David Ménard

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.

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.

Growth, structure, and properties of epitaxial thin films of first-principles predicted multiferroic Bi2FeCrO6

The authors report the structural and physical properties of epitaxial Bi2FeCrO6 thin films on epitaxial SrRuO3 grown on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The 300nm thick films exhibit both ferroelectricity and magnetism at room temperature with a maximum dielectric polarization of 2.8μC∕cm2 at Emax=82kV∕cm and a saturated magnetization of 20emu∕cm3 (corresponding to ∼0.26μB per rhombohedral unit cell), with coercive fields below 100Oe. The results confirm the predictions made using ab initio calculations about the existence of multiferroic properties in Bi2FeCrO6.

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.

Electroluminescence from single-wall carbon nanotube network transistors.

The electroluminescence (EL) properties from single-wall carbon nanotube network field-effect transistors (NNFETs) and small bundle carbon nanotube field effect transistors (CNFETs) are studied using spectroscopy and imaging in the near-infrared (NIR). At room temperature, NNFETs produce broad (approximately 180 meV) and structured NIR spectra, while they are narrower (approximately 80 meV) for CNFETs. EL emission from NNFETs is located in the vicinity of the minority carrier injecting contact (drain) and the spectrum of the emission is red shifted with respect to the corresponding absorption spectrum. A phenomenological model based on a Fermi-Dirac distribution of carriers in the nanotube network reproduces the spectral features observed. This work supports bipolar (electron-hole) current recombination as the main mechanism of emission and highlights the drastic influence of carrier distribution on the optoelectronic properties of carbon nanotube films.

Double ferromagnetic resonance in nanowire arrays

Microstrip line measurements are used to determine the frequency dependent microwave response of 40 nm diameter CoFeB ferromagnetic nanowire arrays, with external static applied field parallel to the nanowire axis. The ferromagnetic resonance (FMR) response of the wires is obtained for applied fields below and above magnetization saturation. For applied magnetic fields above saturation, a single FMR peak is observed, while below saturation, two sets of peaks are obtained. The two FMR peaks below saturation are associated with two magnetization populations, one for nanowires with upward magnetization and one with downward magnetization. A model based on a Maxwell–Garnett homogenization procedure has been established and used to predict the frequency response of the FMR peaks. There is good agreement between the model and experimental results.

Single-walled carbon nanotube thermopile for broadband light detection.

We designed a thermopile based on a PN doping profile engineered in a suspended film of single-walled carbon nanotubes (SWNTs). Using estimates of the film local Seebeck coefficients, the SWNT thermopile was optimized in situ through depositions of potassium dopants. The overall performances of the thermopile were found to be comparable to state-of-the-art SWNT bolometers. The device is characterized at room temperature by a time response of 36 ms, typical of thermal detectors, and an optimum spectral detectivity of 2 × 10(6) cm Hz(1/2)/W in the visible and near-infrared. This paper presents the first thermopile made of a suspended SWNT film and paves the way to new applications such as broadband light (including THz) detection and thermoelectric power generation.