A. Piorra

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.

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.

Self-poled Pb(Zr,Ti)O3 films with improved pyroelectric properties via the use of (La0.8Sr0.2)MnO3/metal substrate heterostructures

Pb(Zr,Ti)O3 (PZT) thin films were deposited on different substrate heterostructures including platinized silicon, stainless steel and Ni-based alloy foils. A buffer layer of (La0.8Sr0.2)MnO3 (LSMO) between PZT and substrate was used. The pyroelectric coefficients were determined using low frequency sinusoidal temperature waves. It is demonstrated that PZT films deposited on metallic foils exhibit high pyroelectric coefficients of up to 760μC∕Km2 in the unpoled state, whereas the films deposited on platinized substrates were found to have pyroelectric coefficients in the range of 30μC∕Km2. These results are explained in terms of elemental diffusion from the substrate together with stressed states of the films.

Low temperature aluminum nitride thin films for sensory applications

A low-temperature sputter deposition process for the synthesis of aluminum nitride (AlN) thin films that is attractive for applications with a limited temperature budget is presented. Influence of the reactive gas concentration, plasma treatment of the nucleation surface and film thickness on the microstructural, piezoelectric and dielectric properties of AlN is investigated. An improved crystal quality with respect to the increased film thickness was observed; where full width at half maximum (FWHM) of the AlN films decreased from 2.88 ± 0.16° down to 1.25 ± 0.07° and the effective longitudinal piezoelectric coefficient (d33,f) increased from 2.30 ± 0.32 pm/V up to 5.57 ± 0.34 pm/V for film thicknesses in the range of 30 nm to 2 μm. Dielectric loss angle (tan δ) decreased from 0.626% ± 0.005% to 0.025% ± 0.011% for the same thickness range. The average relative permittivity (er) was calculated as 10.4 ± 0.05. An almost constant transversal piezoelectric coefficient (|e31,f|) of 1.39 ± 0.01 C/m2 was measured for samples in the range of 0.5 μm to 2 μm. Transmission electron microscopy (TEM) investigations performed on thin (100 nm) and thick (1.6 μm) films revealed an (002) oriented AlN nucleation and growth starting directly from the AlN-Pt interface independent of the film thickness and exhibit comparable quality with the state-of-the-art AlN thin films sputtered at much higher substrate temperatures.

Origin of hysteretic magnetoelastic behavior in magnetoelectric 2-2 composites

The local magnetization behavior of the magnetostrictive phase of ferromagnetic/piezoelectric magnetoelectric composites is compared to the hysteretic response using advanced magneto-optical imaging. Local magnetoelastic relaxation leads to the formation of magnetization modulated branched domain structures in the magnetic phase. This results in a complex field response governed by interlocking domain processes. An interrelation of magnetic domain formation and the piezomagnetic response is derived, revealing the origin of the hysteretic magnetoelectric response. As a result, domain wall induced effects lead to a reduction of magnetoelectric signal. Controlling the magnetic domain formation processes is the foundation for reversible magnetoelectric behavior.

Electrically modulated magnetoelectric sensors

Magnetoelectric thin film composites have demonstrated their potential to detect sub-pT magnetic fields if mechanical resonances (typically few hundred Hz to a few kHz) are utilized. At low frequencies (1–100 Hz), magnetic field-induced frequency conversion has enabled wideband measurements with resonance-enhanced sensitivities by using the nonlinear characteristics of the magnetostriction curve. Nevertheless, the modulation with a magnetic field with a frequency close to the mechanical resonance results in a number of drawbacks, which are, e.g., size and energy consumption of the sensor as well as potential crosstalk in sensor arrays. In this work, we demonstrate the feasibility of an electric frequency conversion of a magnetoelectric sensor which would overcome the drawbacks of magnetic frequency conversion. This magnetoelectric sensor consists of three functional layers: an exchange biased magnetostrictive multilayer showing a high piezomagnetic coefficient without applying a magnetic bias field, a non...

Inverse bilayer magnetoelectric thin film sensor

Prior investigations on magnetoelectric (ME) thin film sensors using amorphous FeCoSiB as a magnetostrictive layer and AlN as a piezoelectric layer revealed a limit of detection (LOD) in the range of a few pT/Hz1/2 in the mechanical resonance. These sensors are comprised of a Si/SiO2/Pt/AlN/FeCoSiB layer stack, as dictated by the temperatures required for the deposition of the layers. A low temperature deposition route of very high quality AlN allows the reversal of the deposition sequence, thus allowing the amorphous FeCoSiB to be deposited on the very smooth Si substrate. As a consequence, the LOD could be enhanced by almost an order of magnitude reaching 400 fT/Hz1/2 at the mechanical resonance of the sensor. Giant ME coefficients (αME) as high as 5 kV/cm Oe were measured. Transmission electron microscopy investigations revealed highly c-axis oriented growth of the AlN starting from the Pt-AlN interface with local epitaxy.

Adaptive Acoustic Noise Cancellation for Magnetoelectric Sensors

Sensors based on the magnetoelectric (ME) effect have the potential to be genuine alternatives for measuring bio-magnetic signals. Unfortunately, the sensor structure usually inhibits the problem that several non-magnetic types of noise couple mechanically into the sensor: in this contribution, we will focus on undesired acoustic coupling. Therefore, an adaptive cancellation approach based on a computationally efficient gradient estimation algorithm with a pseudo-optimally control scheme is proposed. The approach is using a microphone as a noise reference sensor and is implemented in real time. An evaluation in terms of measurements is performed inside a magnetically shielded chamber. For a particular scenario, which is characterized by double excitation, an algorithm with binary control-scheme improves the signal-to-noise ratio (SNR) only by around 4dB. If the proposed control scheme is used instead, an improvement of the SNR of around 13dB is achieved.

Thermal-Mechanical Noise in Resonant Thin-Film Magnetoelectric Sensors

Thin-film magnetoelectric sensors, i.e., composites of magnetostrictive and piezoelectric materials, are able to measure very low magnetic flux densities in the picotesla range. In order to further improve the limit of detection it is of high importance to understand and quantify the relevant noise sources. In this paper, a common model for the deflection noise in vibrational structures is applied to the cantilever structure of resonant magnetoelectric sensors. By means of deflection and noise measurements the existence of thermal-mechanical noise even in sensor structures with a size in the centimeter range is proven. Based on these findings a noise equivalent circuit is suggested which allows not only the distinction between the impact of different sensor-intrinsic noise sources and also the involvement of the preamplifier noise. We found that the thermal-mechanical noise is the dominant noise source if direct signal detection is performed at the first bending resonance frequency of the sensor. However, this kind of noise is not the limiting influence when applying magnetic frequency-conversion techniques.

(La0.8,Sr0.2)MnO3∕Ti-metal foil substrate heterostructure effects on the ferroelectric and piezoelectric properties of lead zirconate titanate thin films

The ferroelectric and piezoelectric properties of lead zirconate titanate thin films deposited on a (La0.8,Sr0.2)MnO3(LSMO)∕Ti-metal foil heterostructure are reported. The results are compared to those of films of similar thickness deposited on platinized silicon substrate. It is shown that the films deposited on LSMO∕Ti are characterized by square ferroelectric and piezoelectric hysteresis loops with high remnant polarization and strain of 26μC∕cm2 and 0.8%, respectively. The effective piezoelectric strain constant, d33, obtained for films deposited on platinized silicon amounts to 103pm∕V. The piezoelectric behavior of the film deposited on LSMO∕Ti is analyzed in terms of a unimorph bending actuator. The extracted effective piezoelectric strain constant, d33, amounts to approximately 600pm∕V.