Optical anisotropy of polyisobutene

The distribution of chemical bonds in polyisobutene results in a theoretically zero optical and electrical anisotropy of the polymer according to bond polarizability additivity principles as conventionally applied, irrespective of polymer chain conformation. Non-zero values of the segmental anisotropy (～4.4 × 10^?40 C²m²/J) derived from strain-optical birefringence on the polymer in the rubbery state have been recently reported by Liberman et al. who suggest qualitatively that severe crowding of the chemical groups in the polymer affects the anisotropies of the group polarizabilities and gives the observed non-zero segmental anisotropy. We have measured optical, electrical and magnetic anisotropies for the pure liquid polymer using techniques of flow birefringence, depolarized light scattering, electro-optic birefringence and magneto-optic birefringence. Segmental optical anisotropies so derived agree with those measured by Liberman et al. and results of the above measurements taken as a whole lead to an origin for the observed anisotropy which requires locality dependent values of optical and electrical bond polarizability anisotropies to be used in the calculation of the polyisobutene polarizability anisotropy. Our explanation of the polarizability locality dependence differs from the steric overlap suggestion of Liberman et al. and depends upon the electric field interaction between dipoles induced in polarizable bonds.     

Preparation of multilayered Co/Pd nanostructure films by electroplating and their magnetic properties

Multilayered fcc Co/Pd nanostructure films, prepared by electrodeposition on an electropolished polycrystalline copper substrate using a dual cell method, exhibit a remanent perpendicular magnetic anisotropy which depends on the Co film thickness. By decreasing the Co layer thickness and setting the Pd monolayer thickness as 1 nm, the remanent magnetic ratio (the ratio of the remanent magnetization perpendicular to the film plane in relation to that found on the plane of the film) became large and exceeded 1 for a Co layer thickness of about 0.4 nm, which indicates that the easy axis of magnetization of the multilayered film changed from a direction parallel to the film plane to a direction perpendicular to it at this Co film thickness. The magnetic behavior of the heat treated Co/Pd multilayered film was measured and discussed.     

Origin of the uniaxial magnetic anisotropy in cobalt ferrite induced by spark plasma sintering

Cobalt ferrite (CFO) is a promising candidate for magnetostrictive applications like actuators or sensors. We have recently shown that uniaxial magnetic anisotropy can be induced in CFO by reactive sintering using spark plasma sintering (SPS), which leads to an improvement of its magnetostrictive properties. However, the origin of the anisotropy and the formation mechanism remain unexplained so far. In this study, different SPS processes have been conducted to determine which parameter is responsible for the induced uniaxial anisotropy. We demonstrate that the magnetic anisotropy arises during the cooling step when done under SPS’s uniaxial compression. In addition, we also investigate the fundamental origin of the magnetic anisotropy induced during the SPS process. We show that the polycrystalline anisotropic cobalt ferrite obtained after SPS exhibits no texture. However, the SPS samples turn isotropic after being annealed in air at 400 °C/2 h, as shown by magnetic and magnetostrictive measurements. A change in ionic distribution after the annealing is also observed by Mössbauer spectroscopy. Our findings suggest that the induced magnetic anisotropy results from the ionic distribution of the Co²⁺ in the CFO’s spinel lattice, a mechanism previously observed in magnetic annealed CFO. This study advances the in-depth understanding of the relationship between SPS processing and magnetic properties of cobalt ferrite.     

Electric field-controlled anisotropic magnetoresistance effect with four-fold-symmetric magnetic anisotropy

The use of electric fields to control magnetism based on the multiferroic effect provides the possibility to optimize the performance of magnetoresistive sensors. In this work, the electric field of a Ni₈₀Co₂₀/PMN-PT multiferroic heterostructure was used to control the anisotropic field of a Ni₈₀Co₂₀ film with four-fold-symmetric magnetic anisotropy and its anisotropic magnetoresistance. The electric field can be used to control the magnetic field response characteristics of the Ni₈₀Co₂₀ film and change the four-fold-symmetric anisotropy to non-volatile uniaxial anisotropy. Application of an electric field of 6 kV/cm increases the magnetic field response range to 300 Oe when the magnetic field is parallel to the current direction. Once the applied electric field is removed, the uniaxial anisotropy has good stability that can be maintained for 90 days. These results lay the foundation for the research and development of anisotropic magnetoresistive sensors with a stable and wide magnetic field measurement range.     

