Mössbauer-effect study of Mg-Al ferrites having a rectangular hysteresis loop

A Mössbauer-effect study was made of Mg-Al ferrites having a rectangular hysteresis loop. The replacement of Fe³⁺ ions by A1³⁺ ions produces an additional component in the Mössbauer spectra, due to second-phase inclusions. The phase composition of the Mg-Al ferrite has a marked effect on the shape of the hysteresis loop. As the second-phase inclusions increase in relative importance, rectangularity coefficient D decreases. Local lattice distortion has a lesser effect on D. At high Al₂O₃ concentrations (above 15 mole%) in the Mg ferrite there is a sharp increase in the coercive force with increasing Al₂O₃ content. This increase is also due to second-phase inclusions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 2, pp. 105–109, February, 1971.In conclusion the authors thank I. I. Sil'vestrovich and B. A. Pavel'ev for furnishing the samples.     

An investigation of electrodeposited cylindrical iron-nickel films of supercritical thickness IV. Magnetic properties of films

The saturation magnetization in cylindrical nickel and iron-nickel films with a high nickel content is significantly lower than in pyrometallurgical metal. This is due to the presence of a high proportion of amorphous nonferromagnetic phase in the film. The application of an axial magnetic film during the electrodeposition of the films increases the total number of Barkhausen discontinuities in axial reverse magnetization, reduces the coercive force, shifts the field at which discontinuities appear into the region of smaller re verse-magnetization fields, and increases the permeability and the rectangularity of the hysteresis loops. The variation of the magnetic characteristics with the alloy composition is correlated with the spontaneous internal stresses in the films.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 18–22, March, 1972.The authors thank V.M. Rudyak for discussion of the results, and B. S. Zolotkovskii and B. L. Nechaevfor carrying out the test experiments with flat films and for participation in discussion.     

MAGNETIC VISCOSITY AND DEMAGNETIZATION BEHAVIOUR IN ISOTROPIC NANOCRYSTALLINE Pr12Fe82B6 RIBBONS

We present the experimental results of the magnetic viscosity, demagnetization curve and recoil loop for isotropic nanocrystalline Pr₁₂Fe₈₂B₆ ribbons prepared by melt-spinning. The thermal fluctuation field, activation volume and irreversible demagnetization are discussed. The coercivity mechanism is mainly determined by the inhomogeneous nucleation rather than a simple nucleation of reverse domain.     

Modeling exchange bias microscopically

Exchange bias is a horizontal shift of the hysteresis loop observed for a ferromagnetic layer in contact with an antiferromagnetic layer. Since exchange bias is related to the spin structure of the antiferromagnet, for its fundamental understanding a detailed knowledge of the physics of the antiferromagnetic layer is inevitable. A model is investigated where domains are formed in the volume of the AFM stabilized by dilution. These domains become frozen during the initial cooling procedure carrying a remanent net magnetization which causes and controls exchange bias. Varying the anisotropy of the antiferromagnet, we find a non-trivial dependence of the exchange bias on the anisotropy of the antiferromagnet.     

Source of dislocations in the field of discrete stoppers

The formation of a dislocation loop by the Frank-Read mechanism in an obstacle field is simulated on a computer in the approximation of constant linear tension. The dependence of the loop formation time on the voltage, the source length, and the temperature are studied. It is shown that the bending time of a dislocation segment to the critical configuration makes the main contribution to the loop formation time. The effective activation energy and the effective activation volume are calculated from the results of the simulation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 20–24, December, 1990.     

The influence of niobium on the magnetic properties of the columnar alnico 5 permanent magnets

An investigation of the influence of Nb addition (0–1.24%) on Alnico 5 columnar alloy's magnetic properties showed that Nb content up to 0.5% slightly decrease remanent induction while at the same time increasing coercivity and improving the rectangularity of the demagnetization curve. The addition of niobium above 0.5% causes a further increase in coercivity simultaneously decreasing magnetic induction in every point of the demagnetization curve.     

Piezoelectric voltage of barium titanate with stabilized domains

In BaTiO₃ with unidirectional stabilized domains mechanical excitation causes a piezoelectric stress that is strongly asymmetrical with respect to the orientation of the electric field applied to the sample. The magnitude of the piezoelectric stress measured in the direction of polarization of the stabilized domains is usually greater than in the opposite direction. At a cyclic change in the weak electric field the dependence of the piezoelectric stress on this field is reflected in the shape of the asymmetric hysteresis loop.     

