Magnetoelectric Composite Sensor Based On Magnetostrictive Multilayers
Magnetoelectric composites have been studied intensively in the recent past because of their unsurpassed magnetoelectric coupling, opening the path to highly sensitive magnetic field sensors operating at room temperature . Moreover, this sensor approach yields the advantage that their constituting phases can be optimized almost independently. The presented work focuses on thin film magnetostrictive phase composites, composed of (Fe90Co10)78Si12B10 multi-layers. The piezoelectric component is a 2 µm thick AlN layer. The constituents are grown on a Si substrate, one on each side, by RF and pulsed DC sputter deposition respectively. Using UV lithography combined with dry and wet etching, magnetoelectric cantilever structures are fabricated with typical lateral dimensions of 3 mm x 25 mm.
In the context of magnetic field sensing, the role of the magnetostrictive component is to respond to an external magnetic field by generation of mechanical stress as high as possible. At least as important, irregular magnetization changes resulting from hysteretic effects have to be avoided in order to prevent significant noise contribution to the sensor output . The multi-layer approach increases the parameter space, which can be used to jointly attune the magnetoelastic response towards high sensitivity and improved control of magnetization reversal .
The presentation reports on recent advances achieved by utilizing magnetostrictors that are exchange biased layer by layer. Magnetoelectric coefficients as large as 4000 V/cmOe have been reached with optimal magnetic fields with excitation frequencies matching the mechanical resonance frequency. Applying magnetic frequency conversion yields minimum detectable magnetic fields as low as 50 pT/ Hz1/2 at 10 Hz  frequency, demonstrating the performance of such macroscopic magnetoelectric field sensors.
Funding of the project work by the German Research Foundation through the CRC 1261 Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics is gratefully acknowledged.