Robert Jahns

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.

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.

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.

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.

Microelectromechanical magnetic field sensor based on ΔE effect

We present a fully integrated microelectromechanical magnetic field sensor based on the ΔE effect. The vacuum encapsulated sensor extends our previous approach [B. Gojdka et al., Appl. Phys. Lett. 99, 223502 (2011); Nature 480, 155 (2011)] and now involves an intermediate piezoelectric AlN layer between a SiO2 cantilever and a magnetostrictive FeCoBSi top layer. The AlN layer serves two functions: It drives the resonator, and it is used for electrical read out. The limit of detection was strongly enhanced to 12 nT/ Hz at 10 Hz.