Anisotropic interaction between self-interstitial atoms and 1/2<111> dislocation loops in tungsten

We investigate the interaction between 111 self-interstitial atoms(SIAs) and 1/2111 self-interstitial dislocation loops in tungsten(W) via atomistic simulations. We explore the variation of the anisotropic distribution of binding energies with the shapes and sizes of the 1/2[111] loop and the nonequivalent configurations of 111 SIAs. For an arbitrarily shaped loop, SIA can be more easily trapped in the concave region of the loop than the convex region, which forms a loop whose curvature is closer to that of a circular loop. The direction of SIAs can largely affect the interaction behaviors with the loop. The capture distance of an SIA by the edge of a circular-shaped 1/2[111] loop is clearly elongated along the direction of the SIA; however, it weakly depends on the size of the loop. Then, we analyze the slanted ring-like capture volume of 111 SIAs formed by the circular loop based on their generated anisotropic stress fields. Furthermore, the binding energies obtained from the elastic theory and atomistic simulations are compared. The results provide a reasonable interpretation of the growth mechanism of the loop and the anisotropic interaction that induces irregular-shaped capture volume, affording an insight into the numerical and Object Kinetic Monte Carlo simulations to evaluate the long-term and large-scale microstructural evolution and mechanical properties of W.     

基于一维耦合腔光子晶体的声光可调谐平顶滤波器的研究

本文利用一维耦合腔光子晶体,提出了一种声光可调谐平顶滤波器.该滤波器利用声光效应,通过改变超声波频率使一维耦合腔光子晶体透射谱的平顶滤波器的中心波长产生漂移,从而实现可调谐的滤波功能.基于传输矩阵法和声光效应理论,建立了这种平顶滤波器的理论模型;利用COMSOL软件,对平顶滤波器的矩形度、通带带宽、插入损耗、可调谐特性、加工精度进行仿真研究.研究结果表明,通过施加频率为6-11 MHz的超声波,可实现通带带宽为5-6 nm及中心波长在1514-1562 nm范围内可调谐的平顶滤波器;在可调谐范围内通带带宽内插入损耗不超过2.23 dB,最低仅为0.78 dB,矩形度最低可达1.4;加工误差在±10 nm内平顶滤波器的中心波长、矩形度、插入损耗、通带带宽出现的偏差很小.该平顶滤波器具有易于设计和集成、通带平坦、可调谐范围宽、通带带宽稳定、插入损耗低、品质因素高的特点,在光开关、可调谐光纤激光器、光纤传感等光通信领域有重要应用.     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

Formation and evolution of giant dynamic domains in an ac magnetic field

Magnetization reversal in ferrite-garnet films placed in an ac magnetic field, bringing about the formation of metastable dynamic domains with sizes exceeding those of quasi-static domains by an order of magnitude or greater, was studied using a stroboscopic method. The formation of giant dynamic domains (GDDs) is due to the finite domain wall velocity and depends on the density of domain nucleation centers. It was shown that the GDD comblike boundary forms during the part of a field period near the moment of field polarity change. GDDs arise when the dynamic hysteresis loop shape changes from a triangle to an ellipse.     

区熔硅单晶中一种位错源

用化学侵蚀法研究了区熔法生长的硅单晶体中的杂质条,以及由此杂质条的体印压产生的位错环列。实验结果表明,杂质条处在硅中{111}面的〈110〉方向上,杂质条的长度约为5—230μm;其横向尺寸约为2—3μm。我们研究了杂质条体印压产生的位错环列的几何结构。杂质条的尺寸和形状决定了位错环的尺寸和形状。还观察和分析了位错环列交叠产生的位错网络。 关键词：     

Anisotropic Brownian motion in ordered phases of DNA fragments

Using Fluorescence Recovery After Photobleaching, we investigate the Brownian motion of DNA rod-like fragments in two distinct anisotropic phases with a local nematic symmetry. The height of the measurement volume ensures the averaging of the anisotropy of the in-plane diffusive motion parallel or perpendicular to the local nematic director in aligned domains. Still, as shown in using a model specifically designed to handle such a situation and predicting a non-Gaussian shape for the bleached spot as fluorescence recovery proceeds, the two distinct diffusion coefficients of the DNA particles can be retrieved from data analysis. In the first system investigated (a ternary DNA-lipid lamellar complex), the magnitude and anisotropy of the diffusion coefficient of the DNA fragments confined by the lipid bilayers are obtained for the first time. In the second, binary DNA-solvent system, the magnitude of the diffusion coefficient is found to decrease markedly as DNA concentration is increased from isotropic to cholesteric phase. In addition, the diffusion coefficient anisotropy measured within cholesteric domains in the phase coexistence region increases with concentration, and eventually reaches a high value in the cholesteric phase.     

