Magneto-Impedance behavior of Co-Fe-Nb-Si-B-based ribbons

The giant magneto-impedance of melt spun Co_xFe_72−xNb₄Si₄B₂₀(x=10, 20, 36, 50) amorphous and nanostructured ribbons have been investigated. Alloys have been optimized at the driving current amplitude, frequency and found that amorphous ribbon of nominal composition of Co₃₆Fe₃₆Nb₄Si₄B₂₀ shown maximum GMI ratio of 13%. The behaviour of the driving current amplitude on the GMI behaviour was studied and the sample was optimized for driving current amplitude, I_ac=10 mA. The frequency dependence of the GMI behaviour was studied for the ribbon sample Co₃₆Fe₃₆Nb₄Si₄B₂₀ at frequency in the range of 100 kHz-1.2 MHz of the optimized driving current amplitude and it was found that the sample showed the maximum GMI behaviour at f=700 kHz. The optimized samples were Joule heated at the current density J=0-35 A/m² for a period of 1 min. The GMI ratio initially increased then progressively deteriorated with J, but after a certain range it shows up to 16% of improvement in the magneto-impedance value due the increase of nanocrystalline volume fraction. The asymmetry in the GMI profile was observed for the sample Joule heated at J=1-5 A/m² for 1 min.     

Generation of 3rd and 5th harmonics in a thin superconducting film by temperature oscillations and isothermal nonlinear current response

The generation of harmonics of the voltage response is considered when an AC current is applied through a superconducting film above T_c. It is shown that almost at all temperatures the mechanism of the temperature oscillations created by the AC current and the temperature dependence of the resistance dominates over the isothermal nonlinear electric conductivity. Only in a narrow critical region close to T_c the latter is essential for the generation of the harmonics. A detailed investigation of harmonics generation provides an accurate method for measuring the thermal boundary conductance between the film and the insulating substrate. The critical behaviour of the third harmonic will give a new method for the determination of the lifetime of metastable Cooper pairs above T_c. The comparison of the calculated fifth harmonics of the voltage with the experiment is proposed as an important test for the applicability of the employed theoretical models. Received 8 September 2001     

Influence of current amplitude on the nonlinear asymmetric ac volt-ampere characteristics in amorphous ribbons with GMI effect

Asymmetric giant magnetoimpedance (GMI) effect in amorphous ribbons is very promising due to its possible application in developing the highly sensitive linear magnetic-field sensors. In this paper, the model of the nonlinear asymmetric volt-ampere characteristics in field-biased Co-based amorphous ribbon has been established through the time derivative of the longitudinal magnetization component in the ribbon. Its harmonics are also developed by Fourier analysis. The influence of the current amplitude on the nonlinear asymmetric effect has been studied. It is found that when the current amplitude is 8.54 mA, the sensitivity of the first harmonic voltage on the external field is equal to that of the second harmonic voltage. The results obtained are useful for developing the high-sensitive magnetic-field sensors.     

Shot noise in a quantum dot with the coulomb interaction

We study the noise in a quantum dot which is coupled to metallic leads by using the non-equation of motion technique at the Kondo temperature T_K. We compute the out of equilibrium density of states, the current and the shot noise. We find that the shot noise exhibits a nonmonotonic dependence on the voltage when variation of ε_d values of the QD energy in the absence of the external magnetic field occurs. We also find that the amplitude of current exhibits a saturation behavior when driving field is increased.     

Harmonic generation in superconducting Bi(Pb)SrCaCuO thick films

We present a detailed study of harmonic generation in Bi(Pb)SrCaCuO superconducting thick film prepared by screen printing technique. A comparative study of amplitude of harmonics of two films of different J_c have been carried out. The variation of amplitudes of the harmonics are studied as a function of magnitude of ac and dc field. The temperature dependence of amplitude of third harmonic (V³) is studied with increasing amplitude as well as frequency of ac field. These results are analyzed in the frame work of critical state model. V₃ -T curve also indicates the presence of two phases in Bi(Pb)SrCaCuO film.     

