Electric and magnetic properties of PMMA/manganite composites

We present the synthesis and characterization of the La_2/3Sr_1/3MnO₃ manganite in the form of tapes using polymethyl methacrylate (PMMA) as binder. We have studied their electric and magnetic properties as a function of temperature and magnetic field. The magnetization results have been shown to be dominated by the intrinsic magnetic properties of the manganite. Resistivity measurements showed an insulating behavior in the whole range of temperatures measured, indicating that the percolation threshold of manganite grains has not been reached even for the sample with 35% of PMMA relative content. The obtained magnetoresistance is largest in the sample with 35% of PMMA relative content.     

Effect of a change in the optical density of a magnetic emulsion in electric and magnetic fields

The change in the optical density of an emulsion of magnetic fluid drops suspended in a mineral oil under the action of electric and magnetic fields has been investigated. It is found that the sign of the change in transparency in an ac electric field depends on the field frequency. It is shown that, joint action of codirectional low-frequency electric and dc magnetic fields can compensate for the change in the optical density. Calculation of the change in the emulsion optical density within the anomalous diffraction approximation showed that this effect can be explained by small field-induced deformation of microdrops.     

Polymer creep in a static magnetic field

By noting changes in the color of samples in a polarization microscope with varying degree of birefringence, the effect of a weak static magnetic fields (2 and 4 kOe) on molecules of glassy polymers has been observed. Variations in the nonmonotonicity of the rate of discontinuous creep at +18° were studied interferometrically. It is shown that the abruptness of the deformation jumps varies in a magnetic field, where this abruptness is assumed to be due to the existence of strong physical junctions between the kinetic units of deformation. The reactivity of the polymers to the magnetic field stands in satisfactory correspondence with their magnetic susceptibilities. Results show that such nonmagnetic materials as glassy polymers can noticeably alter their deformational properties in response to the action of magnetic fields. Fiz. Tverd. Tela (St. Petersburg) 39, 1690–1692 (September 1997)     

强磁与应力场耦合作用下AZ31镁合金塑性变形行为

研究强磁场对AZ31镁合金塑变能力和微观组织的作用,在3 T脉冲强磁场条件下对合金进行磁场耦合应力时的拉伸实验.采用电子背散射衍射、Ⅹ射线衍射和透射电镜分析等方法研究材料的微观组织.结果表明:与0 T拉伸试样相比,3 T拉伸试样抗拉强度和延伸率分别提高了2.2%和28.7%,说明将强磁场耦合作用于材料塑性变形过程时,能在不降低材料强度的同时提高镁合金的塑性变形能力,有助于同步改善材料强韧性.磁场作用机理主要表现为磁致塑性效应,计算表明主要合金相β(Mg_(17)Al_(12))为顺磁性,有助于发挥磁场作用效果.磁场提高了位错运动灵活性并促使位错增殖,晶界处位错堆积和应力集中促进了再结晶形成,晶粒发生细化,发挥细晶强韧化效果;同时磁场诱发塑性变形时的晶粒转动,新生成非基面取向的晶粒弱化了镁合金(0001)基面织构,该组织特征有助于提高材料的塑变能力.     

Transient magnetoviscosity of dilute ferrofluids

The magnetic field induced change in the viscosity of a ferrofluid, commonly known as the magnetoviscous effect and parameterized through the magnetoviscosity, is one of the most interesting and practically relevant aspects of ferrofluid phenomena. Although the steady state behavior of ferrofluids under conditions of applied constant magnetic fields has received considerable attention, comparatively little attention has been given to the transient response of the magnetoviscosity to changes in the applied magnetic field or rate of shear deformation. Such transient response can provide further insight into the dynamics of ferrofluids and find practical application in the design of devices that take advantage of the magnetoviscous effect and inevitably must deal with changes in the applied magnetic field and deformation. In this contribution Brownian dynamics simulations and a simple model based on the ferrohydrodynamics equations are applied to explore the dependence of the transient magnetoviscosity for two cases: (I) a ferrofluid in a constant shear flow wherein the magnetic field is suddenly turned on, and (II) a ferrofluid in a constant magnetic field wherein the shear flow is suddenly started. Both simulations and analysis show that the transient approach to a steady state magnetoviscosity can be either monotonic or oscillatory depending on the relative magnitudes of the applied magnetic field and shear rate.     

Enhancement of hydrogen gas permeability in electrically aligned MWCNT-PMMA composite membranes

The multi-walled carbon nanotube (MWCNT) dispersed polymethylmethacrylate (PMMA) composite membranes have been prepared for hydrogen gas permeation application. Composite membranes are characterized by Raman spectroscopy, optical microscopy, X-ray diffraction, electrical measurements and gas permeability measurements. The effect of electric field alignment of MWCNT in PMMA matrix on gas permeation has been studied for hydrogen gas. The permeability measurements indicated that the electrically aligned MWCNT in PMMA has shown almost 2 times higher permeability for hydrogen gas as compare to randomly dispersed MWCNT in PMMA. The enhancement in permeability is explained on the basis of well aligned easy channel provided by MWCNT in electrically aligned sample. The effect of thickness of membrane on the gas permeability also studied and thickness of about 30 μm found to be optimum thickness for fast hydrogen gas permeates.     

