Off-diagonal magnetoimpedance in stress-annealed amorphous ribbons

The influence of creep-induced magnetic anisotropy on the off-diagonal magnetoimpedance in amorphous Co₆₇Fe₄Cr₇Si₈B₁₄ ribbon is investigated. Hard-ribbon-axis anisotropy is produced by continuous stress annealing. On applying DC bias current, the off-diagonal impedance becomes an antisymmetric function of applied field with a quasi-linear part around the zero field. Theoretical explanation of the phenomenon is based on classical electrodynamics. It is shown that the maximum on the frequency dependence of off-diagonal impedance, observed around 1 MHz, results from competition between the electromagnetic induction and the skin effect. The quasi-linear characteristic can be utilized in low-cost high-sensitive magnetic field sensors.     

Magnetoimpedance effect in Nanoperm alloys

The influence of isothermal annealing (1 h at 600 °C in Ar atmosphere) on the soft magnetic properties and magnetoimpedance (MI) effect has been studied in ribbons of the following Nanoperm alloys: Fe₉₁Zr₇B₂, Fe₈₈Zr₈B₄, Fe₈₇Zr₆B₆Cu₁ and Fe₈₀Zr₁₀B₁₀. A maximum MI ratio of about 27% was measured for the nanocrystalline alloy Fe₈₇Zr₆B₆Cu₁ at a driving frequency of 0.2 MHz. The thermal annealing led to magnetic softening for this alloy, while a hardening is observed for the Fe₈₀Zr₁₀B₁₀ alloy.     

The effect of surface crystalline layers on asymmetric off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured using a pick-up coil wound around the sample. One side of a ribbon was etched in hydrofluoric acid solution during various times in order to change the thickness of the surface crystalline layer appearing after annealing. The asymmetric two-peak field dependence of the off-diagonal impedance was observed for all samples. The evolution of the off-diagonal magnetoimpedance with the change in the ribbon thickness is analyzed.     

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.     

Effect of dc-Joule-heating thermal processing on magnetoimpedance of Fe72Al5Ga2P11C6B4 amorphous alloy

The effect of multistep dc-Joule-heating thermal processing on magnetoimpedance (MI) of Fe₇₂Al₅Ga₂P₁₁C₆B₄ ribbons is presented. After material optimization significant increase of MI response up to value ΔZ/Z≈55% as well as sensitivity of about 6%/kA/m (for H?3–4 kA/m), were recorded in still amorphous samples at driving frequencies 2–3 MHz. On-line and post-annealing electrical resistivity together with Mössbauer spectra analysis and frequency dependence of the penetration depth were used for characterization of MI improvement.     

Influence of hydrogen distribution on magnetic properties of amorphous samples

We have studied the effect of the homogeneity of hydrogen distribution on magnetic properties of hydrogen charged amorphous samples (Metglass 2826MB: Fe₄₀Ni₄₀Mo₄B₁₈). The results suggest that, in high magnetostrictive samples, the induced internal stresses and hence the magnetic anisotropy are related to the hydrogen distribution and not to the total quantity of hydrogen inside the sample. To perform this study, the homogeneity of hydrogen distribution in the sample was improved by using a very low rate of hydrogen charging by means of a pulsed electrolytic current. Furthermore, this pulsed electrolytic current allows us to introduce a high quantity of hydrogen without breaking the sample. Received 7 May 1999 and Received in final form 11 October 1999     

Lattice effect on magnetism in CMR perovskite La0.85Sr0.15MnOz

0.85 Sr_0.15MnO₃ system has been investigated. Enlargements of lattice parameters with grain growth have been observed. Associated with this growth are a series of magnetic changes, including magnetization, Curie temperature T_c and superparamagnetic behavior. These phenomena may provide another route to understanding the relationship between structure and magnetism in the hole-doped manganese perovskites. Received: 20 January 1997/Accepted: 28 February 1997     

Development of high catalytic activity disordered hydrogen-storage alloys for electrochemical application in nickel–metal hydride batterie

