunnelling probability in BFO FTJs. This work will be useful for further study on achieving a giant ON/OFF ratio and developing insights into the barrier profile and transport mechanism of metal/ferroelectric/semiconductor-type FTJs.

Barkhausen noise in disordered striplike ferromagnets: Experiment versus simulations.

In this work, we present a systematic comparison of the results obtained from the low-frequency Barkhausen noise recordings in nanocrystalline samples with those from the numerical simulations of the random-field Ising model systems. We performed measurements at room temperature on a field-driven metallic glass stripe made of VITROPERM 800R, a nanocrystalline iron-based material with an excellent combination of soft and magnetic properties, making it a cutting-edge material for a wide range of applications. Given that the Barkhausen noise emissions emerging along a hysteresis curve are stochastic and depend in general on a variety of factors (such as distribution of disorder due to impurities or defects, varied size of crystal grains, type of domain structure, driving rate of the external magnetic field, sample shape and temperature, etc.), adequate theoretical modeling is essential for their interpretation and prediction. Here the Random field Ising model, specifically its athermal nonequilibrium version with the finite driving rate, stands out as an appropriate choice due to the material's nanocrystalline structure and high Curie temperature. We performed a systematic analysis of the signal properties and magnetization avalanches comparing the outcomes of the numerical model and experiments carried out in a two-decade-wide range of the external magnetic field driving rates. Our results reveal that with a suitable choice of parameters, a considerable match with the experimental results is achieved, indicating that this model can accurately describe the Barkhausen noise features in nanocrystalline samples.

A device for the recording of the dynamic domain structure of ferromagnets in rotating magnetic fields

The principle of operation of a device for the observation and recording of the type of domain structure (DS) of ferromagnets that are remagnetized in rotating magnetic fields is described. This makes it possible to reliably fix the type of the domain structure of a ferromagnet under single cycles of magnetization reversal in the frequency range of 20–400 Hz and in the induction interval of 0.25–2.0 T. The observed features of the rearrangement of the DS in the samples of anisotropic electrotechnical steel allowed us to find a method for reducing the level of its magnetic losses in rotating magnetic fields and thus to improve its performance characteristics significantly.

Symmetry versus Commensurability: Epitaxial Growth of Hexagonal Boron Nitride on Pt(111) From B-Trichloroborazine (ClBNH)3

The growth of hexagonal boron nitride (h-BN) on a Pt(111) surface by decomposition of B-trichloroborazine (ClBNH)3 leads to two different sets of domain structures as indicated by low-energy electron diffraction (LEED). In addition to a domain structure, for which the top, hcp, and fcc sites of the substrate lattice are occupied by the BN lattice, there is a new type of domain structure with B and N alternatingly occupying the top and bridge sites of the Pt(111) substrate, a structural motif which has not been observed so far. The new domain structures are a result of the precursor chemistry (B-trichloroborazine vs borazine) as well as the lattice mismatch of the substrate [Pt(111) vs h-BN; mismatch ∼11%].

Numerical simulations of magnetization reversal processes in an ultrathin cobalt film

Numerical simulations are developed to describe consistently the magnetization reversal processes and magnetic domain structures in ferromagnetic ultrathin Au/Co(0.8 nm)/Au sandwiches deposited on glass substrate. The ultrathin cobalt film shows: (i) an easy magnetization axis perpendicular to film plane; (ii) magnetic after-effect and irreversibility of magnetization processes; (iii) a lacunar Swiss cheese type of domain structure; (iv) different initial magnetization curves depending on the demagnetizing procedure for in-plane or out-of-plane magnetic field. Experimental results are well described by numerical simulations considering the detailed sample morphology. The magnitude of the magnetic coupling between cobalt crystallites located at the gold crystallites boundaries is found far weaker than for other cobalt crystallites.

Capabilities of a muon method for studying the diamagnetic domains accompanying the de Haas-van Alphen effect

This paper analyzes the capabilities of a muon (μSR) method for studying the de Haas-van Alphen effect and the diamagnetic domain structure accompanying it. It is shown that, unlike the NMR method, the μSR method makes it possible to observe the formation of a diamagnetic domain structure in all metals. It is not currently known what type of domain structure accompanies the de Haas-van Alphen effect: one-dimensional (laminar) or two-dimensional. It is shown that the line shape of the Fourier spectrum of the signal makes it possible to determine both the character of the domain structure (two-dimensional or laminar) and the magnetic field distribution in the domains.