Deformation of NaCl crystals under combined action of magnetic and electric fields

The paper reports observation of a strong macroplastic effect of weak magnetic and electric fields (B≤0.4 T, E～1 kV/m) when applied simultaneously to NaCl samples in the course of their active deformation at constant rate ?=const. In the absence of magnetic field, electric effects in the macroplasticity of the same crystals become manifest at fields E≥10³ kV/m. Quantitative dependences of the macroplasticity on magnetic and electric fields and on the strain rate have been measured. A physical interpretation of the observed phenomena is proposed.     

Multilevel character of deformation of polymers

The inhomogeneity in the creep rate of polymers on different scales of deformation has been studied by laser interferometry. The main results have been obtained for the amorphous-crystalline polymer polytetrafluoroethylene. The deformation characteristics are the oscillation periods of the rate (jumps of deformation), oscillation amplitudes of the rate, and the scatter of these quantities. Application of computer methods for processing of the results has made it possible to determine the difference and similarity between jumpwise deformations on different structural levels, including the nanolevel. For a more distinct separation of deformation levels, the measurements have been made in a magnetic field and outside the magnetic field. Deformation jumps have been found on five levels: from 4 nm to more than 10 μm. Introduction of a sample into a magnetic field changes the characteristics of jumps; in this case, the scatter in the values of jumps always increases, whereas their average value varies differently on different scale levels. The measurement of the parameters of deformation jumps on different scales allows one to study the laws of the development of the deformation process and the evolution of structural inhomogeneities.     

Nanoscale deformation irregularities of γ-irradiated poly(methyl methacrylate)

Development of deformation jumps in the creep of poly(methyl methacrylate) (PMMA) has been studied. The structural levels of deformation have been determined from the creep rate oscillation periods (deformation jumps) measured by the interferometric method. Special attention is given to a new method of data processing, which enables one to reveal previously undetectable nanoscale deformation jumps. By the example of PMMA specimens preliminarily exposed to γ radiation with doses D=55–330 kGy and unexposed specimens, the presence of nanoscale deformation jumps with the values dependent on the dose D and time of creep has been shown. The obtained results confirm the existence of 10–20-nm domains in amorphous polymers and make it possible to study the multilevel organization of the deformation process, starting from the nanoscale.     

The influence of transverse magnetic field on Gunn effect threshold

The effect of magnetic field has been studied on the Gunn effect threshold electric field for short planar Gunn devices. For low magnetic fields, experimental results are in good agreement with the theoretical results but a large increase in threshold electric field has been observed for higher magnetic fields.     

Biological effects of magnetic fields: chronic exposure of the nematode Panagrellus redivivus

The Panagrellus redivivus bioassay, an established monitor of adverse toxic effects of different environments, has been used to study the biological effects of exposure to static and time-varying uniform and gradient magnetic fields, and to time-varying magnetic field gradients superimposed on a static uniform magnetic field of 2.35 Tesla. Temporally stationary magnetic fields have no effect on the fitness of the test animals. Time-varying magnetic fields cause some inhibition of growth and maturation in the test populations. The combination of pulsed magnetic field gradients in a static uniform magnetic field also has a small detrimental effect on the fitness of the test animals.     

Comparative study of the structural and magnetic properties of bulk and nanostructured Fe2CrAl

Nanostructured Fe₂CrAl has been prepared by mechanical alloying using a high energy ball mill. The structural and magnetic properties of the as-milled and mechanically alloyed sample annealed in the presence of a magnetic field have been studied by means of X-ray diffractometry, ⁵⁷Fe Mössbauer spectroscopy and DC magnetization. The observed properties are compared to that of the bulk sample. The magnetic hyperfine fields are related to structural disorder. There is a very good enhancement of bulk magnetic properties in the nano-sized samples, the most notable one being a dramatic increase in the Curie temperature of these samples and are explained in terms of structural disordering and size effect.     

Magnetocaloric effect in the intermetallic compound DyNi

Magnetic and heat capacity measurements have been carried out on the polycrystalline sample of DyNi, which crystallizes in the orthorhombic FeB structure (space group Pnma). This compound is ferromagnetic with a Curie temperature of 59 K. Magnetization-field isotherms at low temperatures show a multi-step behavior characteristic of metamagnetic transitions. The magnetocaloric effect has been measured both in terms of isothermal magnetic entropy change and adiabatic temperature change for various applied magnetic fields. The maximum values of the entropy change and the temperature change are found to be 19 J kg⁻¹ K⁻¹ and 4.5 K, respectively, for a field of 60 kOe. The large magnetocaloric effect is attributed to the field-induced spin-flop metamagnetism occurring in this compound, which has a noncollinear magnetic structure at low fields.     