Structural and magnetic characterization of BaTiO3–BaFe12O19 bilayer thin films: Interface effects on the magnetic properties of barium hexaferrite layer

We report the successful growth of BaFe₁₂O₁₉–BaTiO₃ (BaM-BTO) bilayer thin films using pulsed laser deposition, considering different crystallographic textures; BTO on (0001)-BaM and BaM on (100)-BTO. Our study involved the bilayers, the individual ferrite and titanate films, and the targets used in their growth. Raman spectroscopy and X-Ray diffraction were used to examine the structure of BaM-BTO thin films, indicating that there is no formation of impurity phases. The morphological characterization was made by scanning electron microscopy, and the magnetic behaviour was studied using SQUID magnetometry. The spontaneous magnetization, magnetic anisotropy constant, and anisotropy field were determined simultaneously from the magnetic hysteresis loop. In addition, we also studied the influence of different annealing temperatures over the magnetic behaviour of bare BaM and covered with BTO thin film. This allows to discern between the different magnetization reversal processes in bilayer systems, indicating a strong correlation between the anisotropy field and the coercive field, and an unusual linear relation of the anisotropy constant with the spontaneous magnetization. Our results indicate a direct influence of the BTO on the magnetic properties of the BaM phase, which places these composite bilayers as excellent candidates for the development of multifunctional devices.     

Magnetocrystalline anisotropy in the Co/Fe codoped Aurivillius oxide with different perovskite layer number

Perpendicular magnetic anisotropic materials are of great interest for their huge potential to realize high‐density nonvolatile memory and logic chips. Designing such materials with lower cost and better magnetoelectric coupling still remains major challenges. In this work, aurivillius oxide ceramics with highly [00l]‐oriented grains were prepared by a facile pressureless sintering method, and the perovskite layer number was modified by varying the cobalt content. Afterwards, the unique perpendicular magnetic anisotropy and the intriguing anisotropic ferroelectric properties have been observed. The magnetic properties of the Aurivillius oxides with different layer numbers have been carefully investigated by field cooling, zero field cooling, and the magnetization with a varying field. The magnetic anisotropy in the oriented ceramics is demonstrated to be caused by the magnetocrystalline anisotropy with the easy magnetization direction along the c‐axis, which possibly arises from the unquenched 3d orbitals combined with the special layered crystal structures. The magnetocrystalline anisotropy becomes weaker in the ceramics with lower numbered perovskite layers, whereas the orientation degree of the ceramics and the ferroelectric anisotropy show quite opposite trends. Furthermore, the weak magnetoelectric coupling is also observed in the ceramics. This special anisotropic multiferroic properties may open up a new window for the Aurivillius materials.     

90°-Domain reversal in Pb(Zrx, Ti1-x)O3 ceramics

In this study, a simple and direct method has been proposed, which can to be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. By using this method, the 90° -domain reorientation in the Pb(Zr_xTi_1-x)O₃ ceramic under the electric field was examined through the X-ray diffraction analysis. It was found that the polarization switching in the PZT ceramic with the composition near the morphotropic phase boundary is predominantly controlled by the two successive 90° domain processes rather than only the 180° domain reversal process. Experimental results also indicate that the coercive field of ferroelectric ceramics is related to the cooperative deformation associated with each grain. This cooperative deformation arises from 90°-domain reversal process.     

