Surface acoustic wave (SAW) magnetic field sensors are promising candidates for medical applications and problems, e.g. magnetic detection of biomarkers, localization of magnetic capsules for gastrointestinal diagnosis or cardiological applications and medical device localization. The use of amorphous magnetostrictive thin films in the magnetic layer system, like the alloy (Fe_{90}Co_{10})_{78}Si_{12}B_{10}, appears to be especially interesting as the sensitive element in SAW sensors due to their high magnetostriction and low magnetic anisotropy. The ability to detect magnetic fields below 28 pT/\sqrt(Hz) over a large range of frequencies from 10 Hz to several 10 kHz has been demonstrated for Love wave-based sensors. However, these sensors require the application of an external magnetic bias to show best performance.
For sensor application in particular in a sensor array zero-biasing is necessary. An important research aspect is therefore the control and manipulation of the magnetic anisotropy during the deposition of an exchange biased magnetic multilayer system
(Ta | FeCoSiB | NiFe | MnIr | Ta | FeCoSiB | NiFe | MnIr | Ta). This approach allows to reduce magnetic noise sources and to operate the sensors with maximum sensitivity in zero bias field.
The presented SAW sensors are characterized in the aspects of sensitivity, i.e. generated phase shift with applied magnetic field and noise behavior as a function of the signal frequency. Different anisotropy alignment configurations and approaches to increase the sensor performance will be discussed. ST cut quartz-based magnetic SAW sensors with a scissor-like anisotropy alignment show already very high sensitivities over 280 °/mT and a detection limit of 120 pT/\sqrt(Hz) at 10 Hz and 30 pT/\sqrt(Hz) at 100 Hz with zero biasing.
This work was funded by the German Research Foundation (DFG) through the Collaborative Research Centre CRC 1261 “Magnetoelectric Sensors – From Composite Materials to Biomagnetic Diagnostics”.