A. García-Arribas

Giant magnetoimpedance biosensor for ferrogel detection: Model system to evaluate properties of natural tissue

Thin-film based magnetoimpedance (MI) sensors were used for quantitative determination of the concentration of magnetic nanoparticles (MNPs) in ferrogels. Ferrogels (model systems for biological tissue) were synthesized by radical polymerization of acrylamide in a stable aqueous suspension of γ-Fe2O3 MNPs fabricated by laser target evaporation. MI [FeNi/Ti]3/Cu/[Ti/FeNi]3/Ti sensitive elements were prepared by sputtering. Thorough structural and magnetic studies of MNPs, ferrogels, and multilayered sensitive element insure the complete characterization of biosensor prototype. The MI response of the sensitive element was carefully evaluated in initial state and in the presence of ferrogels with different concentration of iron oxide MNPs from 0 to 2.44 wt. %, which produced systematic changes of the MI in a frequency span of 300 kHz to 400 MHz.

Transition from quasistatic to ferromagnetic resonance regime in giant magnetoimpedance

Detailed measurements of giant magnetoimpedance (GMI) in an amorphous ribbon and a magnetic/nonmagnetic multilayer are presented for frequencies up to 3GHz. Through this frequency range, the transition from quasistatic to dynamic regime of GMI can be clearly distinguished, due to the appearance of the ferromagnetic resonance (FMR). The unambiguous experimental evidence presented mediates between conflicting interpretations of GMI: the ones that assume the existence of FMR even for low frequencies and the ones that consider that it is irrelevant. The frequency at which the transition takes place is shown to be related with the width of the resonance, which is substantially different for both samples. It is concluded that the large increase of permeability caused by the ferromagnetic resonance can be advantageous for GMI-based devices only for samples with a small, very well-defined perpendicular anisotropy.

Influence of magnetization processes and device geometry on the GMI effect

Summary form only given. The use of Giant Magneto-Impedance as a mechanism for sensor design has been stressed in the past few years. Nowadays, some devices are already in the market and many different materials with various shapes have been tested for these sensors. In this paper we review the different geometries (wires, ribbons, films) and structures (either homostructures or sandwiches and plated wires) as well as the different materials (either amorphous or crystalline) and magnetization processes occurring in the GMI elements. The different character of the magnetic anisotropy in crystalline and amorphous alloys determines the type of magnetization process dominant in the material (either wall movement or magnetization rotation) as well as the single peak or double peak behaviour of the GMI. The wall movement results typically in larger permeability values and then in improved GMI ratios. However the damping of the walls rapidly reduces the effective permeability that becomes the one corresponding to that of rotation processes.

Neural network-based micropositioning control of smart shape memory alloy actuators

Shape memory alloys (SMA) are a special kind of smart materials whose dimensions change because of a temperature-dependent structural phase transition. This property can be used to generate motion or force in electromechanical devices and micromachines. However, their highly nonlinear hysteretical stimulus-response characteristic fundamentally limits the accuracy of SMA actuators. The purpose of this work is to design nonlinear control methods suitable for SMA-based positioning applications. To account for the hysteresis effects, inverse hysteresis models are inserted in proportional integral with antiwindup control loops. The inverse hysteresis models are obtained both using a linear phase shift approximation and by training neural networks using experimental data. It is found that neural networks are excellent tools perfectly capable of learning the hysteresis effects. Several control strategies, with and without compensation, are experimented on a laboratory SMA actuator and it is found that neural networks successfully improve the closed-loop response, leading to position accuracies close to the micron.

FeNi-based magnetoimpedance multilayers: Tailoring of the softness by magnetic spacers

The microstructure and magnetic properties of sputtered permalloy films and FeNi(170 nm)/X/FeNi(170 nm) (X = Co, Fe, Gd, Gd-Co) sandwiches were studied. Laminating of the thick FeNi film with various spacers was done in order to control the magnetic softness of FeNi-based multilayers. In contrast to the Co and Fe spacers, Gd and Gd-Co magnetic spacers improved the softness of the FeNi/X/FeNi sandwiches. The magnetoimpedance responses were measured for [FeNi/Ti(6 nm)]2/FeNi and [FeNi/Gd(2 nm)]2/FeNi multilayers in a frequency range of 1–500 MHz: for all frequencies under consideration the highest magnetoimpedance variation was observed for [FeNi/Gd(2 nm)]2/FeNi multilayers.

