The frequency spectrum of a voltage measured in an amorphous wire magnetized in alternating magnetic field

We have studied the process of magnetization reversal in amorphous microwires under the action of a longitudinal (axial) high-frequency magnetic field. The amplitudes of harmonics in the frequency spectrum of the response voltage between the wire ends were measured as functions of the high-frequency field amplitude and a dc axial magnetic field strength. For relatively large amplitudes of the high-frequency field, the first several harmonics in the frequency spectrum of the response voltage are highly sensitive with respect to the dc magnetic field. The experimental results are interpreted within the framework of a simple electrodynamic model.     

Tailoring of Magnetic Properties of Magnetostatically-Coupled Glass-Covered Magnetic Microwires

We report on tailoring of magnetic properties of Fe- and Co-rich microwires through magnetostatic coupling among them. We studied hysteresis loops of the arrays containing different number of the Co₆₇Fe_3.9Ni_1.5B_11.5Si_14.5M_0.6 and Fe₇₄B₁₃Si₁₁C₂ amorphous microwires. Fe₇₄B₁₃Si₁₁C₂ microwires have rectangular hysteresis loop, while Co₆₇Fe_3.9Ni_1.5B1_1.5Si_14.5M_0.6 with vanishing magnetostriction constant posses inclined hysteresis loop with low coercivity. The presence of neighboring microwire (Fe either Co-based) significantly modifies hysteresis loop of whole microwire array. In a microwire array containing Fe-based microwires, we observed splitting of the initially rectangular hysteresis loop with a number of Barkhausen jumps correlated with number of Fe-rich microwires. In Co?CCo arrays, we observed a change of inclination of overall hysteresis loop, and consequently magnetic anisotropy field under influence of the additional of Co-based microwire. In the case of mixed arrays containing Fe and Co-rich microwires, we were able to obtain irregular hysteresis loops with unusual shape. In this case, considerable increase of harmonics has been observed. Magnetic field amplitude and frequency affect the behavior of all studied arrays. Increasing the amplitude the shape of hysteresis loop of microwire array containing Fe-based microwires transforms from multi-step to single above certain magnetic field amplitude. In the array with Co-based microwires, we observe a change of coercivity. Observed dependences have been attributed by us to the magnetostatic interaction between the microwires with different magnetic domain structure. Together with the conventional method, such as thermal treatment, designing of arrays containing different types of microwires can serve for tailoring of their magnetic properties.     

From Manipulation of Giant Magnetoimpedance in Thin Wires to Industrial Applications

We present results on our studies of soft magnetic properties and Giant magnetoimpedance, GMI, effect in thin microwires at elevated frequencies paying special attention to tailoring the GMI effect and achievement of high GMI effect with low hysteretic behavior. We measured magnetic field, H, dependence of real part, of the longitudinal wire impedance up to frequency, f, 4 GHz in Co-rich microwires. Amorphous microwires of appropriate composition exhibit extremely soft magnetic properties with low coercivity (generally below 10 A/m) with well defined magnetic anisotropy field, H _k . We report a number of interesting for sensor applications phenomena such as stress-impedance effect and stress sensibility of overall hysteresis loop shape. Field dependence of the off-diagonal voltage response of pulsed GMI effect in nearly-zero magnetostriction (λ _s≈?3×10^?7) microwires exhibits anti-symmetrical shape, suitable for industrial applications. We observed that the magnetic field dependence of GMI ratio can be tailored either controlling magnetoelastic anisotropy of as-prepared microwires or by heat treatment. Composite character of such microwires results in the appearance of additional magnetoelastic anisotropy. We found that if the surface anisotropy is not circumferential, then the magnetization, and consequently, the MI curve Z(H) present hysteresis. This hysteresis can be suppressed by application of sufficiently high DC bias current I _B that creates a circumferential bias field H _B.     