Magnetic anisotropy in isotropic and nanopatterned strongly exchange-coupled nanolayers

In this study, the fabrication of magnetic multilayers with a controlled value of the in-plane uniaxial magnetic anisotropy field in the range of 12 to 72 kA/m was achieved. This fabrication was accomplished by the deposition of bilayers consisting of an obliquely deposited (54°) 8-nm-thick anisotropic Co layer and a second isotropic Co layer that was deposited at a normal incidence over the first layer. By changing the thickness value of this second Co layer (X) by modifying the deposition time, the value of the anisotropy field of the sample could be controlled. For each sample, the thickness of each bilayer did not exceed the value of the exchange correlation length calculated for these Co bilayers. To increase the volume of the magnetic films without further modification of their magnetic properties, a Ta spacer layer was deposited between successive Co bilayers at 54° to prevent direct exchange coupling between consecutive Co bilayers. This step was accomplished through the deposition of multilayered films consisting of several (Co_{8 nm-54°}/Co_{X nm-0°}/Ta_{6 nm-54°}) trilayers.     

Magnetization reversal of Co/Pd multilayers on nanoporous templates

By making use of an e-beam deposition system, the [Co(2 Å)/Pd(10 Å)]₁₅ multilayers were prepared on a Si(100) substrate and anodized aluminum oxide [AAO] templates with average pore diameters of around 185, 95, and 40 nm. The mechanism of magnetization reversal of the Co/Pd multilayers was investigated. Wall motion was observed on the Co/Pd multilayers grown on the Si substrate. A combination of wall motion and domain rotation was found in the sample grown on the AAO template with a 185-nm pore diameter. For the samples grown on the AAO templates with pore diameters of around 95 and 40 nm, the reversal mechanism was dominated by domain rotation. The rotational reversal was mainly contributed from the underlying nanoporous AAO templates that provided an additional pinning effect.PACS: 75.30.Gw, magnetic anisotropy; 78.67.Rb, nanoporous materials; 75.60.Jk, magnetization reversal mechanisms.     

Study on magnetic properties of a nano-graphene bilayer

A nano-graphene bilayer is described by a transverse Ising model with single-ion anisotropy. The upper layer consists of spin 3/2 with an antiferromagnetic exchange coupling, whereas the bottom layer composes of spin 5/2 with a ferromagnetic exchange coupling. Based on the effective-field theory with correlations the general formula for the magnetization is given. Magnetic properties of the system have been studied, such as the magnetization, the magnetic susceptibilities and the blocking temperature. The exchange coupling, the single-ion anisotropy, the transverse field and the external magnetic field have important effects on the blocking temperature.     

Impact of magnetic field gradients on the free corrosion of iron

The influence of magnetic fields oriented parallel and perpendicular to the electrode surface on the corrosion behaviour of differently shaped iron samples in low concentrated sulphuric acid solutions has been studied. It is demonstrated, that the relative sample-to-magnet configuration, which determines the magnetic flux density distribution in front of the electrode surface, is decisive for the free corrosion activity. In a configuration generating low magnetic flux density gradients the Lorentz force driven micro-convection leads to an anodic shift of the free corrosion potential. In contrast, a configuration yielding high magnetic flux density gradients causes a cathodic potential shift and leads to a suppression of the corrosion reaction. These effects are discussed on the basis of the Lorentz force and the magnetic field gradient force acting on the partial reaction steps during the corrosion process.     

TbCo非晶垂直磁化膜的磁各向异性来源研究

采用不加偏压的磁控溅射法制备了带有和不带有金属Cr底层的TbCo非晶垂直磁化膜 ,并对TbCo非晶垂直磁化膜的磁各向异性来源进行了研究。理论分析结果表明 ,TbCo非晶垂直磁化膜的垂直磁各向异性主要来自于Tb离子的非球对称分布电荷与膜内晶场之间的库仑相互作用以及膜内的应力。扫描电镜的观测结果表明 ,带有Cr底层的TbCo薄膜具有柱状结构。柱状结构的存在导致了其磁各向异性的增强。     

Analysis of sintering behavior of alumina green bodies molded under a strong magnetic field based on master sintering curve theory