Effect of magnetic field on the microplastic strain rate for C60 single crystals

Microplastic deformation in a magnetic field and in a zero field, as well as after preliminary action of a magnetic field on C₆₀ crystals, is studied with the help of a laser interferometer, which makes it possible to measure the strain rate on the basis of linear displacements of 0.15 µm. It is shown that the introduction of a sample into the field and its removal from a field of 0.2 T directly during sample deformation lead to a change in the strain rate, the decrease in the rate being accompanied by a brief interruption of deformation. The sign of the effect depends on temperature: the magnetic field accelerates deformation at room temperature and slows it down at 100 K. Preliminary holding of a sample in a field of 0.2 or 2 T produces a similar effect on the strain rate. Possible reasons for the observed manifestations of the magnetoplastic effect in C₆₀ and the relation between the sign of the effect and the phase transition at 260 K are considered.     

Fe–Ag granular multilayers and heterostructures studied in applied magnetic field

An (0.2 nm ⁵⁷Fe / 2.6 nm Ag)₇₅ granular multilayer sample and heterostructures with additional continuous Fe layers in different sequences were studied in magnetic field applied at different temperatures. The broadening of the superparamagnetic lines was found to be very similar for the three samples in applied fields both parallel and perpendicular to the sample plane. While the layer sequence has no significant effect on the superparamagnetic properties, the continuous magnetic layers follow a different approach to saturation in perpendicular magnetic fields.     

Visualization of the domain structure of ferromagnetic films using the magnetochemical effect

Iron film surfaces have been studied after their chemical treatment in a magnetic field. The chemical treatment of a metal leads to the appearance of a relief image that is interpreted as a “photograph” of magnetic scattering fields. The morphology of the relief image formed is shown to correlate with the domain structure of a magnetic film. The magnetochemical effect revealed allows the visualization of the dependence of the scattering field distribution at the sample surface on external magnetic field.     

Verification by finite element modeling for the origin of the apparent image effect in closed-circuit magnetic measurements

Closed-circuit magnetic measurements using a hysteresisgraph have generally been considered free from errors associated with the demagnetizing factor, as well as from the image effect, both of which can occur in open-circuit measurements. However, measurements on magnetic samples clamped between the pole pieces of an electromagnet may show an apparent drop in magnetization with increasing applied field, similar to the image effect found in open-circuit measurements. We have shown that as the saturation magnetization of the sample increases, the drop in apparent magnetization becomes greater and appears at lower fields; the effect also increases as the length-to-diameter ratio L/D of the sample decreases. This behavior has been attributed to distortion of the magnetic field distribution around the sample resulting from localized saturation of the electromagnet pole pieces. This paper presents the results of computer modeling using finite element method, FEM, which confirm this explanation and show that the field acting on the sample is highly non-uniform when the sample L/D is small. The modeling results are in good agreement with experimental data, and show that the apparent drop in magnetization is caused by measurement of the applied field that does not accurately reflect the field acting on the sample.     

Serrated creep of zinc single crystals under compression in a magnetic field

The compressive creep rate of zinc single crystals was measured for sample deformation increments of 150 nm, which permits the measurement of deformation jumps larger than 300 nm. A weak magnetic field B = 0.2 T is shown to increase the average creep rate and decrease the height and sharpness of submicron-sized deformation jumps. Preliminary holding of a sample in a magnetic field also influences the creep rate and the characteristics of deformation jumps. The data are explained in terms of a model relating the effect of a magnetic field to the destruction of barriers to dislocation motion.     

Effect of magnetic fields on out-of-plane orientations of nematic liquid crystalline polymers under simple shear flow

The effect of magnetic fields on out-of-plane orientations of liquid crystalline polymers (LCPs) under simple shear flows is numerically analyzed using the Doi–Hess equation. The evolution equation for the probability distribution function of the LCP molecules is directly solved without any approximation closure. The initial director is parallel to the vorticity direction. Two cases of the magnetic fields are considered (1) the magnetic field parallel to the flow direction, and (2) the magnetic field parallel to the velocity gradient direction. For both cases a log-rolling orientation state is detected at low shear rates. However, the director is quickly aligned along the direction of magnetic fields because of the deformation of molecules. The field affects on the scalar order parameter rather than the major orientation direction for the magnetic field parallel to the flow direction. On the other hand regarding the magnetic field along the vorticity gradient direction, the effect of the magnetic field is more remarkable on the major orientation in comparison with the effect on the scalar order parameter. Also it is be found that the order parameter is increased obviously with increasing the magnetic fields. It is an efficient way to improve the performance of LCP materials.     

Motion of electron-hole drops in low magnetic fields

The effect of magnetic fields on the motion of electron-hole drops in germanium is studied. A non-uniform strain is used to provide a known and controllable driving force for drop motion in the sample plane. Contrary to the results of earlier experiments in which drop motion was normal to the sample plane, the results are consistent with conventional models of drop-phenomenon interaction and weak magnetic fields have no observable effects on this motion.