A Ground State Monte Carlo Approach for Studies of Dipolar Systems with Rotational Degrees of Freedom

We have developed a path integral ground state Monte Carlo (PIGSMC) algorithm for quantum simulations of rotating dipolar molecules, using a highly accurate sixth-order algorithm. The method allows us to calculate unbiased estimates of ground state properties of dipolar molecules in a variety of geometries, with or without an external electric field. To demonstrate the capability of the approach, we calculate the orientational phase diagram of a one dimensional lattice system of rotating point dipoles in the absence of any external electric fields. We find that for finite lattice size, this system exhibits an order?Cdisorder transition at finite dipolar interaction strength in contrast to the well-known orientational disorder of the corresponding one dimensional O(3) quantum rotor models. Comparison of the quantum Monte Carlo results with a self-consistent field estimate of the phase transition shows the emergence of an ordered phase at non-zero dipolar strength, confirming the symmetry breaking role of the anisotropic dipole?Cdipole interaction.     

Collective modes and spin waves in superfluid3He-B

Making use of a model Hamiltonian, which includes, besides the kinetic energy and the BCS-like pairing energy, the spin exchange energy as well as the dipolar interaction energy, we study theoretically the collective modes of the Balian - Werthamer state in the collisionless limit. In the absence of the dipolar interaction energy, the collective modes are classified according to their total angular momentumJ. Among 18 distinct modes, one mode withJ=0 and three modes withJ=1 are gapless. The latter modes couple with the spin fluctuation and are identified with spin wave modes. The effect of the dipolar interaction on these modes is discussed.Supported by the National Science Foundation.     

Surface and Bulk Magnetic Hysteresis Loops of Co-Rich Glass Covered Microwires

The magnetization reversal process in the surface and volume areas of Co-rich glass covered microwires has been investigated. The value of the helical anisotropy has been determined in two wires with different thickness of glass covering. The dependence of the coercive field on the frequency of the external axial magnetic field was studied in the surface and volume of the microwires     

Influence of the Competition Between Dipolar and Exchange Interactions on the Magnetic Structure of Single-Wall Nanocylinders. Monte Carlo Simulation

In this work, we address the Metropolis Monte Carlo simulation of single-wall magnetic nanocylinders with zig-zag edges on the basis of a three-dimensional classical Heisenberg model with short-range nearest neighbors ferromagnetic exchange interactions and long-range dipolar interactions. A study of the magnetic properties (thermal dependence of the magnetization per site, specific heat, and magnetic susceptibility) with different strengths of dipolar interaction relative to the exchange energy is carried out. Our results reveal a strong dependence of the low-temperature magnetic structure of the nanotubes with the strength (??) of the dipolar interaction. Comparisons with the case without considering dipolar interaction are carried out and a summary of our results in terms of the ??-dependence of the low-temperature magnetization is also presented and discussed.     

Higher-Order Line Element Analysis of Potential Field with Slender Heterogeneities

Potential field due to line sources residing on slender heterogeneities is involved in various areas, such as heat conduction, potential flow, and electrostatics. Often dipolar line sources are either prescribed or induced due to close interaction with other objects. Its calculation requires a higher-order scheme to take into account the dipolar effect as well as net source effect. In the present work, we apply such a higher-order line element method to analyze the potential field with cylindrical slender heterogeneities. In a benchmark example of two parallel rods, we compare the line element solution with the boundary element solution to show the accuracy as a function in terms of rods distance. Furthermore, we use more complicated examples to demonstrate the capability of the line element technique.     

First principles simulations of kink defects on the SP 90° partial dislocation in silicon

In this paper we study kink defects on the single period (SP) reconstructed 90° partial dislocation in silicon. First, we use the Tersoff potential in order to determine how to set up the supercells we are employing so that we can extract meaningful results. In particular, we consider the elastic interactions between all dislocation segments and their effect on the energetics of the cells. We have identified a new long-range elastic field associated with the SP structure and we model it as arising from a dipolar line of force along the dislocation line. We also study the effect of this field on kink energies. With this information we then perform first principles simulations. Our results indicate that the only stable kink defects are those that separate regions of the dislocation with opposite reconstruction senses. We study the structure of these defects or complexes, as they are called, and we obtain the formation energy of a complex pair. We also investigate the changes in electronic structure and the rebonding that take place during the motion of these defects. We obtain the energy of migration of a single complex. Using a tight binding approach, we obtain the bandstructures associated with the complexes both at stable positions and at the migration saddle point and we discuss their role in the dislocation dopant effect.     