On the effect of low-temperature demagnetization on TRMs and pTRMs

SUMMARY Low-temperature demagnetization (LTD) experiments with TRMs and pTRMs using liquid nitrogen were carried out on samples with different (MD and PSD) domain structures to infer and to compare the type of domain structure of samples before and after LTD judged by the thermomagnetic criterion. This is based on testing materials against Thellier’s law of independence of pTRMs. The results demonstrate good agreement between the commonly used criteria for differentiating domain structure, such as Qrt, Hc,/Hc and ZrS/Zs factors, and more sophisticated ones, such as the thermomagnetic criterion and LTD treatment. It is noteworthy that there is no observable tendency of MD or PSD samples to display SD-type pTRM =f(7‘) curves after LTD treatment. Hence our experiments failed to detect a quasi-SD component of remanence acquiring TRM by thermofluctuations in MD or PSD grains in the manner of Verhoogen (1959) and Stacey & Banerjee (1974) hypotheses

Toward a better understanding of the Lowrie‐Fuller test

We develop a theory of acquisition and alternating field (AF) demagnetization of anhysteretic remanence (ARM) and saturation isothermal remanence (SIRM) in multidomain (MD) grains in order to better understand the Lowrie‐Fuller test. Our theory shows that the relative stabilities of low‐field ARM and high‐field SIRM against AF demagnetization are determined by the distribution ƒ(hc) of microcoercivity hc in a sample, as found earlier by Bailey and Dunlop. When f(hc) is nearly constant, weak‐field ARM is more resistant to AF demagnetization than SIRM. In contrast, when ƒ(hc) varies exponentially or is a Gaussian distribution, SIRM is more AF resistant than ARM. These contrasting stability trends are conventionally called single‐domain (SD)‐type and MD‐type Lowrie‐Fuller results, respectively, but in reality, both types occur in the MD size range. We propose instead the descriptive terms L‐type result (low‐field remanence, i.e., ARM, more stable) and H‐type result (high‐field remanence, i.e., SIRM, more stable). The Lowrie‐Fuller test does not distinguish one type of domain structure from another, but it does depend indirectly on grain size. We show that the distribution ƒ(hc) in a given sample is determined primarily by the grain size d and the dislocation density ρ. A nearly constant ƒ(hc) occurs in grains with small d and/or ρ, but a Gaussian f(hc) is approached with increasing d and/or ρ. The transition from L‐type to H‐type behavior in the Lowrie‐Fuller test occurs at a critical grain size dt ≈ 2/(ρw), where w is the domain‐wall width. The lower the dislocation density, the larger the transition size in the Lowrie‐Fuller test. This simple relationship explains the increase in the transition size from about 5–10 μm observed for crushed magnetite grains to ≈ 100 μm for hydrothermally grown magnetites, which have lower dislocation densities than crushed grains.

Phase transition and domain structure of heteroepitaxial PbTiO3/(001) MgO films

Abstract Heteroepitaxial PbTiO3 films were studied by X-ray diffraction and electron microscopy methods. A model allowing to predict the type of domain structure that forms as a result of the film-substrate mechanical interaction and to determine the c − and a-domain concentrations is suggested.

Spinach glycolate oxidase and yeast flavocytochrome b2 are structurally homologous and evolutionarily related enzymes with distinctly different function and flavin mononucleotide binding.

A comparison of the three-dimensional structures of the flavin mononucleotide (FMN)-dependent enzymes glycolate oxidase, flavocytochrome b2, and trimethylamine dehydrogenase is presented. Their flavin-binding domains all have the same structural motif, the 8-fold beta/alpha-barrel domain, which is also present in a large number of other enzymes. FMN is bound in a similar fashion in all three enzymes. The binding site is at the carboxyl-terminal end of the eight beta-strands of the barrel where the active site is invariably found in this type of domain structure. The similarity of the structures of glycolate oxidase and flavocytochrome b2 extends to the loop regions and even outside the beta/alpha-barrels with a root mean square deviation of 0.93 A for 311 superimposed C alpha-atoms and with a sequence identity of 37%. A detailed analysis of their active sites shows, however, that the orientation of FMN is significantly different in the two structures due to different conformations of residues in the end of strand one. Thus, in flavocytochrome b2 a hydrogen bond is formed between the FMN N-5 position and the main chain amide of Ala-198, while in glycolate oxidase, the ring system is tilted away from the strand, creating a pocket on the re-side of the FMN ring where a water molecule is bound. Model building shows that this site could accommodate the hydroperoxide moiety of a FMN-4a-hydroperoxide intermediate. Thus, in the course of evolution, a few mutations in, and close to, the active sites have fine tuned these enzymes to exert their specific functions as an oxidase or transferase, respectively.

Effects of magnetic history on the domain structure of small NiFe shapes

Buckled domain structures, usually with charge on both surfaces and domain walls, are created after applying a large field (historic field Ĥh) to rectangles and other odd-shaped elements and returning to the remnant state. The type of domain structure can be predicted by imposing the boundary condition that Ĥh⋅ Medge ≥0, where Medge is the magnetization at the edge of the shape. The origin of the boundary condition comes from the high demagnetizing energy necessary to reverse the magnetization at the edges of a thin film. Details of the metastable state are determined by wall and anisotropy energies as well as the demagnetizing energy from surface and domain wall charging.

