Reinhard Knöchel

Exchange biasing of magnetoelectric composites

Magnetoelectric composite materials are promising candidates for highly sensitive magnetic-field sensors. However, the composites showing the highest reported magnetoelectric coefficients require the presence of external d.c. magnetic bias fields, which is detrimental to their use as sensitive high-resolution magnetic-field sensors. Here, we report magnetoelectric composite materials that instead rely on intrinsic magnetic fields arising from exchange bias in the device. Thin-film magnetoelectric two-two composites were fabricated by magnetron sputtering on silicon-cantilever substrates. The composites consist of piezoelectric AlN and multilayers with the sequence Ta/Cu/Mn(70)Ir(30)/Fe(50)Co(50) or Ta/Cu/Mn(70)Ir(30)/Fe(70.2)Co(7.8)Si(12)B(10) serving as the magnetostrictive component. The thickness of the ferromagnetic layers and angle dependency of the exchange bias field are used to adjust the shift of the magnetostriction curve in such a way that the maximum piezomagnetic coefficient occurs at zero magnetic bias field. These self-biased composites show high sensitivity to a.c. magnetic fields with a maximum magnetoelectric coefficient of 96 V cm(-1) Oe(-1) at mechanical resonance.

Low damping resonant magnetoelectric sensors

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.

Noise Performance of Magnetometers With Resonant Thin-Film Magnetoelectric Sensors

Sensors based on materials with a giant magnetoelectric (ME) effect may be used to measure biomagnetic fields at room temperature. It is necessary to know the noise behavior of the whole detection unit. The noise level of a thin-film ME sensor was measured at room temperature, and suitable types of low-noise amplifiers were investigated. Noise measurements were carried out at room temperature. Results show a sensitivity value of 5.4 pT/ √Hz at a resonance frequency of 330 Hz. Furthermore, the signal-to-noise ratio was investigated in order to improve the sensitivity of the sensor.

Composition of foods including added water using microwave dielectric spectra

Abstract The added water content of fresh and untreated pork, poultry, fish and prawns was adjusted either by dipping in polyphosphate and salt solutions of known concentrations for controlled periods, or by injection (pork) with polyphosphate and salt solutions. Mixtures were prepared from treated and untreated fish and other foods (milk and flour). Liquid uptake was determined by the weight gain of samples. The proximate composition of the samples was determined: water, fat, protein, NaCl and phosphorus (for polyphosphate content). Complex dielectric spectra of each product were measured at known temperatures and at 31 frequencies in the range 0.2–12 GHz using an automatic network analyser (ANA) and a 3.0 mm open-ended coaxial sensor. The spectra were subjected to various procedures. • Principal component analysis (PCA) using the individual complex components. • Regression of the composition data against the principal components to obtain prediction formulae for composition including liquid uptake (internal cross-validation). • Regression of the composition data against raw spectral data and against other composition variables to obtain similar formulae. In order to design a simpler instrument, the effect on accuracy was studied of reducing the number of frequencies in the spectrum and its range. The slight loss of accuracy engendered by using only five or six frequencies was acceptable. The accuracy of the method in predicting liquid uptake and composition was good. Using one of the compositional variables in the calibration made it equivalent to accuracy obtained by proximate analysis, which was the limiting factor. Measurements on solutions of glycerol, NaCl and water with precisely known composition demonstrated that the intrinsic accuracy of the instrument was far better. A prototype instrument was built and validated using samples of prawns and herring.

Giant magnetoelectric effect in vacuum

Magnetoelectric (ME) thin film cantilever type sensors made of AlN and FeCoSiB are operated in vacuum, reducing air damping and thus increasing the ME coefficient and improving the limit of detection (LOD) for ac-magnetic fields. Depending on the sensor geometry, the response is increased by a factor of 5 resulting in a ME coefficient of 20 kV/cmOe at 152 Hz and by a factor of 11 with 12 kV/cmOe at 4.7 kHz and an improvement in LOD by an order of magnitude. Modelling these cantilevers reveals dominant contributions of viscoelastic and molecular damping above and intrinsic damping below 10−2 mbar, respectively.

New density-independent moisture measurement methods using frequency-swept microwave transmission

This paper presents density independent moisture measurement procedures using frequency-swept microwaves. By proper processing changes of attenuation and phase of a transmitted signal versus frequency, calibration curves can be easily generated on-site, without any need of varying density or layer thickness of moist material under test. It is shown, that the new measurement methods are usable up to very high moisture contents of more then 40%, in contrast to previous methods, which are only usable below approximately 25%.

Fully integrable magnetic field sensor based on delta-E effect

A fully integrable magnetic field sensor based on magnetic microelectromechanical systems is presented. The approach yields high application potential since it is compatible with standard micromachining techniques, operates at room-temperature, and provides high bandwidth and vector field capability. The demonstrator presented in this work consists of a tipless commercial atomic force microscope cantilever which is coated with an amorphous thin film layer of (Fe90Co10)78Si12B10. Amplitude and frequency of magnetic fields are measured via the modulation of the oscillation of the microcantilever via the delta-E effect of the FeCoSiB coating.

Giant magnetoelectric effect at low frequencies in polymer-based thin film composites

A polymer-based magnetoelectric 2-2 composite was fabricated in a thin film approach by direct spin coating of polyvinylidenefluoride-co-trifluoroethylene onto a Metglas substrate without the usage of an adhesive for the mechanical coupling between the piezoelectric and magnetostrictive materials. For a prototype single-sided clamped cantilever, a magnetoelectric coefficient as high as 850 V cm−1 Oe−1 is observed at its fundamental bending mode resonance frequency at 27.8 Hz and a detection limit of 10 pTHz−1/2 at its second bending mode resonance frequency at 169.5 Hz.

Resonant microwave sensors for instantaneous determination of moisture in foodstuffs

Abstract Open microwave resonators are very well suited for online moisture monitoring during the production of foodstuffs. In order to allow for rapid measurements, the required information is derived from the dielectric behaviour of the material. Unlike for example the method of Karl Fischer titration, the dielectric microwave method determines the moisture content indirectly and is thus dependent on density changes. The present paper outlines special features of open resonators for density independent moisture measurement. It is demonstrated, how unfavourable design of the window for coupling out the fringing field of the open resonator leads to non-linear behaviour of the characteristics of resonant frequency and reciprocal quality factor vs density and moisture changes. Several engineered open microwave moisture sensors are presented and discussed. More in detail, a surface sensor, a surface sensor equipped with a small container on top, a self contained hand-held surface moisture sensor and a grooving-probe as well as a multi-hole sensor are shown. The electronics and the signal processing of the sensors are briefly described. Finally experimental moisture measurements and a density independent calibration are given for rolled oats. Moisture measurements on whole kernels of oats, barley and wheat demonstrate the only weak dependence of the microwave moisture measurement approach on the special type of material.

Magnetoelectric thin film composites with interdigital electrodes

Magnetoelectric (ME) thin film composites on silicon cantilevers are fabricated using Pb(Zr0.52Ti0.45)O3 (PZT) films with interdigital transducer electrodes on the top side and FeCoSiB amorphous magnetostrictive thin films on the backside. These composites without any direct interface between the piezoelectric and magnetostrictive phase are superior to conventional plate capacitor-type thin film ME composites. A limit of detection of 2.6 pT/Hz1/2 at the mechanical resonance is determined which corresponds to an improvement of a factor of approximately 2.8 compared to the best plate type sensor using AlN as the piezoelectric phase and even a factor of approximately 4 for a PZT plate capacitor.