Flexible piezotronic strain sensor

Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I- V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.     

Role of Defects on Domain Wall Propagation in Magnetically Bistable Glass-Covered Microwires

We are trying to reveal the contribution of local defects on DW propagation in amorphous microwires. Consequently, we present a comparative study of single domain wall dynamics and local nucleation fields in Fe- and Co-rich amorphous glass-coated microwires. For this we modified experimental set-up based on the classical Sixtus?CTonks approach introducing additional pick-up coil in order evaluate defects contribution in domain wall propagation. Below some critical magnetic field, H _N , determined by the microwires inhomogeneities, an almost linear DW velocity,?v, dependence on magnetic field, H, is found. Quite fast DW propagation (v?till 3000?m/s at?H about 65?A/m) has been observed. When the applied magnetic field exceeds H _N , new reverse domains can be nucleated and consequently tandem remagnetization mechanism can be realized. The role of defects existing in magnetically bistable microwires is related with nucleation of new reversed domains.     

Self-assembly of colloidal nanocrystals as route to novel classes of nanostructured materials

Colloidal crystallisation is the only way to obtain three-dimensional ordered materials in which semiconductor, metallic, and magnetic nanocrystals are in close contact. It is expected that the quantum mechanical and dipolar interactions between the nanocrystal units can lead to unseen physical phenomena and materials. Here we review the development of this new and exciting field. We first compare nanocrystal superlattices with regular atomic solids regarding their mechanical strength and opto-electronic properties. We describe how nanocrystal superlattices have been obtained from colloid suspensions in several ways. The thermodynamic driving force for colloidal crystallisation is discussed in terms of inter-particle interactions in a good solvent and entropy. We compare the binary superlattices that have been obtained by solvent evaporation with the predictions of the hard-sphere model and show that semiconductor nanocrystals in a good solvent can behave as hard spheres. Finally, we discuss the quantum mechanical and dipolar interactions in nanocrystal superlattices and review recent studies of the opto-electronic and magnetic properties of novel superlattice materials.     

玻璃包覆金属微丝的快速凝固制备及应用

玻璃包覆金属微丝具有耐蚀、耐高温、抗辐照及高绝缘性等特点,作为精密电子元器件用电磁线,具有重要的应用前景.综述了玻璃包覆金属微丝制备技术的发展现状、基本工艺原理、主要工艺参数以及玻璃包覆金属微丝材料的种类、微丝的形状尺寸与组织,介绍了玻璃包覆金属微丝在电气、电子工业以及复合材料制备等领域的用途.     

A semiconductor exciton memory cell based on a single quantum nanostructure

We demonstrate storage of excitons in a single nanostructure, a self-assembled quantum post. After generation, electrons and holes forming the excitons are separated by an electric field toward opposite ends of the quantum post inhibiting their radiative recombination. After a defined time, the spatially indirect excitons are reconverted to optically active direct excitons by switching the electric field. The emitted light of the stored exciton is detected in the limit of a single nanostructure and storage times exceeding 30 msec are demonstrated. We identify a slow tunneling of the electron out of the quantum post as the dominant loss mechanism by comparing the field dependent temporal decay of the storage signal to models for this process and radiative losses.     

