A. Yelon

Giant magnetoimpedance in a cylindrical magnetic conductor

A rigorous treatment of the giant magnetoimpedance (GMI) in soft magnetic wires is presented. A small-signal approximation is used for a cylindrical magnetic conductor which is saturated along its axis by a static magnetic field. The general analysis of GMI includes a discussion of the influence of different parameters on the GMI and of how the calculation can be extended to nonsaturating fields. The comparison with high frequency impedance spectra of CoFeSiB wires measured with a network analyzer, including the observation of the ferromagnetic resonance peaks, confirms that the proposed model gives a satisfactory explanation for the linear GMI effect over a broad frequency range and opens the way to more refined calculations.

CALCULATIONS OF GIANT MAGNETOIMPEDANCE AND OF FERROMAGNETIC RESONANCE RESPONSE ARE RIGOROUSLY EQUIVALENT

It is simply demonstrated that the giant magnetoimpedance (GMI) response of a plate or ribbon is rigorously equivalent to the response of the same sample in ferromagnetic resonance (FMR) experiment. Thus, all of the solutions for FMR response behavior of metals may be applied to the description of GMI. For situations which have not been studied before, the methods which have been developed over the past 40 years for theoretical description of FMR in metals may be applied to predict the GMI behavior.

Multi-excitation entropy: its role in thermodynamics and kinetics

This review concerns the concept of multi-excitation entropy (MEE) and its consequences. When a fluctuation involving a large number of excitations occurs, for example, when a large activation barrier is overcome, there must be a large entropy associated with this fluctuation. First, the concepts of free energy and entropy, of activated processes and the Arrhenius and Eyring equations are reviewed. The tendency to neglect entropy, whose value is difficult to determine, in modelling kinetic processes, is briefly discussed. We then present a review of the experimental observations of the phenomenon which is variously known as the Compensation Law, the Isokinetic Rule and the Meyer–Neldel Rule (MNR). These observations include examples from chemistry, condensed matter physics, biology and geology. Arguments are then presented for the importance of entropy and particularly of MEE in both kinetics and thermodynamics, when activation energies are large. After a discussion of non-entropic models of compensation, we present results which support the MEE model as an explanation of MNR. The behaviour of systems with low activation energies, or at high temperatures, to which the MEE model does not apply, is then discussed.Several consequences of MEE, including applications to the interpretation of experimental data, particularly the unification of models of dc and ac electrical properties of materials are considered. The high temperature behaviour of systems which obey the MNR at low temperature is then explained, and the idea of a total entropy, of which the MEE is a part, is introduced, as is the correlation between the two empirical parameters encountered in MNR. Finally, these ideas lead to verified predictions of reasonable values of attempt frequencies and cross sections in kinetic processes, which initially appear unreasonable.

S‐passivated InP (100)‐(1×1) surface prepared by a wet chemical process

A highly stable crystalline S‐passivated InP(100) surface has been obtained by a simple wet chemical process, using illumination and heated (NH4)2S solution. Low‐energy electron diffraction studies show a (1×1) diffraction pattern, which persists even after 3 days of exposure to the atmosphere. High‐resolution photoemission studies show that the surface is terminated with a monolayer of sulfur, which forms bridge bonds only to indium. The P 2p core level is identical to that of a vacuum‐cleaved InP surface. A possible structural model is presented.

Theory of longitudinal magnetoimpedance in wires

A theory of giant magnetoimpedance (GMI) in anisotropic magnetic wires is presented. The theory is valid over a broad field and frequency range. The emphasis is put on the GMI response in the low field region, where the wire is not saturated. The behavior of the wire may be described using three magnetic modes of mixed electromagnetic and spin wave character and one nonmagnetic mode, which is uncoupled from the magnetic response of the material. The properties of these four modes are discussed, with particular attention to the exchange-conductivity effects, which play a decisive role at moderate frequency. Application of the theory to real material is examined, with an outline of its applicability and its limitations. Predictions of the model compare well with experimental data on CoFeSiB wire.

