Levitated ferromagnetic magnetometers represent a cutting-edge class of devices engineered for ultrahigh magnetic field resolution. These magnetometers harness the inherent macroscopic spin coherence and robust spin-lattice coupling within levitated ferromagnets. This intrinsic coupling, combined with the exceptional environmental isolation afforded by magnetic levitation platforms, theoretically enables these systems to transcend the conventional energy resolution limit (ERL) of traditional magnetic sensors and the standard quantum limit (SQL) typically associated with gaseous atomic spin magnetometers of comparable spin numbers.
This review systematically elucidates the foundational principles governing levitated ferromagnetic magnetometry. We meticulously analyze the dynamic mechanisms and scaling laws that define the magnetic field resolution limits for two predominant measurement modalities: the precession mode and the librational mode. Furthermore, we provide a comprehensive overview of the current state-of-the-art experimental platforms, specifically focusing on superconducting magnetic levitation and room-temperature diamagnetic levitation techniques.
Recent experimental breakthroughs have showcased remarkable performance, achieving magnetic field resolutions at the fT level and energy resolutions exceeding the ħ by approximately two orders of magnitude. For instance, a notable experiment demonstrated a magnetic field noise power spectral density of approximately 4×10^-28T²/√Hz utilizing a levitated ferromagnetic microsphere with a radius of R = 20.78μm. This yielded an energy resolution of E_R = 6.75×10^-36J·s at a signal-to-noise ratio (SNR) of 1, which is roughly 0.064¯ h, unequivocally surpassing the quantum limit by two orders of magnitude. These significant advancements underscore the profound potential of this technology for precision magnetic field sensing and its broader implications for fundamental physics research. The paper concludes by discussing the prevailing challenges and outlining promising future directions for this rapidly evolving field.