Understanding and controlling the anisotropic sintering shrinkage behavior of the green body in the strong magnetic field molding technique bring out the superior functions of ceramics. This study aimed to elucidate the sintering behavior of green bodies molded under a strong magnetic field using the master sintering curve (MSC) theory. Green bodies were prepared to obtain MSCs in the directions perpendicular and parallel to the applied magnetic field. A unique MSC of the alumina green bodies molded with and without a magnetic field was obtained using sintering shrinkage ratio estimated by various heating rates. The apparent activation energy of sintering in the direction perpendicular and parallel to the applied magnetic field was 663 kJ/mol, independent of the measured direction, which is higher than that without an applied magnetic field (562 kJ/mol). The (0001) planes of the sintered body obtained from the green body molded in the magnetic field were oriented perpendicular to the magnetic field, whereas the samples without a magnetic field were randomly oriented. Consequently, we found that the grain boundaries with high consistency in the sample applied to a magnetic field should increase the apparent activation energy because of its grain orientation.     

Controlling anisotropy of porous B4C structures through magnetic field-assisted freeze-casting

Anisotropic porous boron carbide (B₄C) structures were successfully produced, for the first time, using the magnetic field-assisted freeze casting method. The effect of the magnetic field on the structure and mechanical strength of the formed porous B₄C was compared for two different magnetic field directions that were either aligned with ice growth (vertical), or perpendicular to the ice growth direction (horizontal). It was shown that applying even a weak horizontal magnetic field of 0.1–0.3 T noticeably affected the alignment of mineral bridges between lamellar walls. Both the porosity and the channel widths decreased with increasing horizontal magnetic field strength. In the case of a vertical magnetic field, a larger strength of 0.4 T was required for highly aligned lamellar walls and larger channel widths. Compression strength tests indicated that the application of magnetic fields led to more homogeneously aligned channels, which resulted in increased compression strength in the longitudinal (parallel to the ice growth) direction. Applying a vertical magnetic field of 0.4 T with a cooling rate of 2 °C/min during the freezing step of the magnetic field-assisted freeze-casting method was found to result in the best conditions for producing highly anisotropic structures with large channel widths and fewer mineral bridges, which led to an increase in the mechanical strength.     

Stoner–Wohlfarth rotation or domain wall motion mechanism in W-type magnetic hexaferrite nanoparticles

Highly magnetized SrCo_2−xMn_xFe₁₆O₂₇ (x=0–0.5) W-type ferrite was synthesized by a chemical co-precipitation method followed by a hot press technique. The role of substitution of Mn captions on the thermal, structural and magnetic properties of nanoparticles was characterized by TGA/DTA, X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), electron diffraction pattern and vibrating sample magnetometer (VSM). Single phase of W-type ferrite was formed based on the XRD accuracy. According to the TEM micrograph evaluation, it was found that the particle size was in the range of 15–25 nm, with an irregular configuration. In-plane and out-of-plane magnetic hysteresis loops for bulk ferrite were measured at a room temperature up to a maximum applied field of 24 kOe. It was found that the samples with composition of x=0–0.3 have perpendicular anisotropy, while the rest of samples have in-plane anisotropy. Angular dependence of coercivity and initial magnetization curves proved that the highly magnetized bulked ferrite with x=0 to 0.3 obey the Stoner–Wohlfarth mechanism, while x=0.4 and 0.5 follow the domain wall motion rule.     

不同磁场作用下Fe3O4/water纳米流体层流流动对流传热系数的实验研究

对体积分数为3%的Fe₃O₄/water纳米流体在不同温度、不同磁场大小和方向的均匀磁场和梯度磁场作用下的对流换热进行了详细的实验研究。首先,开展了纳米流体能量方程的量纲1分析,讨论了纳米流体强化换热的机理。发现磁性纳米粒子所受到的磁力远远大于布朗运动力。实验测试结果与量纲1分析相吻合,在垂直均匀磁场作用下,纳米流体层流流动的平均对流传热系数提高了5.2%;在垂直梯度磁场作用下,平均对流传热系数提高了9.2%。而在水平均匀磁场作用下,纳米流体平均对流传热系数下降了4.8%。另外,随着温度的升高,对流传热系数均逐渐升高。     

The resistive switching memory of CoFe2O4 thin film using nanoporous alumina template

A novel conductive process for resistive random access memory cells is investigated based on nanoporous anodized aluminum oxide template. Bipolar resistive switching characteristic is clearly observed in CoFe₂O₄ thin film. Stable and repeatable resistive switching behavior is acquired at the same time. On the basis of conductive filament model, possible generation mechanisms for the resistive switching behaviors are discussed intensively. Besides, the magnetic properties of samples (before and after the annealing process) are characterized, and the distinct changes of magnetic anisotropy and coercive field are detected. The present results provide a new perspective to comprehend the underlying physical origin of the resistive switching effect.