Magnetic properties of biotite micas

The magnetic behavior of eight samples of biotite and a lepidomelane was studied in the temperature range 1.5–300 K and analyzed in relation to their chemical composition. The platy morphology of biotite facilitates magnetic measurements with the applied field either in the plane of the sheets (Xe⊥ ) or perpendicular to the plane (Xe∥). At high temperature, the samples obey Curie‐Weiss laws, showing typical paramagnetic behavior which is due to the presence of both Fe2+ and Fe3+ ions. However, the susceptibilities are anisotropic (χe⊥/χe∥≂1.25), because of a trigonal contribution to the crystal field which affects only the ferrous ions. Formulae are given which permit the ferrous Curie‐Weiss parameters to be derived from the measured susceptibility. At low temperature (T<10 K), three of the biotite samples containing high concentrations of iron (17 to 27 wt. %) undergo an antiferromagnetic transition. The magnetic structure consists of octahedral sheets of iron ions with their moments ferromagnetically co...

Ferromagnetic Nanowire Metamaterials: Theory and Applications

An overview of ferromagnetic nanowire (FMNW) metamaterials is presented. First, FMNW metamaterials are placed in the historical context of antique composites and 20th Century artificial dielectrics, and presented as an example of second-generation metamaterials following the microstructured metamaterials developed in the first part of the decade. Next, the fabrication processes of FMNW metamaterials and subsequent planar devices are detailed. It is then shown how the geometrical properties of the FMNW structure, such as the wire diameter and the wire nanodisk thicknesses, determine the dc and RF responses of the material. Upon this basis, the modeling of the metamaterial is presented, using a two-level approach where the microscopic (with respect to the wires) susceptibility is derived by solving the Landau-Lifshitz equation and the macroscopic (metamaterial) permittivity and permeability tensors are obtained by effective medium theory. Next, a review of FMNW microwave devices, such as circulators, isolators, and phase shifters, is provided, and the example of an FMNW dual-band edge-mode isolator is studied. Finally, spintronic effects and applications of FMNW metamaterials, such as dc to RF generators and detectors based on the spin-torque transfer phenomenon, are reviewed.

Magnetic anisotropy in arrays of Ni, CoFeB, and Ni/Cu nanowires

An effective field model based on intrawire and interwire dipolar interactions has been developed in order to describe the magnetic anisotropy in arrays of homogeneous and multilayer nanowires. Variable angle ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) characterization techniques were used to determine the effective interaction field acting on Ni, CoFeB, and Ni/Cu nanowires. FMR spectra are well described by a rigid magnetization model and VSM data are in rough agreement with a mean longitudinal field model. FMR and VSM values of the effective fields are mutually consistent and in fair agreement with the values calculated with the model. The results show that the anisotropy of our arrays is strongly dominated by the dipolar interactions.

High frequency impedance spectra of soft amorphous fibers

Giant magnetoimpedance (GMI) spectra of soft amorphous magnetic fibers, measured in the 1 kHz–1.2 GHz frequency range, and GMI responses, measured in the field range of ±120 Oe, have reinforced the assumption that linear giant magnetoimpedance and ferromagnetic resonance (FMR) have the same physical origin. The samples, NiCo-rich, CoFe-rich, and Metglas-type fibers, 30–40 μm in diameter, were cast by melt extraction. Their impedance has been measured up to 13 MHz, in the presence of a magnetic field, using an impedance analyzer. These measurements have been extended up to 1.2 GHz by using a network analyzer. The reflection coefficient of a shorted coaxial line whose inner conductor was replaced by a magnetic fiber was measured, and the input impedance per unit length of this line was then calculated. The two impedances above are equivalent and their spectra show a behavior associated with FMR: the real part of the impedance peaks at a frequency where the imaginary part passes through zero.

Fluoropolymer surface modification for enhanced evaporated metal adhesion

Adhesion of evaporated Cu to Teflon PFA (polytetrafluoroethylene-co-perfluoroalkoxy vinyl ether) was greatly enhanced by plasma pretreatment. The efficiency of the treatment decreased in the following order: N2 > O2 > (N2 + H2) > (O2 + H2) > H2. X-ray photoelectron spectroscopy (XPS) showed the loss of fluorine and the incorporation of oxygen and nitrogen at the polymer surface. Among the gases, H2 was found to be the most efficient for fluorine elimination, and (N2 + H2) for surface functionalization. Based on this investigation, it is proposed that Cu reacts with both oxygen and nitrogen to form, respectively, Cu-O and Cu-N bonds at the interface but no reaction occurs with carbon and fluorine. While greater enhancement in polymer surface wettability and stronger interfacial reactions can account for the higher performance of N2 over O2 in improving adhesion, these effects cannot explain the lower efficiency of H2. Several possibilities are discussed, including surface cleaning, oxygen incorporation and...