Ferromagnetic resonance driven by spin transfer torque

We study spin-torque-driven ferromagnetic resonance (ST-FMR) in point contacts. Point contacts as small as a few nanometers in size are used to inject microwave currents into F/N/F spin valves where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer. High densities of injected currents produce the spin-transfer torque on magnetic moments and drive FMR in the F-layers. The resonance is detected electrically when a small rectified dc voltage appears across the point contact. Here we focus on the origin of this rectified signal and study ST-FMR in point contacts to spin valves with different ferromagnets (Py and Co) and single ferromagnetic (Py) films, as well as in spin-valve wires patterned by electron beam lithography. We find that this voltage can be explained by the resistance variations which originate from giant magnetoresistance in point contacts to spin valves and involve effects of anisotropic magnetoresistance and extraordinary Hall effect on the propagation of microwave currents in continuous F-films and microwires.     

Single-crystal relaxor ferroelectric piezoactuators with interdigitated electrodes

We report on the fabrication and performance of (1-x) Pb(Zn(1/3)Nb(2/3))O3-xPbTiO3 (PZN-PT) single-crystal relaxor piezoactuators with interdigitated electrodes patterned on a single surface. An electric field gradient across the sample thickness induces a differential contraction between opposite faces, and it is responsible for the actuation. The samples are poled by energizing the electrodes at 100 degrees C and cooling in a field. Calculations of the piezoelectric response based on a periodically modulated dipolar field yield good agreement with experiment. Discrepancies with the model are ascribed to multidomain formation in the ferroelectric sample as a result of field reversals in the applied electric field along the sample length.     

Quantum Tunneling of the Magnetization In Mn12-Ac

We present new experiments on the zero-field magnetic relaxation in a single crystal of Mn12-acetate. This study shows the important role played by the interactions between the spins and their environment. It suggests the existence of two relaxation regimes. Above 2.0 K, the relaxation time follows the Arrhenius law. The time decay is not strictly exponential due to dipolar interactions. At lower temperatures, the relaxation time depends weakly on the temperature. A square root decay is observed, as recently predicted by a theoretical model in which the tunneling is assisted by dipolar and hyperfine interactions.     

Synthesis of Au microwires by selective oxidation of Au–W thin-film composition spreads

Abstract

We report on the stress-induced growth of Au microwires out of a surrounding Au–W matrix by selective oxidation, in view of a possible application as ‘micro-Velcro’. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au–W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au₄₈W₅₂ composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au₄₈W₅₂ composition are suitable for micro-Velcro applications.     

Ferromagnetic microwires enabled polymer composites for sensing applications

In the present work, sensing functionalities are introduced into structural composites via embedded magnetic microwires. A systematic study on the structure and functionalities of microwires and their composites is performed. The single-wire composite shows a significant giant magnetoimpedance (GMI) effect of up to 320% in a frequency range of 1–100 MHz due to stress enhanced transverse magneto-anisotropy. With increasing quantities of embedded wires from 1 to 3, the maximum GMI ratio is enhanced significantly by more than 35%, making the resultant composite favourable for field sensing applications. The microwire-composite also shows superior stress-sensing resolution as high as 134.5 kHz/microstrain, which is about 26 times higher than the recently proposed SRR-based sensor. As evidenced by the structural examination and tensile tests, the extremely small volume fraction of microwires (～0.01 vol.%) allows the wire-composites to retain their mechanical integrity and performance.     

不同直径玻璃包覆非晶微丝的制备及其磁性能

采用熔融拉丝法制备了直径范围分别在6.1～28.0μm和14.0～35.2μm之间的玻璃包覆非晶Fe基和Co基合金微丝, 测试了不同合金直径和不同玻璃包覆层厚度的玻璃包覆合金微丝样品的静磁性能. 结果表明: 轴向矫顽力和轴向剩磁比随着微丝直径的增大而降低, 随着玻璃包覆层的增大而升高; 径向剩磁比的变化趋势则相反. 微丝合金直径和玻璃包覆层厚度改变, 静磁性能变化的主要原因是作用在合金芯上的内应力的变化, 导致了具有不同磁畴结构的合金内芯区和合金外壳区体积比的改变.