Domain configuration and dielectric properties of Ba0.6Sr0.4TiO3 thin films

The microstructure and electrical properties of Ba_0.6Sr_0.4TiO₃ thin films have been investigated. Nanometer-sized domains, ranging from 8 to about 30 nm, were observed by piezoresponse force microscopy (PFM). The critical size, below which only single domains exist, is found to be about 31 nm. The film exhibits ferroelectric behavior characterized by polarization hysteresis loop and capacitance-voltage curve.     

高压烧结法合成致密纳米BaTiO3陶瓷结构和铁电性能研究

 在6 GPa压力、1 000 ℃温度条件下制备了致密的纳米BaTiO₃陶瓷，合成样品的平均晶粒尺寸为50 nm，理论密度在97%以上。通过介电测量，观察到了样品宽化的相变峰，它与粗晶陶瓷的相变峰大不相同。由于90°电畴的减少和退极化场的存在，观察到了细长的电滞回线，它是样品铁电性存在的有力证据，表明钛酸钡陶瓷的临界尺寸在50 nm以下。     

Phase behavior and structure of an ABC triblock copolymer dissolved in selective solvent

A mean-field lattice theory is applied to predict the self-assembly into ordered structures of an ABC triblock copolymer in selective solvent. More specifically, the composition-temperature phase diagram has been constructed for the system (C)₁₄(PO)₁₂(EO)₁₇/water, where C stands for methylene, PO for propylene oxide and EO for ethylene oxide. The model predicts thermotropic phase transitions between the ordered hexagonal, lamellar, reverse hexagonal, and reverse cubic phases, as well as the disordered phase. The thermotropic behavior is a result of the temperature dependence of water interaction with EO- and PO-segments. The lyotropic effect (caused by changing the solvent concentration) on the formation of different structures has been found weak. The structure in the ordered phases is described by analyzing the species volume fraction profiles and the end segment and junction distributions. A “triple-layer” structure has been found for each of the ordered phases, with each layer rich in C-, PO-, and EO-segments, respectively. The blocks forming the layers are not stretched. The dependence of the domain spacing on polymer volume fraction and temperature is also considered. Received 17 April 2002 Published online: 21 January 2003     

Stray field interaction of stacked amorphous tapes

In this study, magnetic cores made of amorphous rectangular tape layers are investigated. The quality factor Q of the tape material decreases rapidly, however, when stacking at least two tape layers. The hysteresis loop becomes non-linear, and the coercivity increases. These effects are principally independent of the frequency and occur whether tape layers are insulated or not. The Kerr-microscopy was used to monitor local hysteresis loops by varying the distance of two tape layers. The magnetization direction of each magnetic domain is influenced by the anisotropy axis, the external magnetic field and the stray field of magnetic domains of the neighboring tape layers. We found that crossed easy axes (as the extreme case for inclined axes) of congruent domains retain the remagnetization and induce a plateau of the local loop. Summarizing local loops leads to the observed increase of coercivity and non-linearity of the inductively measured loop. A high Q-factor can be preserved if the easy axes of stacked tape layers are identical within the interaction range in the order of mm.     

Irregular frequency chirp in radiation of an injection laser in high-velocity code modulation

The effect of the shape of the excitation pulses on the magnitude of the frequency chirp in an injection laser is investigated. It is established that as the degree of rectangularity of the pulses becomes higher, the frequency chirp increases. Simultaneous consideration of the effect of residual charge in the active region of the laser, the delay of stimulated radiation, and the frequency chirp indicates that in the case of high-velocity code modulation of an injection laser, in the latter an irregular frequency chirp appears that can exert a noticeable influence on the distortion of code trains of pulses having a super-Gaussian shape, when they propagate in media with a dispersion of 100 p.s./nm/km and above. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 45–49, January–February, 1997.