Ferromagnetic torsional oscillator based magnetic field measurement and its applications
Authors: WU Lielie , REN Yichong , XUE Fei ,   School of Physics, Hefei University of Technology, Hefei 230601, China
Published: December 2024

<sec>The ferromagnetic-mechanical system can be used as a magnetometer by monitoring its mechanical response to magnetic signals. This system can exceed the energy resolution limit (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. In this work, a magnetometer is proposed based on a magnetically levitated ferromagnetic torsional oscillator (FMTO), which transforms 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, thus providing enhanced flexibility and control.</sec><sec>By analyzing the influence of fundamental noise, including thermal noise and quantum measurement noise (SQL), the relationship between the magnetic noise floor of the FMTO made of NdFeB and its radius is obtained. The SQL is much lower than both thermal noise and ERL, indicating that thermal noise is a dominant factor affecting the magnetic sensitivity of the FMTO. The magnetic sensitivity of the FMTO system at <inline-formula><tex-math id="M3">\begin{document}$4.2\ \rm{K}$\end{document}</tex-math></inline-formula> exceeds the ERL by three orders of magnitude, confirming the significant potential application of the FMTO system in high-precision magnetic measurements.</sec><sec>Searching for exotic interactions is one of the most promising applications of ultra-high sensitivity magnetic sensors. It 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 challenges in meeting both of these 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 accuracy of exotic interaction detection, while the FMTO, with its excellent sensitivity, is expected to significantly improve the detection of exotic interactions.</sec><sec>If there is an exotic interaction, 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. The lower limit of the coupling constant for the new interaction is determined by measuring the torsional motion. 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, exceeds the ERL by up to five orders of magnitude in sub-centimeter Compton wavelength and the existing experimental results by two to nine orders of magnitude. These results highlight the potential advantages of FMTO-based magnetometers in probing exotic interactions.</sec><sec>All in all, in this work, a magnetometer configuration is proposed based on a levitated FMTO and its mechanical response, fundamental noise, magnetic performance, and applications in fundamental research are analyzed comprehensively.</sec>