Sample size effect in coercivity measurement of epitaxial magnetic garnet films

The full set of characteristic parameters of epitaxial garnet films was measured as a function of the sample size from about 2000 to 5 mm2. The type of domain structure, width of the stripe domain period, characteristic material length, domain wall energy, and anisotropy did not change within the error of measurement on reducing the sample size. The bubble domain collapse field decreased by about 2.5%, and the domain wall coercivity was measured and found to have decreased—almost by an order of magnitude. The experiments showed the lack of dependence of coercivity on the domain wall length. The coercivity and collapse field are suggested as being related to the sample dimensions via the sample-size-dependent derivatives of the total free energy with respect to the domain wall positions. The assumption of the direct effect of the free energy derivatives on the coercivity is supported by a model experiment.

Magnetic domains in construction steel

The magnetic domain structure in constructional steel has been studied using a ferrofluid technique. We observed domains of four types which have been described and classified. A correlation is observed between the type of domain structure and grain size. For a simple antiparallel domain structure, a relationship exists between the grain size and domain wall spacing. The coefficient characterizing the magnetic interaction between grains is suggested, and a model is proposed which gives a linear relation between sqrt (domain length) and domain wall spacing. This model gives us a reasonable agreement with experiment and is compared with the results obtained using different models. The size of a single-domain grain was also estimated using the suggested model.

A new type of domain structure in nematic liquid crystals

A new type of domains, with static aspect, consisting of white and grey bands, or of fine lines, is presented. This structure appears in the dielectric region (till to 8 kHz) by raising the frequency at constant voltage up to the disappearance of the chevron structure.

A new type of domain structure in the superconducting intermediate state

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Observations and Properties of a New Domain: Hollow Bubble

A new type of domain structure, a hollow bubble or ring domain, has been studied. Calculations with zero Bloch-wall pinning force show that these ring domains can exist in platelets of uniaxial materials within a narrow region of the magnetic field normal to the platelet. Within this region the dimensions of the ring change enormously; they can be changed by a factor 2 by a field variation, which is about 100 times less than the variation needed to change the bubble radius by the same amount. We observed that the presence of a pinning force strongly influences the field region for stable ring domains and their behavior under a varying magnetic field. For a high-coercivity sample an experiment is reported, in which each cycle H1-H2-H1 of a varying magnetic field normal to the platelet diminishes the size of a ring. This experiment is in some sense the microscopic analogy to the ``bascule'' and ``reptation'' described by L. Néel.

Stripe Domains in Thin Co–Al Films

Stripe domains were observed in sputtered Co–Al films 350–700-Å thick. This type of domain structure has been reported previously only in much thicker ferromagnetic films. The structure of the films determined by electron diffraction showed the stripe domains were associated with a bcc structure similar to iron. Saturation magnetization was measured with a vibrating sample magnetometer and was found to vary linearly with film composition, but had no relationship to the formation of stripes. The domains were observed by Lorentz microscopy and could be removed by a saturating field, but reappeared, contrary to the case reported on Ni–Fe films when an ac field was applied.

New Type of Domain Structure of Permalloy “Stripe-Films”

The domain structure of Permalloy “stripe-films” of thickness 2.5–10 μ was investigated by the magneto-optic method. Three types of measurements were used: (1) measurements of static hysteresis loops from the surface layer of the films, (2) measurements of the ratio IR/IS for the surface layer of the films by means of the dynamic method, and (3) measurement of the magnetization vector orientation in a single surface domain by a device with micron resolution. The comparison of these results with the static hysteresis loops of the whole film justifies the assumption of a new type of domain structure of the “stripe-films” with closed magnetic flux, the residual magnetization of which is determined essentially by the surface domains. The orientation of the magnetization vector has been determined in the surface and internal domains of the film. The formation of this domain structure is evidently due to the energy dependence of the domain boundaries.

A New Type of Magnetic Domain Structure in Negative Magnetostriction Ni-Fe Films

Ni-rich Ni-Fe and Ni evaporated polycrystalline films, the thickness of which is larger than a critical value, were found to be composed of stripe domains running parallel to the field previously applied to the films. B-H loops and magnetoresistance effect revealed that the magnetization in each domain points in one sense and deflects from the plane of the film upwards and downwards alternately. This type of domain structure is to be caused by the anisotropy whose easy axis is normal to the plane of the film, the origin of which is attributed to the magnetostrictive effect due to strains or defects in the film. The width of the domain increases with the thickness of the film, but does not depend sensitively on substrate temperatures and compositions. The critical thickness increases with an elevation of substrate temperatures and with decreasing Ni concentration. The stripe domain is no longer observable for the film containing Ni less than 80% where the magnetostriction constant is positive. The relation...

Domain structure of cobalt whiskers

An experimental investigation is made into the domain structure of cobalt whiskers by the colloid technique. A new type of domain structure was found; the width of the domains is studied as a function of the thickness of the whisker. This dependence follows a two-thirds power law and does not agree with existing theories which predict a half power law.