PACS

68.37.-d; 73.40.Rw; 73.61.-r     

Mechanical anisotropy in injection-molded polypropylene

By use of a mod with a film gate, two straight polypropylenes (PP) with different melt flow index (MFI) and a glass fiber-reinforced polypropylene (FRPP) were injection molded at various molded at various temperatures into square plates with orientational anisotropy. The anisotropies of tensile property, tensile impact strength, and flexural property were studied on the molded sample cut mainly in the machine direction (MD), 45°-direction (45°), and transverse direction (TD). Both the orders of the yield strength and tensile impact strength of the FRPP, and those of the necking stress and tensile impact strength of the straight PP, were MD >45° >TD, which are reasonable tendencies. The orders of the yield strength and flexural modulus of the straight PP were MD > TD > 45°, which suggests the presence of shear deformation between the lamellae in the skin layer. The variation of the flexural modulus with the angle to the MD fitted well to Hearmon's equation. Generally, for straight PP, the anisotropy of various properties increased as the MFI and cylinder temperature became lower, or as the skin layer became thicker. For the FRPP, the anisotropy increased as the cylinder temperature became higher, or as the degree of the orientation of glass fibers became higher.     

Development of high-performance magnetic thin film for high-density magnetic recording

This overview describes our recent study on fabrication processes of high-performance magnetic thin films for high-density magnetic recording. Particularly, it is emphasized that electrochemical processes play significant roles in fabricating the recording heads and media used for the high-density recording. Newly developed electrodeposition methods for fabricating CoNiFe and CoFe soft magnetic thin films with high-saturation magnetic flux density are shown, and the key points for obtaining them are highlighted. It is summarized that the effective seedlayers for the sputter-deposited [Co/Pd]_n multilayered films, which are promising candidates as magnetic recording media with strong perpendicular magnetic anisotropy for the high-density magnetic recording, have been developed. We have recently succeeded in developing the novel electroless deposition methods for the CoNiFe-based soft magnetic underlayers of double-layered perpendicular magnetic recording media and for the patterned medium consisting of CoNiP magnetic nano-dot arrays, which are briefly explained.     

Magnetic field effect on the anodic behaviour of a ferromagnetic electrode in acidic solutions

The magnetization of a ferromagnetic electrode in an external homogeneous magnetic field leads to a stray field in front of the electrode. This stray and its gradients can alter the anodic behaviour of the electrode significantly. Potentiodynamic polarisation measurements of an iron wire in a 0.5 M sulfuric acid solution (pH 0.25) and in a 0.5 M phthalate buffer solution (pH 5) without and with applied magnetic fields up to 0.6 T in different orientations to the electrode surface were performed. In sulfuric acid solution an increase of the diffusion-limited dissolution current density and a shift of the active-passive transition potential to more noble potentials was observed when the magnetic field was applied parallel to the electrode surface. In contrast, in perpendicular field configuration the diffusion-limited current density is lowered and the active-passive transition potential is shifted to less noble values. In phthalate buffer no significant influence of the magnetic field on the current density was observed in the active region, but a shift of the active-passive transition to less noble potentials occurred irrespective of the magnetic field configuration.The observed effects of a superimposed magnetic field on the anodic behaviour of iron are discussed with respect to an increase of the mass transport due to the Lorentz-force-driven magnetohydrodynamic (MHD) effect, the magnetic field gradient force and its interaction with the paramagnetic iron ions. The results of this paper show that the effect of the field gradient force can become very important due to the high magnetic field gradient at ferromagnetic electrodes.