Ultrasensitive magnetometers based on spin resonances have led to remarkable achievements. However, the field responsivities of these spin resonances are inherently constrained by these particles’ gyromagnetic ratios, such as the electron, with a constant gyromagnetic ratio of ${\gamma }_e=2\pi \times 28\mathrm{GHz}/\mathrm{T}$. Here, we demonstrate an ultrasensitive magnetometer based on the cusp point (CP) of photon-magnon synchronization modes (PMSMs). The PMSM’s field responsivity at the CP is enhanced to $37{\gamma }_e$ and further amplified to $236{\gamma }_e$ by utilizing the sixth-order oscillating mode of the PMSM. Moreover, the emission linewidth of the PMSM can be reduced to 0.06 Hz, resulting in excellent sensitivity to weak magnetic fields. These outstanding properties position our magnetometer to potentially achieve superior sensitivity to conventional magnetometers. Our Letter introduces a cost-effective prototype for the next generation of magnetometry and may advance scientific research and technologies that rely on ultrasensitive magnetic field detection.

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• Magnetization dynamics
• Magnons
• Spintronics
• Magnetic insulators
• Ferromagnetic resonance
• Microwave techniques