Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> A complete theoretical analysis is presented of the operation of a magnetoelastic amorphous metal low-frequency magnetic field sensor. This directional magnetometer is a hybrid device consisting of a piezoelectric plate, a field-annealed amorphous metal ribbon, and a viscous fluid, and it exhibits a low-frequency magnetic field detection level of 8.0 pT/ square root Hz at 1.0 Hz. The sensor may also be configured as a first- and second-order gradiometer. The device analysis focuses on the influence of the constituent materials on the magnetometer performance, identifying potential noise sources and optimal design parameters. This analysis may be applied to a variety of magnetoelastic amorphous metal sensors, e.g. stress, strain, and torque sensors, and is also useful in research concerning fundamental aspects of magnetoelasticity. Experimental data are presented demonstrating the performance of magnetometers constructed with amorphous metal ribbons exhibiting striped and closure domain structures. > <s> BIB001 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> The signal of magnetic sensors based on resonant cantilevers comprised of elastically coupled piezoelectric and magnetostrictive materials increases as the damping decreases. Here, we demonstrate that air damping which normally is suppressed by evacuation can also be substantially reduced by lowering the resonance frequency. We show that a Si-cantilever structured to include a seismic mass features a resonant magnetoelectric coupling coefficient of 1.8 kV/cmOe at 330 Hz in air. <s> BIB002 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> An anisotropic magnetorheological composite formed by dispersions of silver-covered magnetite microparticles (Fe3O4@Ag) in polydimethylsiloxane (PDMS) displaying electrical conduction only in one preferred direction is presented. A set-up for applying and detecting electrical conduction through the composite is described and applied to characterize the behavior of the system in on-off commutation cycles. The composite is obtained by loading the polymer with relatively low concentration of fillers (5% w/w of the total weight) and curing it in the presence of a uniform magnetic field. The fillers appear in the final composite as an array of needles, i.e. pseudo-chains of particles aligned in the direction of the magnetic field. Using Fe3O4 nanoparticles (13 nm) it is possible to obtain cured composites in a superparamagnetic state, that is, without magnetic hysteresis at room temperature. Hysteresis is not found in the elastic properties either; in particular, Mullins effects (change of physical properties after the first strain-stress cycle) were not observed. No measurable transversal electrical conduction was detected (transversal resistivity larger than 62 Mohms∙cm). Thus, significant electrical conductivity is present only between contact points that are exactly facing each other at both sides of the composites in the direction parallel to the needles. The I-V curves in that direction have ohmic behavior and exhibit both piezoresistance and magnetoresistance, that is, the electrical conductivity in the direction parallel to the pseudo-chains increases when a pressure (i.e. compressive stress) is applied at constant magnetic field and/or when a magnetic field is applied at constant pressure. The materials do not exhibit magnetoelectric or piezoresistive hysteresis. These characteristics illustrate the high potentiality of these systems in elastic connectors where electrical conduction can be varied by external mechanical or magnetic forces. <s> BIB003 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> Abstract For high spatial resolution, deep body region measurement of biomagnetic signals in the pico- and femtotesla regime, dense vector-field sensor arrays are required. Current state-of-the-art sensors, like SQUIDs, are too bulky and not applicable. In this paper for the first time MEMS sensors based on magnetoelectric (ME) composites with vacuum encapsulation using wafer-level packaging technology are presented. Silicon device and cap wafers with 150 mm diameter were fabricated using micromachining processes and bonded afterwards for hermetic sealing. The device wafer contains rectangular cantilever beams with a stack composed of SiO 2 /Pt/AlN/FeCoSiB, Au metal-bond frames and conduction lines. The lateral dimensions of the cantilever were 200 μm × 900 μm with an overall thickness of 7.8 μm. The cap wafer comprised the vacuum cavities, alignment marks and Au/Sn metal-bond frames. For avoiding degradation of the temperature sensitive amorphous magnetic material [(Fe 90 Co 10 ) 78 Si 12 B 10 ] a special developed low-temperature, hermetic sealed transient-liquid phase bonding process was used to package the sensor devices. Characterization of a sensor showed a giant ME coefficient of 2390 (V/m)/(A/m) and a corresponding sensitivity of 3800 V/T in resonance at 7.1 kHz. A minimum resolution of 30 pT and noise levels as low as 27 pT/Hz 1/2 have been reached in resonant operation. <s> BIB004 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> Abstract Theory and results of a systematic study on the nature of nonlinear magnetoelectric (ME) interactions in layered ferromagnetic and ferroelectric composites are discussed. The model that considers the nonlinearity associated with magnetostriction of the ferromagnet is to result in (i) a dc component and (ii) frequency doubling when the composite is subjected to an ac magnetic field. In the presence of two ac magnetic fields of different frequencies, nonlinear effects give rise to generation of ME voltages at the sum and difference of the fields frequencies. The efficiencies of nonlinear ME interactions are shown to be a function of the second derivative of the magnetostriction with respect to the bias magnetic field. The predictions of the model are compared with data for bilayers of lead zirconate titanate (PZT) and ferromagnetic layers with wide variations in saturation magnetostrictions and saturation magnetic fields, i.e., an amorphous ferromagnetic (AF) alloy, Ni, or permendur. Under linear excitation conditions an enhancement in the ME voltage is measured when the ac magnetic field is applied at the acoustic mode frequencies. Under nonlinear excitation conditions the mechanical deformation and the ME response occur at twice the