Giant magneto-impedance effect in CoMnSiB amorphous microwires

The giant magneto-impedance (GMI) ratio, ΔZ/Z=[(Z(H)−Z(H_max)]/Z(H_max), in a nearly zero magnetostrictive Co_68.5Mn_6.5Si₁₀B₁₅ amorphous microwire has been investigated for the frequency range 0.5–10 MHz, driving current amplitude of 0.5–2.5 mA, bias DC magnetic field up to 2400 A/m and under applied tensile stress up to 132 MPa. A maximum relative change in the GMI ratio up to around 130% is observed at a frequency of 10 MHz, magnetic DC field of about 180 A/m, driving current amplitude of 1 mA and under tension of 60 MPa. The tensile stress dependence of the magnetic field, H_m, corresponding to the maximum ΔZ/Z ratio allows to estimate the magnetostriction constant (λ_s≈−2×10⁻⁷) to be in good agreement with λ_s values estimated by different methods and in amorphous alloys with similar compositions.     

Effects of helical anisotropy on nonlinear voltage response in amorphous wires

The nonlinear voltage response in soft magnetic amorphous wires exciting by an alternating current is studied. The frequency spectrum of the voltage in the pick-up coil wound around the wire with a helical anisotropy is found in the framework of a model based on quasi-static Stoner−Wohlfarth magnetization reversal. The effect of a deviation of the anisotropy axis from the azimuthal direction on the field dependences of amplitudes of voltage harmonics is analyzed. It is shown that the field sensitivity of even harmonics increases with the anisotropy axis deviation angle. The current amplitude range to obtain a maximal field sensitivity of the second harmonic is found. The influence of the skin effect on the frequency spectrum of the pick-up coil voltage is discussed. The results obtained may be of importance for the development of sensors of a weak magnetic field.     

Quantum oscillations of rectified dc voltage as a function of magnetic field in an “almost” symmetric superconducting ring

Periodic oscillations in the dependence V _dc(B) of rectified dc voltage on the perpendicular magnetic field have been experimentally observed near the critical temperature in a single superconducting aluminum ring with slight geometric inhomogeneities (without specially formed circular asymmetry), biased by an external ac current (without a dc component). With a change in the external current and temperature, the voltage V _dc(B) behaves like the corresponding voltage on a circularly asymmetric ring but has a much smaller amplitude. The Fourier spectrum of the function V _dc(B) contains the fundamental frequency, corresponding to the ring area, and its highest harmonics. “Satellite” frequencies, dependent on the structure geometry and external parameters, were unexpectedly found in the spectrum.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

On the harmonic analysis of non-linear dielectric effects

We investigate the nonlinear dielectric effects in a polar viscous liquid, propylene carbonate, by analyzing the voltage and current traces obtained for a sinusoidal electric field at a frequency of 1 kHz and field amplitudes as high as 162 kV/cm. The main source of non-linear behavior results from the energy absorbed from the field and is understood quantitatively. However, there is a qualitative difference in the behavior of the field induced change, Δε^′′(E), and the third harmonic amplitude of the current, I_3ω. Although both Δε^′′(E) and I_3ω are considered reliable measures of non-linear behavior, we show here that the third harmonic signal reflects only those non-linear responses that are instantaneous on the time scale of the test frequency.     

AC susceptibility and intergranular critical current density study in pure and Ag doped (La1−xYx)2Ba2CaCu5Oz superconductors

The temperature and AC field amplitude variations of AC susceptibility have been measured on pure and 5 wt% Ag doped (La_1−xY_x)₂Ba₂CaCu₅O_z superconductors. The AC susceptibility as a function of field have been analyzed using Kim's critical state model. The temperature dependence of intergranular critical current density and the effective volume fractions of the grains have been estimated. The Ag doped samples show relatively large critical current density due to the improved intergranular coupling. The exponent of temperature variation of critical current density suggests that the weak links form superconductor-normal metal-superconductor (SNS) type of junctions for all the samples.     