Multi-element, multiphase disordered metal hydride alloys have enabled the widespread commercialization of nickel–metal hydride (NiMH) batteries by allowing high capacity and good kinetics while overcoming the crucial barrier of unstable oxidation/corrosion behavior to obtain long cycle life. Alloy-formula optimization and advanced materials processing have been used to promote a high concentration of active hydrogen-storage sites vital for raising NiMH specific energy. New commercial applications demand fundamentally higher specific power and discharge-rate kinetics. Disorder at the metal/electrolyte interface has enabled a surface oxide with less than 70 ? metallic nickel alloy inclusions suspended within the oxide, which provide exceptional catalytic activity to the metal hydride electrode surface. Received: 14 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Role of interparticle coupling on the tunneling magnetoresistance of granular films in interacting superparamagnetic system

A phenomenological model is employed to explain the departure of H/T scaling of tunneling magnetoresistance (TMR) in Co_0.43(ZrO₂)_0.57 granular films (see [B.J. Hattink, M. García del Muro, Z. Konstantinovi?, X. Batlle, A. Labarta, Phys. Rev. B 73 (2006) 045418]). The model bases on the modification of Langevin function by taking into account the field-dependent correlation length, in the framework of Inoue–Maekawa model. Both TMR–H and TMR–H/T curves can be reproduced very well by this model, indicating that the interparticle coupling plays an important role on TMR in the interacting superparamagnetic systems. The influence of anisotropy and spin polarization of the magnetic particles on TMR–H/T curves is also discussed.     

Measurement of the stress sensitivity of magnetostriction in electrical steels under distorted waveform conditions

Electrical steel laminations used in the construction of transformer cores are subject to stresses introduced during their construction and analysis of the effect of this on the magnetostriction of the lamination has been investigated previously. It has been shown that higher harmonics of magnetostriction are of greater importance than the fundamental when considering transformer noise. Whereas previous studies have concentrated on the magnetostriction harmonics generated by sinusoidal magnetization, this investigation seeks to understand the relationship between harmonics present in the magnetization waveform and those in the magnetostriction waveform. A measurement system has been designed based on a similar principle to one previously described. In this case, a single Labview Virtual Instrument (VI) is used for the control of the applied stress, controlled magnetization and measurement of magnetostriction together with other magnetic parameters such as specific total loss, specific apparent power, permeability, coercivity and remanence. An adaptive digital feedback algorithm is utilized for control of arbitrary waveform which may be constructed from discrete harmonics or read from an input waveform. As well as measuring peak magnetostriction the software utilizes an FFT to calculate the harmonics of magnetostriction at each stress point. The effect of harmonics introduced into the magnetization waveform on the magnetostriction harmonics will be shown at various applied stresses. A harmonic, Harm_B in the flux density waveform is shown to have the effect of producing a dominant harmonic in the magnetostriction given by (Harm_B+1)/2.     

Impacts of amorphous metal-based transformers on energy efficiency and environment

Magnetic properties of a recently developed Fe-based amorphous alloy with a high saturation induction of 1.65 T are reviewed. The increased saturation induction is fully utilized in transformers with reduced magnetic losses, physical sizes and audible noises, minimizing some of the drawbacks of amorphous metal-based transformers based on a currently available amorphous alloy. Impacts of this on the worldwide energy savings and reduction of greenhouse gas emissions are discussed. A recent effort in achieving a saturation induction beyond 1.65 T in nanocrystalline alloys is mentioned.     

Effect of gas pressure on the growth and structure of carbon nanotubes by chemical vapor deposition

The effect of gas pressure on the structure of carbon nanotubes (CNTs) has been systematically investigated in the chemical vapor deposition process. The yield of CNTs (defined as the weight ratio of CNTs vs. catalyst) increases significantly with the gas pressure, reaches 600% at 600 Torr, then decreases with further increase of gas pressure. At low reacting gas pressure the CNTs have completely hollow cores, whereas at high pressure the CNTs have a bamboo structure. The density of the compartments in the bamboo-structured CNTs increases dramatically with the increase of the gas pressure. This result shows that the structure and yield of carbon nanotubes are strongly affected by the growth gas pressure. Received: 10 May 2001 / Accepted: 10 May 2001 / Published online: 20 June 2001     

Effect of temperature on growth and structure of carbon nanotubes by chemical vapor deposition

