Temperature Stability of Magnetic Materials for Highly Sensitive Magnetoelectric Sensors

The detection of magnetic signals is a significant research area where numerous magnetoelectric sensor systems, such as antennas, filters, and magnetometers, are being developed and tested [1]. The use of magnetic signals has the advantage over electrical signals that they are not affected by water, for example, which simplifies communication with underwater vessels and the measurement of biomagnetic signals [1, 2].<br/>The development and fabrication of magnetoelectric cantilever sensors, based on microsystem techniques, are among the primary research focuses of the Collaborative Research Center 1261 at Kiel University [2, 3]. To construct such highly sensitive magnetic field sensors, a wide range of material parameters must be considered. For instance, piezoelectric layers require high piezoelectric coefficients, while magnetic layers should possess soft magnetic properties combined with high magnetostriction. Commonly used magnetostrictive materials are FeCoSiB and FeGaB. Amorphous magnetic materials have the advantage of avoiding undesired crystal anisotropies. To further enhance the magnetic behavior, exchange-bias multilayer systems are employed to minimize magnetic noise resulting from domain wall activity. Imprinting anisotropy in the multilayers is crucial for optimal performance, which can be achieved through sputter deposition in a magnetic field, as well as subsequent heat treatment at approximately 250 °C for FeCoSiB. However, higher annealing temperatures may be necessary depending on the blocking temperature of the selected antiferromagnet for biasing. which could lead to crystallization of the amorphous magnetic layers.<br/>Another potential source of unintentional crystallization is the packaging of the devices at the end of production to protect them from environmental influences and facilitate handling. This common method typically requires temperatures of about 400 °C to create a vacuum-sealed capsule.<br/>This study focuses on investigating different magnetostrictive materials such as FeCoSiB, FeGaB, FeGaC, and CoFeB, which are fabricated through sputter deposition with multicomponent targets and co-deposited layers [1]. The materials are evaluated using XRD, VSM, and magnetostriction measurements at different temperatures. The study aims to determine the temperature ranges at which a negative magnetic change occurs. For example, significant changes in the magnetic behavior of FeCoSiB are observed at temperatures above 300 °C, resulting in a degradation of sensor properties and rendering them unsuitable for vacuum encapsulation by standard wafer bonding.<br/>[1]Tu, C.; Chu, Z.-Q.; Spetzler, B.; Hayes, P.; Dong, C.-Z.; Liang, X.-F.; Chen, H.-H.; He, Y.-F.; Wei, Y.-Y.; Lisenkov, I.; el al. <b>Mechanical-Resonance-Enhanced Thin-Film Magnetoelectric Heterostructures for Magnetometers</b>, Mechanical Antennas, Tunable RF Inductors, and Filters. <i>Materials</i> 2019, <i>12</i>, 2259<br/>[2]E. Elzenheimer, P. Hayes, L. Thormählen, E. Engelhardt, A. Zaman, E. Quandt, N. Frey, M. Höft, G. Schmidt: <b>Investigation of Converse Magnetoelectric Thin Film Sensors for Magnetocardiography</b>, IEEE Sensors Journal, Volume 23, Number 6, Pages 5660-5669, 2023<br/>[3]P. Hayes, M. Jovičević Klug, S. Toxværd, P. Durdaut, V. Schell, A. Teplyuk, D. Burdin, A. Winkler, R. Weser, Y. Fetisov, M. Höft, R. Knöchel, J. McCord & E. Quandt: <b>Converse Magnetoelectric Composite Resonator for Sensing Small Magnetic Fields, </b><i>Sci Rep</i> 9, 16355, 2019