Giant magnetoimpedance strip and coil sensors

We present noticeable values of giant magnetoimpedance on a cobalt-rich amorphous alloy, together with a new approach to giant magnetoimpedance devices of reduced size. Values as high as 260% of impedance variation as a function of the applied magnetic field are obtained with the samples in the usual form of long ribbons. The benefits of specific thermal treatments in reaching these high values are studied through the magnetic domain structure. In order to reduce the size of the system and therefore extending the practical use of giant magnetoimpedance devices, we present the results obtained in wound ribbons in the form of coils. Though the magnetoimpedance is considerably reduced, it is still large enough to be applicable in sensor devices.

Sensor applications of soft magnetic materials based on magneto-impedance, magneto-elastic resonance and magneto-electricity.

The outstanding properties of selected soft magnetic materials make them successful candidates for building high performance sensors. In this paper we present our recent work regarding different sensing technologies based on the coupling of the magnetic properties of soft magnetic materials with their electric or elastic properties. In first place we report the influence on the magneto-impedance response of the thickness of Permalloy films in multilayer-sandwiched structures. An impedance change of 270% was found in the best conditions upon the application of magnetic field, with a low field sensitivity of 140%/Oe. Second, the magneto-elastic resonance of amorphous ribbons is used to demonstrate the possibility of sensitively measuring the viscosity of fluids, aimed to develop an on-line and real-time sensor capable of assessing the state of degradation of lubricant oils in machinery. A novel analysis method is shown to sensitively reveal the changes of the damping parameter of the magnetoelastic oscillations at the resonance as a function of the oil viscosity. Finally, the properties and performance of magneto-electric laminated composites of amorphous magnetic ribbons and piezoelectric polymer films are investigated, demonstrating magnetic field detection capabilities below 2.7 nT.

Magnetic Properties and Giant Magnetoimpedance of FeNi-Based Nanostructured Multilayers With Variable Thickness of the Central Cu Lead

The magnetoimpedance effect is attractive for thin film-based magnetic sensor applications. Recently a significant progress has been made in the development of appropriate theories, preparation and characterization of MI thin film-based structures. In the present work FeNi(100 nm)/Cu(3.2 nm)]4/FeNi(100 nm)/Cu(LCu)/[FeNi(100 nm)/Cu(3.2 nm)]4/FeNi(100 nm) multilayered structures with open magnetic flux have been prepared by RF-sputtering. Their magnetic properties and MI were studied as a function of the thickness of the central Cu lead. It was shown that the thickness of the Cu lead is an important parameter. The highest sensitivity (≈ 50%/Oe, f=160 MHz) was observed for the sample with a central Cu layer thickness of about a half-thickness of a magnetic layer (LCu ≈ 250 nm). The maximum sensitivity of the real part of the impedance was also obtained for this thickness (≈ 75%/Oe).

Anisotropy field distribution in amorphous ferromagnetic alloys from second harmonic response

We present a new procedure to obtain the anisotropy distribution in a ferromagnetic amorphous alloy. The method is based on the existence of second‐order harmonics in the differential magnetization around the anisotropy field. We derive a mathematical expression relating the strength of the second‐harmonic component and the anisotropy distribution function. An experimental setup is described and the optimum operating conditions discussed. The apparatus is based on the detection of the voltage induced in a coil using a lock‐in amplifier tuned to a frequency double to the exciting one. Results obtained on a stress annealed sample show the existence of well defined values of the anisotropy in addition to a continuous background.

Nanostructured giant magneto-impedance multilayers deposited onto flexible substrates for low pressure sensing.

Nanostructured FeNi-based multilayers are very suitable for use as magnetic sensors using the giant magneto-impedance effect. New fields of application can be opened with these materials deposited onto flexible substrates. In this work, we compare the performance of samples prepared onto a rigid glass substrate and onto a cyclo olefin copolymer flexible one. Although a significant reduction of the field sensitivity is found due to the increased effect of the stresses generated during preparation, the results are still satisfactory for use as magnetic field sensors in special applications. Moreover, we take advantage of the flexible nature of the substrate to evaluate the pressure dependence of the giant magneto-impedance effect. Sensitivities up to 1 Ω/Pa are found for pressures in the range of 0 to 1 Pa, demostrating the suitability of these nanostructured materials deposited onto flexible substrates to build sensitive pressure sensors.