Christine Kirchhof

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.

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.

Phase modulated magnetoelectric delta-E effect sensor for sub-nano tesla magnetic fields

We present a resonant micromechanical magnetic field sensor, which utilizes the magnetically induced change in elastic modulus, i.e., the delta-E effect. The sensor is based on magnetoelectric thin film composites, resulting in high sensitivity at room temperature and at low frequencies. The cantilever is electrically excited and read out by a 2 μm AlN piezoelectric layer. Depending on its magnetization, the 2 μm thin film of amorphous (Fe90Co10)78Si12B10 changes its elasticity, which results in a shift of the cantilevers resonance frequency. The sensor is operated in the first or second transversal bending mode at 7.6 kHz or 47.4 kHz. With a limit of detection of 140 pTHz−0.5 at 20 Hz under a magnetic bias field and 1 nTHz−0.5 without external bias field, this sensor exceeds all comparable designs by one order of magnitude.

Dual wavelength magneto-optical imaging of magnetic thin films

A magneto-optical imaging approach for the simultaneous imaging of multiple magnetization components is demonstrated. The method is applied to investigate complex magnetization reversal processes in single crystal iron and patterned amorphous magnetostrictive ferromagnetic structures. The use of a splitted optical illumination and observation path allows for the direct extraction of different complementary magnetic information. Real-time in-plane vector magnetization imaging reveals complicated domain arrangement processes in magnetostrictive films due to locally varying stress induced magnetic anisotropy. Magnetic domain features concealed by standard domain imaging techniques are directly exposed.

Adaptive Readout Schemes for Thin-Film Magnetoelectric Sensors Based on the delta-E Effect

Thin-film magnetoelectric sensors, i.e., composites of magnetostrictive and piezoelectric materials, are able to measure very low magnetic fields. As a consequence, an application of such sensors could be, e.g., the measurement of biomagnetic fields in the near future. To measure these signals, typically characterized by low-frequency components, techniques, such as the delta-E effect, are utilized. The limit of detection (LoD) of such sensor systems did not reach the required level until now. In order to improve this, an adaptive readout scheme is proposed for sensor systems based on the delta-E effect. The basis is a simultaneous measurement with a single sensor at different frequency ranges close to the resonance frequencies. The signals are combined optimally in regard to their signal-to-noise ratio. Two combination approaches are presented and evaluated. An improvement up to 6 dB in terms of LoD is achieved. Due to an adaption of the weighting coefficients with time, the proposed method can be interpreted as a noise reduction technique, which increases the usability of such sensors in realistic measurement scenarios.

Multimode delta-E effect magnetic field sensors with adapted electrodes

We present an analytical and experimental study on low-noise piezoelectric thin film resonators that utilize the delta-E effect of a magnetostrictive layer to measure magnetic fields at low frequencies. Calculations from a physical model of the electromechanical resonator enable electrode designs to efficiently operate in the first and second transversal bending modes. As predicted by our calculations, the adapted electrode design improves the sensitivity by a factor of 6 and reduces the dynamic range of the sensor output by 16 dB, which significantly eases the requirements on readout electronics. Magnetic measurements show a bandwidth of 100 Hz at a noise level of about 100 pTHz−0.5.

Tunable Strain in Magnetoelectric ZnO Microrod Composite Interfaces

The intrinsic strain at coupled components in magnetoelectric composites plays an important role for the properties and function of these materials. In this in situ X-ray nanodiffraction experiment, the coating-induced as well as the magnetic-field-induced strain at the coupled interface of complex magnetoelectric microcomposites were investigated. These consist of piezoelectric ZnO microrods coated with an amorphous layer of magnetostrictive (Fe90Co10)78Si12B10. While the intrinsic strain is in the range of 10-4, the magnetic-field-induced strain is within 10-5, one order of magnitude smaller. Additionally, the strain relaxation distance of around 5 μm for both kinds of strain superposes indicating a correlation. The value of both intrinsic and magnetic-field-induced strain can be manipulated by the diameter of the rodlike composite. The intrinsic interface strain within the ZnO increases exponentially by decreasing the rod diameter while the magnetic-field-induced strain increases linearly within the given range. This study shows that miniaturizing has a huge impact on magnetoelectric composite properties, resulting in a strongly enhanced strain field and magnetic response.

Modeling and Analysis of Noise Sources for Thin-Film Magnetoelectric Sensors Based on the Delta-E Effect

We present a comprehensive noise model for an electromechanical resonator that is utilized as a magnetic field sensor. The cantilever-type sensor is coated with a magnetostrictive film that exhibits a change in elastic modulus E with a magnetic field and therefore detunes the resonator—the so-called delta-E effect. The noise model contains all relevant noise sources from the operational electronics, the wiring, and the sensor itself. Measurements show good agreement up to a certain excitation voltage, where an additional dominant noise source appears. It is identified as originating from the magnetic film. With the results of the model, the operational parameters of such sensors are discussed. The model predicts that the limit of detection at 10 Hz for the present sensors can be improved to 60 pT/(Hz)

Noise of a JFET Charge Amplifier for Piezoelectric Sensors

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Giant Magnetoelectric Effect in Thin-Film Composites

if the magnetic noise is eliminated.