Surface Acoustic Wave Magnetic Field Sensors for Biomagnetic Applications

The measurement of biomagnetic signals i.e. magnetic fields generated by currents in the human brain or heart is of high interest for medical applications. Surface acoustic wave devices are promising candidates for measuring these biomagnetic signals due to their ability to detect magnetic fields below 100 pT/Hz^1/2 over a large frequency range of 1 Hz to 100 kHz As the sensitive element in these devices amorphous magnetostrictive thin films such as the alloy (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ seem to be particularly promising due to their high magnetostriction with simultaneously having a low magnetic anisotropy. Operating in a delay line configuration the magnetoelastically-induced change of shear modulus of the sensitive layer yields in a corresponding phase shift of the acoustic wave. Different approaches to improve the signal-to-noise ratio of these sensors are investigated. Particularly, by exchange biasing the ferromagnetic phase by the coupling to an antiferromagnetic phase a single or nearly single magnetic domain state can be achieved. The formation of magnetic domains and thus an important source of magnetically induced sensor phase noise are thereby reduced. These exchange biased devices show equivalent detection limits of 28 pT/Hz^1/2 at 10 Hz and 10 pT/Hz^1/2 at 100 Hz.