Higher harmonics in the voltage on a superconducting wire carrying AC electrical current

The problem of determining the harmonic content in the voltage that appears on a superconducting wire carrying cosine-like AC current was resolved theoretically, using two approaches. First, the Fourier components of the voltage spectrum were found by numerical integration. Importance of individual terms was established, leading to two conclusions: a) it is the cosine component of the 3rd harmonic that represents the bulk of harmonic distortion, b) for the practical purposes it is sufficient to consider higher harmonics with n ≤ 7. Then, the analytical formulas were derived. While for the sine components a general expression containing an infinite series was found, closed-form formulas were derived for the cosine components of the harmonics 1, 3, 5, 7. Consequences of the results to the experimental technique used to study the AC transport properties of superconductors are discussed.     

Preparation of meander thin-film microsensor and investigation the influence of structural parameters on the giant magnetoimpedance effect

A novel GMI-based microsensor with symmetrical meander structure has been fabricated by MEMS technology and mutual perpendicularity between longitudinal external field and easy axis was established by electroplating process for reaching a higher GMI. The GMI effects of the 3 and 6 turns sample with different Cu line width has been investigated systematically. The longitudinal frequency dependence has been given and a new frequency characteristic was observed in this paper. The experimental results show the increasing of Cu line width in appropriate range and more turns can respectively lead to greater GMI effect, the positive GMI ratio peak in 6 turns sample is nearly 4.5 times over the 3 turns sample, the peak frequency is greatly increased with the increasing of turn number, and the GMI ratio in 3 and 6 turns sample both exhibits negative maximum value when H _L>50 Oe. In this work, the GMI ratio peak appears at f _AC=4.8 MHz in 6 turns sample with the Cu line width of 140 μm, almost reaches 183.7 % under H _L=17 Oe.     

Enhancement of giant magneto-impedance effect in Ni80Fe20/SiO2/Cu composite wires

Ni₈₀Fe₂₀/SiO₂/Cu composite wires of Cu core 60 μm in diameter and coated with layers of SiO₂ and Ni₈₀Fe₂₀ were prepared by RF magnetron sputtering. The influences of the insulator layer thickness, the measurement mode and the magnitude of the driving current on the giant magneto-impedance (GMI) effect were investigated. The results showed that there was an optimum thickness of the insulator layer and the driving current can influence the shape of the MI curve. Resonance enhancement of the GMI was found in the new measurement mode. The results are discussed by taking account of the electromagnetic interactions.     

Effective nonlinear AC response to graded cylindrical composites

The perturbation method is used to study the effective nonlinear response of graded cylindrical composites with power-law gradient inclusions under a sinusoidal alternating current (AC) external field of finite frequency ω. In dilute limit, the local potentials and the formulae of effective nonlinear AC responses are derived at the fundamental frequency and the third harmonics. Furthermore, the general effective nonlinear responses are given and compared with the effective nonlinear AC responses of fundamental frequency and the third harmonics, and the relationships between the nonlinear effective AC response and the general effective nonlinear response are also determined.     

Coupled relaxation oscillators for frequency multiplication

Relaxation oscillators with microbridges as hysteretic elements are investigated in a frequency range of up to 1 GHz. The signal form of the voltage pulses and the resulting frequency versus bias current dependence are calculated, as well as the frequency spectrum of the harmonics. To demonstrate the possibility of a frequency multiplication, several relaxation oscillators are coupled in a serial or ring configuration. Superconducting delay lines are used to synchronize the oscillators to each other with the proper phase shift. As our measurements show, a frequency multiplication, or at least an enhancement in the power of the desired harmonic is possible in this way.     

