Exchange biased magnetoelectric composites for magnetic field sensor application by frequency conversion

A comparison is presented between magnetoelectric composite sensors based on AlN and FeCoSiB with and without exchange bias coupling. All layer stacks were fabricated by thin film deposition on Si substrates. Whereas sensors without exchange bias exhibit a low limit of detection in the 1 pT/√Hz regime for resonant AC fields, such sensors fail at the detection of low frequency signals. Accordingly, their detection limit increases to about 10 nT/√Hz for alternating magnetic fields with 10 Hz frequency and an integration time equal to 3 s. A frequency conversion technique based on magnetic modulation of the sensors improves their detection limit by about one order of magnitude. However, frequency conversion can be applied more effectively to magnetoelectric sensors with exchange biased multilayers as a magnetostrictive phase. As a result, their limit of detection is about 180 pT/√Hz for 10 Hz signals and an integration time of 1 s. This is in contrast to the magnetoelectric coefficient αME of the two types of sensors. Whereas αME equals 6.9 kV/cm Oe for non-biased composites and resonant fields, it is by a factor of 8 smaller for the exchange biased composites. The nonetheless arising improvement of sensor performance with regard to the limit of detection for magnetic fields with low frequencies can be explained by a significantly lower magnetic noise contribution during modulation of the exchange biased sensors.