Francisco Alves

The BNM Watt balance project

A new watt balance project is now developed by the BNM: the general configuration and the main parts of the experimental set-up in development are presented. Its aim is to contribute to the international effort in monitoring the kilogram towards a new definition of the mass unit with an accuracy of 10-8 or better

Magnetic circuit design for the BNM Watt balance experiment

BNM has started since 2001 the development of a watt balance experiment. The authors present in this paper the design of the magnetic circuit intended to equip it

Anisotropy and domain patterns of flash stress-annealed soft amorphous and nanocrystalline alloys

Abstract Domain structures and stress-induced anisotropy in some soft ferromagnetic materials are reported. Stress-Joule heating performed on amorphous Co66Fe4B12Si16Mo2 induces an anisotropy perpendicular to the ribbon axis, as in nanocrystalline FeSiBCuNb alloys. However, in the case of FeZrB–Cu–Nb systems, large longitudinal domains are observed. We think that the origin of this anisotropy comes from the so-called back stress effect as previously proposed by Herzer. As magnetostrictions of both α-Fe and α-FeSi crystallites are negative, the frozen-in stress exerted on α-Fe nanograins is expected to be compressive-type. We suggest that this surprising behaviour is related to more pronounced creep because of the smaller hardness of α-Fe nanograins.

Feasibility of a Giant MagnetoImpedance Sandwich magnetometer for space applications

Space experiments require very efficient magnetic field sensors. Until now, fluxgate remains the most competitive way to measure weak magnetic field in space in spite of some drawbacks, like their offset drift. Since few years, giant magneto-impedance (GMI) effect were intensively studied and have shown promising results. Thus, we present a new GMI sandwich magnetometer for space application. The GMI sandwich transducer was fabricated with nanocrystalline alloy films and copper film. Very low noise amplifier is combined with an AM demodulation to extract weak dc-low frequency magnetic field from carrier. Noise equivalent magnetic induction (NEMI) is improved thanks to an experimental optimization method enables to increase the intrinsic sensitivity and use of a ratio transformer optimization to reduce noise electronic. We have obtained a flat transfer function in the frequency range [DC-1 kHz]. In this broad range, sensitivity is equal to 1000 V/T and NEMI reaches few nT/sqrt(Hz).

Ultrasoft Finemet thin films for magneto-impedance microsensors

FeCuNbSiB thin films have been deposited using RF sputtering. Characterizations have shown that oxygen contamination and residual stress are mainly responsible for magnetic hardening. The sputtering and annealing conditions have been optimized and films with coercive field as low as 10 A m-1 (0.125 Oe) have been achieved. In addition, the influence of film thickness on the magnetic properties has been studied. Thus, magnetic field microsensors based on the magneto-impedance effect have been fabricated by stacking up Finemet/copper/Finemet films. The highest sensitivity (4000 V/T/A) is reached for 750 nm thick films. It is in the same range as cm-sized macroscopic devices realized using 20 μm thick ribbons.

Magnetoelastic effect in soft amorphous and nanocrystalline FeCuNbSiB thin films

Thin films of FeCuNbSiB have been sputtered on SiO2/Si substrates with thickness varying from 50 nm to 1.8 microns, then annealed at temperature ranging from 200 to 500 °C. The coercivity globally decreases with the film thickness down to 100 A m−1 for as-deposited samples and 10 A m−1 for annealed samples. However, it appears a local maximum for thickness close to 1 micron (i.e. the exchange length), regardless the annealing temperature. This behavior is supposed to be related to a modification of the magnetic domain structure, as the RAM applied to thin films and the dependence of the transitions points (glass state temperature Tg and crystallization temperature Tx) on the film dimension are monotonous. In addition, the films have been annealed under magnetic field and strong anisotropy energy has been reached, in the range of 150 J m−3.

Improvement of HF RFID Tag Detection With a Distributed Diameter Reader Coil

This letter focuses on 13.56 MHz high-frequency radio frequency identification (RFID) in the case of small tags detection, with an effective area below 1 cm2. In such an identification system, based on load modulation principle, the magnetic coupling coefficient k and quality factor of the RFID reader coil are the key parameters. The main goal of this letter is to improve the detection of small tags over a given surface of 10 × 10 cm2 by modifying the reader coil structure, and consequently the coupling coefficient k. Several coil designs are compared experimentally by distributing the diameters of their turns among three possible values. The design of the coils is based on empirical formulas that are in good agreement with experimental measurements. Electromagnetic simulations are performed to confirm the magnetic field distribution of the different designs. The results show that distributed diameter coil (DDC) as RFID reader coil is clearly efficient in this context of the RFID detection. The DDC structures determine the k factor, and, as k is low, the quality factor Q is a second parameter that can improve, in a second step, the RFID detection performances in function of the tag position and orientation.

Influence of Oxygen Contamination on Magnetic Properties of Amorphous and Nanocrystallized FeCuSiNbB Thin Films

Thin films of amorphous FeCuSiNbB alloy have been deposited by RF sputtering with various deposition rates. The bulk oxygen content has been characterized using EDS and XPS. Its dependence on deposition rate shows that water vapour in the sputtering chamber is at the origin of the contamination. It allows also estimating the adsorption coefficient of the oxygen on the sample to be around 15 % at 350 K. The magnetic hardness and the resistivity increase with the contamination in oxygen. In devitrified films, this increase is also related to an enrichment of the residual amorphous matrix in oxygen.

Performances of a Newly High Sensitive Trilayer F/CU/F GMI Sensor

We have selected stress-annealed nanocrystalline Fe-based ribbons for magnetic/copper/magnetic sensors exhibiting high magneto-impedance ratio. Longitudinal magneto-impedance DeltaZ/Zsat reaches 400% at 60 kHz and longitudinal magneto-resistance DeltaR/Rsat increases up to 1100% around 300 kHz.

Fabrication and AC characterization of magneto-impedance microsensors for alternating magnetic field measurement

Multilayered magneto-impedance microsensors (Finemet/Copper/Finemet) were elaborated by microfabrication pro- cess using bi-layers lift-off method. A post-annealing step was carried out at 300 ◦ C for 1h under magnetic field, which led to induce a longitudinal or transversal anisotropy in the magnetic films. A method based on a double amplitude demodulation was proposed for the sensitivity characterization corresponding to the AC magnetic field measurement capability. According to these alternating magnetic field characterizations, the sensitivity, the linearity and the bandwidth of the microsensor were obtained. The highest sensitivity is around 4200 Ω/T and the measurable sensor bandwidth reaches up to 300 kHz. The sensor presented no hysteresis since a DC bias field larger than anisotropy field is applied.