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.
For the measurement of biomagnetic signals in the pico-and femtotesla regime superconducting interference devices (SQUIDs) are commonly used. Their major limitation comes from helium cooling which makes these sensors bulky and expensive. We show that MEMS sensors based on magnetoelectric (ME) composites could be capable as a replacement for biomagnetic measurements. Using surface micromachining processes a cantilever beam with a stack composed of SiO 2 /Ti/Pt/AlN/Cr/FeCoSiB was fabricated on a 150 mm Si (1 0 0) wafer. First measurements of a rectangular micro cantilever with a thickness of 4 μm and lateral dimensions of 0.2 mm × 1.12 mm showed a giant ME coefficient α ME = 1000 (V m −1 )/(A m −1 ) in resonance at 2.4 kHz. The resulting static ME coefficient is α ME = 14 (V m −1 )/(A m −1 ). In resonance operation a sensitivity of 780 V T −1 and noise levels as low as 100 pT Hz −1/2 have been reached.
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
Highly sensitive AC magnetic field sensors are presented using magnetoelectric composites consisting of magnetostrictive and piezoelectric phases. They are offering passive nature, high sensitivity, large effect enhancement at mechanical resonance, and large linear dynamic range. Thin-film magnetoelectric 2-2 composites benefit from perfect coupling between the piezoelectric and magnetostrictive phases and from the reduction in size which is essential for high spatial resolution. Their design uses AlN and a plate capacitor or PZT with interdigital electrodes and magnetostrictive amorphous FeCoSiB single layers or exchanged biased multilayers. At mechanical resonance and depending on the geometry, extremely high ME coefficients of up to 9.7 kV/cm Oe in air and up to 19 kV/cm Oe under vacuum were obtained. To avoid external DC magnetic bias fields, composites consisting of exchanged biased multilayers serving as the magnetostrictive component with a maximum magnetoelectric coefficient at zero magnetic bias field are employed. Furthermore, the anisotropic response of these exchanged biased composites can be utilized for three-dimensional vector field sensing. Sensitivity and noise of the sensors revealed limits of detection as good as to 2.3 pT/Hz 1/2 at mechanical resonance. Sensitivity between 0.1 and 1000 Hz outside resonance can be enhanced through frequency conversion using AC magnetic bias fields.
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