Directional magnetic field detection with zero-bias surface acoustic wave sensors

Abstract Surface acoustic wave magnetic field sensors are promising candidates in detecting low-amplitude magnetic fields over a broad frequency range. For sensor arrays and portable systems, operating without an external magnetic bias field is essential for reducing system complexity and power consumption. One approach to achieve an internal magnetic bias is to intentionally tilt the magnetic anisotropy of the sensing layer relative to the acoustic wave propagation direction. In this work, we present two types of internally biased surface acoustic wave magnetic field sensors based on exchange-biased multilayer structures. These consist of two multilayer stacks of Ta | (Fe 90 Co 10 ) 78 Si 12 B 10 | Ni 81 Fe 19 | Mn 80 Ir 20 | Ta layers that enable a stable magnetic configuration with low magnetic noise in a nearly single magnetic domain state. The best fabricated sensor reaches a detection limit of 69 pT/Hz 1/2 at 10 Hz and 22 pT/Hz 1/2 at 100 Hz, which is to our knowledge the lowest detection limit reported for SAW magnetic field sensors operated without an external bias field. The sensors exhibit a sinusoidal directional response fitting to an analytical magnetic model that includes magnetic energy contributions from magnetostatic interactions and unidirectional anisotropy.