The ferromagnetic-mechanical system can be employed as a magnetometer by monitoring its mechanical response to magnetic signals. Such a system can surpass the ERL in terms of sensitivity, due to the ultra-high spin density and strong spin-lattice interactions inherent in ferromagnetic materials. A levitated ferromagnetic-mechanical system can further enhance its quality factor by eliminating clamp dissipation, thus achieving higher magnetic sensitivity. This paper proposes a magnetometer based on a magnetically levitated ferromagnetic torsional oscillator (FMTO), which converts magnetic signals into torque to drive the oscillator. An optical method is then used to measure the torsional motion and extract the magnetic signal. The resonance frequency of this FMTO system can be controlled by modifying the bias field, offering enhanced flexibility and control.
After analyzing the influence of fundamental noise, including thermal noise and quantum measurement noise (SQL), the magnetic noise floor of the FMTO made from NdFeB versus its radius is illustrated in Fig.3.(b). The quantum measurement noise (SQL) is much lower than both thermal noise and ERL, indicating that thermal noise is the dominant factor affecting the magnetic sensitivity of the FMTO. The magnetic sensitivity of the FMTO system at 4.2 K surpasses the ERL by three orders of magnitude, confirming the significant potential of the FMTO system for high-precision magnetic measurements.
One of the most promising applications of ultra-high sensitivity magnetic sensors is the search for exotic interactions, which is typically achieved by measuring pseudo-magnetic fields. The accuracy of detecting exotic interactions depends on two main factors: the magnetometer’s sensitivity and the distance between the sensor and the source. The ERL presents a challenge in satisfying both factors simultaneously. Improving magnetic sensitivity typically increases the radius of the sensor, which in turn increases the distance between the sensor and the source, limiting the accuracy of detecting exotic interactions. Thus, ERL limits the precision of exotic interaction detection, while the FMTO, with its superior sensitivity, is expected to significantly advance the detection of exotic interactions.
Fig.4.(a) illustrates the fundamental principle behind exotic interaction detection. If an exotic interaction is present, the BGO nuclei oscillating perpendicular to the paper will generate a pseudo-magnetic field along the vertical direction. This pseudo-magnetic field will induce torsional motion in the FMTO, allowing for the determination of the lower limit of the coupling constant for the new interaction through measurement of the torsional motion. The exotic interaction probe is shown in Fig.4.(b). Existing experiments have approached the ERL at Compton wavelengths on millimeter and micrometer scales. However, the FMTO system, with a bias field of 1 µT, surpasses the ERL by up to five orders of magnitude in sub-centimeter Compton wavelengths and exceeds existing experimental results by two to nine orders of magnitude. These results highlight the potential advantages of FMTO-based magnetometers in probing exotic interactions.
In conclusion, this paper proposes a magnetometer configuration based on a levitated ferromagnetic torsional oscillator (FMTO) and provides a comprehensive analysis of its mechanical response, fundamental noise, magnetic performance, and applications in fundamental research. It is demonstrated that the FMTO-based magnetometer can surpass the ERL by approximately three orders of magnitude in sensitivity at a temperature of 4.2 K. Furthermore, an FMTO with a bias magnetic field of 1 µT and a temperature of 50 mK significantly outperforms the ERL in probing exotic interactions, exceeding existing experiments by two to nine orders of magnitude.