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.

We present a comprehensive study of a magnetic sensor system that benefits from a new technique to substantially increase the magnetoelastic coupling of surface acoustic waves (SAW). The device uses shear horizontal acoustic surface waves that are guided by a fused silica layer with an amorphous magnetostrictive FeCoSiB thin film on top. The velocity of these so-called Love waves follows the magnetoelastically-induced changes of the shear modulus according to the magnetic field present. The SAW sensor is operated in a delay line configuration at approximately 150 MHz and translates the magnetic field to a time delay and a related phase shift. The fundamentals of this sensor concept are motivated by magnetic and mechanical simulations. They are experimentally verified using customized low-noise readout electronics. With an extremely low magnetic noise level of ≈100 pT/, a bandwidth of 50 kHz and a dynamic range of 120 dB, this magnetic field sensor system shows outstanding characteristics. A range of additional measures to further increase the sensitivity are investigated with simulations.

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 cantilever's 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.