Observation of a sharp lambda peak in the third harmonic voltage response of high-T superconductor thin films

In this paper, we report on the sharp peak observed in the third harmonic voltage response generated by a bias sinusoidal current applied to several strips patterned in a YBa₂Cu₃O _{7 - δ} thin film and in two La_2-xSr_xCuO₄ thin films, when the temperature is close to the normal-superconductor transition. The lambda-shaped temperature dependence of the third harmonic signal on the current characteristics is studied. Several physical mechanisms of third harmonic generation are examined. Received 13 November 2002 / Received in final form 21 February 2003 Published online 7 May 2003     

Voltage/current characteristics of high-current vacuum arc at a small gap length

Measurements of arc voltage for semiwave currents of the amplitude up to 13 kA and frequencies 0.9 kHz and 6.0 kHz were done for CuCr electrodes whose separtion was 2 mm. The obtained data were used to determine voltage/current and voltage-time characteristics of high-current vacuum arc. They enabled to state as:

1. The shape and values of arc voltage during a sinusoidal current semiwave flow depend on the current amplitude and frequency. As the current amplitude increases the arc voltage rises, and its shape changes from relatively low and quiet to high with considerable fluctuations of high frequency and amplitude (HAF).
2. Arc voltage at a given current instantaneous valuei _a1 depends on the current waveform before reachingi _a1 . The arc voltage beyond the range of HAF occurrence, at the same instantaneous value of the sinusoidal current, is higher during the current increase than during its decrease and higher for higher current frequency.
3. Mean arc voltage and current value at the moment of HAF onset, rise with the increase of current peak and frequency.

This article presents qualitative correlations between the measured arc voltage and an expected discharge mode in the intercontact gap.     

电流退火铁基薄带巨磁阻抗效应的影响

研究了电流退火工艺对铁基非晶合金Fe₈₄Zr₈Nb_3.5B_3.5Cu_{1〈/sub>薄带巨磁阻抗效应的影响.实验结果表明,该铁基合金的巨磁阻抗ΔZ/Z随退火电流的增加而增强,当电流为820mA时,ΔZ/Z达到最大值62％,阻抗变化灵敏度可达约0.23％（A/m)^-1.结合此合金在电流退火燧后电性能和软磁性能的演变,讨论了材料的巨磁阻抗效应借助趋肤效应与交流频率和外加纵向磁场的关系. 关键词：}     

High-pressure effect on elastic constants, stacking fault energy and correlation with dislocation properties in MgO and CaO

The elastic properties and the generalized-stacking-fault-energy (GSFE) in MeO (Me = Mg, Ca) under different pressures have been calculated using the first principle calculations. In the anisotropic elasticity theory approximation, by using the Foreman’s method, the core structure and Peierls stress of $\tfrac{1} {2}\left\langle {110} \right\rangle \left\{ {110} \right\}We report structural, DC magnetization, detailed linear/non-linear AC susceptibility, (with applied frequency and amplitude) isothermal and thermoremanent magnetization (TRM) behavior for RuSr₂Y_1.5Ce_0.5Cu₂O₁₀ (YRu-1222) magneto-superconductor to understand its complex magnetism. Studied sample is synthesized through the novel solid state high pressure (6?GPa) high temperature (1450?°C) (HPHT) technique. The compound is crystallized in tetragonal structure with space group I4/mmm (No.?139). DC magnetic susceptibility shows that studied YRu-1222 is magneto-superconducting with Ru spins magnetic ordering at around 110?K and superconductivity (SC) in the Cu-O₂ planes below ?~?30?K. Frequency and field dependent detailed AC magnetic susceptibility measurements confirms the spin-glass (SG) behavior with homogeneous/non-homogeneous ferromagnetic (FM) clusters in this system. Variation of cusp position with applied AC frequency follows the famous Vogel-Fulcher law, which is commonly accepted feature for spin-glass (SG) system with homogeneous/non-homogeneous ferromagnetic clusters embedded in spin-glass (SG) matrix. Above the freezing temperature (T _f ), first and third harmonics AC susceptibility analysis indicated possibility of the co-existence of spin cluster ferromagnetism with superparamagnetism (SPM). The M-H loops at low temperature exhibit the ferromagnetic behavior with rather small coercive field (H _c ) and remnant magnetization (M _r ). Summarily, the magnetic (DC and AC) susceptibility measurements and their analysis have enabled us to unearth the complex magnetism in terms of successive SG-FM-SPM transitions with temperature.