Highly Sensitive Surface Acoustic Wave Magnetic Field Sensor Based on a Strongly Coupled Magnon‐Phonon System

Strong coupling between quasiparticles enables breakthroughs in quantum technologies. Surface acoustic waves (SAWs), carrying coherent gigahertz‐frequency phonons, provide a platform to achieve strong coupling between SAW phonons and quasiparticles like magnons. SAWs devices also utilize the delta‐E or delta‐G effect for magnetic field sensing, achieving an ultra‐low limit of detection (LoD) down to tens of picotesla. Concomitant disadvantages include unidirectional response, millimeter size, complex structure, and fabrication still remain. The strong magnon‐phonon coupling has great potential to overcome these challenges for weak magnetic detection. Here, an ultra‐compact and highly sensitive SAW magnetic field sensor based on a strongly coupled magnon‐phonon system is demonstrated, featuring a thin FeGaB film embedded in an acoustic cavity. In the strong coupling regime, the SAW frequency exhibits anti‐crossing and responds dramatically to external magnetic fields, enabling high sensitivities. Notably, within the anti‐crossing region, a frequency sensitivity of 600 kHz/Oe and a phase sensitivity of 44 °/Oe are achieved. Combined with phase noise evaluation, the LoD is determined to be 126 pT/Hz0.5 @ 10 Hz and 27 pT/Hz0.5 @ 100 Hz. Besides its fundamental significance for hybrid quasiparticles, this work proposes a brand‐new sensing mechanism for developing miniaturized, highly sensitive, and low‐LoD SAW magnetic field sensors.