Response Model of Naturally Abundant Rubidium SERF Magnetomete

Spin-exchange relaxation-free (SERF) atomic magnetometers are widely utilized due to high sensitivity and promising miniaturization capability. Alkali vapor cells filled with ⁸⁷Rb are commonly employed as the core component, while facing challenges of high cost and difficulty in scalable production. Replacing ⁸⁷Rb with naturally abundant rubidium (^natRb) in vapor cell fabrication can substantially reduce the cost of isotopic enrichment. In this study, we conduct comparative analyses of rubidium isotopes and determine that the gyromagnetic ratio difference between natRb and 87Rb arises from their distinct nuclear spins, which results in the polarization-dependent slowing-down phenomenon. Under the SERF regime, a hybrid slowing-down factor of the natRb ensemble is formulated based on equilibrium mass-dependent isotopic fractionation theory. We then analyze relaxation rate differences between ^natRb and ⁸⁷Rb vapor cells and ultimately establish the response model of the ^natRb SERF magnetometer. The result demonstrates that optimizing the buffer gas pressure allows the ^natRb magnetometer to achieve a response signal close to that of the ⁸⁷Rb device, enabling comparable sensitivity. Experiments conducted using 8 mm cubic vapor cells as a representative case revealed that the response signal of the ^natRb magnetometer was approximately only 19% lower than that of the ⁸⁷Rb device, with a comparable increase in probe noise. This led to a slight yet engineering-tolerable difference in sensitivity. These results indicate that ^natRb provides a practical and cost-effective alternative to ⁸⁷Rb in high-sensitivity applications. The presented model can be extended to arbitrary cell sizes and other alkali species.