excitation frequency and the AF-PZT composite shows a much higher nonlinear ME effects. In addition, the AF-PZT shows an efficient frequency mixing than the samples with Ni or permendur when subjected to two ac magnetic fields. The frequency mixing is shown to be of importance for magnetic field sensor applications. <s> BIB005 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (d) Hybrid Magnetometers-Magnetoelectric Sensors <s> Abstract Composite material constituted by Fe micro-particles homogeneously dispersed in a silicone matrix, at a volume concentration slightly above the percolation threshold but separated by a thin silicone layer, was produced. The particle magnetic softness and their average size, have been properly improved with respect to previous investigations in order to maximize the piezo-resistive and the piezo-magnetic effects. The optimal combination of magneto-elasticity and piezo-resistivity enables to achieve a record value of magneto-piezo-resistivity sensitivity. An analytical model is proposed to simulate the theoretically expected behavior of electric resistance vs. the applied induction field gradient, so to predict the magneto-piezoresistive response and explain the obtained material tailoring. The experimental results have been in good agreement with the theoretically predicted behaviors, so validating the employed model and the interpretation of the phenomenon. A simple basic application in position sensing is also reported. The analytical model presented in this paper has demonstrated its potentiality to project further improvements, while the experimental results allow for different innovative applications. <s> BIB006
In the last decade a set of hybrid magnetic sensors has been developed of piezoelectric and ferromagnetic material as an alternative to traditional sensors BIB001 , they possess competitive characteristics for the present-day market. Piezoelectric materials respond to electric excitation with relative size changes and ferromagnetic materials respond likewise in the presence of a magnetic field. The range of these relative changes depends on the material's intrinsic characteristics. Moreover, the size of a ferromagnetic substance only changes when magnetization occurs by means of rotation; when it occurs through wall movements of 180° there is no global change of size since the latter separate already saturated domains. In order to produce in a magnetostrictive ferromagnetic substance a significant change of size, it is convenient to put it previously under a thermal treatment that positions the easy magnetization direction perpendicularly to the longitudinal axis (axis in which the sensor is magnetized). When a piezoelectric substance and a highly magnetostrictive ferromagnetic substance are united mechanically, the size changes produced in the piezoelectric material resulting from an electric field are transmitted to the ferromagnetic substance, affecting its inner structure and hence, its magnetic behaviour. Logically the opposite effect is also possible. This way magnetic type changes take place with electrical excitation or vice versa. These cross effects can be used to detect external magnetic or electric fields and to measure some qualities of the hybrid's constituting materials. In order to detect weak magnetic fields by means of a piezoelectric ferromagnetic hybrid, various settings are possible. The most common configuration is the following: the piezoelectric material is a support with quite a bigger size than the ferromagnetic material, excited by an electric field outside its longitudinal resonance frequency (at this frequency the relative size changes on the longitudinal axis are considerably greater than at any other frequency). Ferromagnetics have a high level of magnetostriction, so size changes affect them a lot. They are also magnetically soft, this means that they are easily magnetized in presence of weak fields. Both materials are united by a viscous fluid that allows vibration transmission of the piezoelectric material, avoiding the mechanic tensions, typical of rigid glue, in both of them, but especially in the ferromagnetic material which is the thinnest one. So, each time the piezoelectric base stretches or contracts, an anisotropy axis is induced in the ferromagnetic material, this is, a privileged axis where magnetization tends to accommodate to minimize energy. For example, if the ferromagnetic material has a positive magnetostriction, each time the piezoelectric material stretches, the magnetization tends to follow a longitudinal direction and each time it contracts, the magnetization will occur in any direction perpendicular to the longitudinal axis. In the presence of a magnetic field, the ferromagnetic material will magnetize more easily when the piezoelectric material stretches and it will magnetize less easily when the piezoelectric material contracts. These size changes happen at the resonant frequency (tens of kHz), accordingly, if a secondary coil is rolled around the core, the signal induced is a sine wave with an amplitude that depends on the piezoelectric vibration range, on the external magnetic field range and on various other parameters of both materials. Using these data, a magnetic field sensor with excellent features can be made. With this kind of sensor, magnetic fields as small as 100 pT can be measured, thanks to its great sensitivity and low noise level. The frequency range in which the hybrid sensor operates extends to frequencies close to the resonance of piezoelectric materials. It is also highly stable in time, being its stability limited by thermal effects that damage the interface's sticky fluid. Furthermore, it is small of size, easy to produce, low cost and a low consumption (of the order of mW). More recently, some authors have developed different magnetoelectric (ME) sensors BIB005 BIB006 BIB003 . In [40] a theory for non-linear ME effects in planar ferromagnetic-piezoelectric composite structures is described. It was shown that non linearity of magnetostriction results in the generation of dc voltage across the piezoelectric, doubling of ac magnetic field frequency, and generation of ac voltages with sum and difference frequencies. In BIB004 we can see a ME sensor built by encapsulation in vacuum. This micro-sensor has a sensitivity of 3800 V/T and a resolution of 30 pT, which is comparable to larger sensors ME BIB002 and magnetic biosensors.