The effect of temperature on growth and structure of carbon nanotubes (NTs) using chemical vapor deposition (CVD) has been investigated. Iron embedded silica was used to grow NTs in large quantity at various temperatures from 600 to 1050 °C with gas pressure fixed at 0.6 and 760 Torr, respectively. The growth and structure of the NTs are strongly affected by the temperature. At low gas pressure, the NTs are completely hollow at low temperature and bamboo-like structure at high temperature. While at high gas pressure, all the NTs are bamboo-like structure regardless of temperature. The diameter of NTs increases significantly with temperature. At low gas pressure the diameter gets bigger by mainly increasing the number of graphene layers of the wall of NTs, whereas at high gas pressure the diameter gets bigger by increasing both the number of graphene layers of the wall and the inner diameter of the NTs. This result indicates that the growth temperature is crucial in synthesizing NTs with different structures. The findings here are important for realizing controlled growth of NTs for their applications in different fields. Received: 20 November 2001 / Accepted: 21 November 2001 / Published online: 4 March 2002     

Influence of relaxation phenomena in dynamic magnetization processes on magnetic power losses of non-oriented electrical steel sheets

We have reported that the magnetic power losses per magnetizing cycle change with the magnetization pausing time in a constant rate of magnetization change and some relaxation phenomena exist in the magnetization pausing period. We discuss the influence of the relaxation phenomena on the dynamic magnetic power losses and suggest that the relaxation phenomena seems to be causes of the frequency dependence of the magnetic losses and of the variation of losses that cannot be expressed only by magnetization speed dependence of losses.     

Magnetic couplings vs. stress and strain in epitaxial (La, Sr)MnO3

Based on ab initio calculations, we quantify the magnetic couplings and the stress tensor in ferromagnetic Sr-doped LaMnO₃ upon combined application of built-in epitaxial and external uniaxial strains. We suggest YAlO₃ as a substrate suited to change magnetic order in manganite film with practicable external strains. The effect could lead to strain-activated switches based on piezoelectric-piezomagnetic heterojunctions.     

Effect of MgO on the enhancement of ultraviolet photoluminescence in ZnO

By glycine-nitrate combustion route and followed by 900 °C annealing in air, ZnO-MgO nanocomposite with heterojunction-like structures between ZnO and MgO phase was successfully produced. The ultraviolet photoluminescence band from ZnO is enhanced by the incorporation of MgO, as compared to the pure ZnO synthesized via the similar route. The charge transfer required by electronic equilibrium across the junction creates an electron depletion region in ZnO phase, which greatly changes the electron states of visible emission-related defects, as a result, the band-edge emission is enhanced while the visible emission in ZnO is suppressed. This mechanism may provide an effective way to modify the emission property of nanomaterials.     

High-resolution electron microscopy of individual metallofullerene molecules on the dipole orientations in peapods

Electron microscopy with atomic sensitivity enables us to obtain a direct image of the intra-molecular structure of metallofullerenes encapsulated inside single-walled carbon nanotubes. By a comparison of high-resolution images with a simulation to extract the relative atom positions for encaged metal atoms in each molecule, the distribution of the molecular orientations and interactions between adjacent molecules in metallofullerene peapods have been statistically analyzed. The results are suggestive of strong interactions between fullerene–fullerene and fullerene–tube in peapods at room temperature. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6310, E-mail: suenaga-kazu@aist.go.jp     

Interplay of metamagnetic and structural transitions in Ca2-xSrxRuO4

Metamagnetism in layered ruthenates has been interpreted as a novel kind of quantum critical behavior. In an external magnetic field, Ca_2-xSr_xRuO₄ undergoes a metamagnetic transition accompanied by a pronounced magnetostriction effect. In this paper we present a mean-field study for a microscopic model that naturally reproduces the key features of this system. The phase diagram calculated is equivalent to the experimental T-x phase diagram. The presented model also gives a good basis to discuss the critical metamagnetic behavior measured in the system.     

General plasma behavior in the energy relaxation of electrons in highly p-doped semiconductors

We present a systematic study of the dependence of the energy relaxation of photo-excited minority electrons on the doping concentration in highly p-doped GaAs. A nonmonotonic dependence is found in the region where the characteristics of the carrier-carrier interaction changes from plasmon-mediated to quasistatically screened. Using a detailed Monte-Carlo study we are able to attribute this observation to a general property of plasmas at high density. Received: 1st April 1998 / Revised: 6 May 1998 / Accepted: 18 May 1998