Search for Ultralight Axion Dark Matter with a Levitated Ferromagnetic Torsional Oscillator

Abstract

We present a search for ultralight axion dark matter coupled to electron spins using a levitated ferromagnetic torsional oscillator (FMTO). This platform directly measures axion-induced torques on a macroscopic spin-polarized body, combining large spin density with strong mechanical isolation to probe magnetic fluctuations below 10 Hz while suppressing gradient-field noise. In a first implementation, the experiment yielded 18000 s of analyzable data at room temperature under high vacuum with optical readout and triple-layer magnetic shielding. A likelihood-based statistical framework, incorporating stochastic fluctuations in the axion-field amplitude, was used to evaluate the data. No excess consistent with an axion-induced pseudo-magnetic field was observed near 2e-14 eV. To account for possible shielding-induced signal attenuation, we quantify its effect and report both the uncorrected (g_aee < 1e-7) and attenuation-corrected (g_aee < 6e-5) 90% CL limits on the axion-electron coupling. Looking ahead, improvements guided by both noise-budget analysis and shielding-attenuation considerations, including optimized levitation geometry, cryogenic operation, and superconducting shielding, are expected to boost sensitivity by multiple orders of magnitude.

Figures (8)

• Figure 1: Schematic of the FMTO system. The system consists of a levitated magnet, a sensing magnet, and a connecting Teflon rod. A pyrolytic graphite disk provides stable levitation of the assembly. The motion of the FMTO is monitored using a camera combined with a centroid-tracking algorithm. A pair of coils generates the bias magnetic field $\vec{B}_{\mathrm{bias}}$, inducing the torsional eigenmode of the sensing magnet around its symmetry axis, while the axis perpendicular to this coil pair defines the detector’s sensitive axis $\vec{\xi}$ for the axion-wind effective magnetic field $\vec{B}_a$. Another pair of coils, aligned along $\vec{\xi}$, is used to apply calibration signals to characterize the system response.
• Figure 2: The magnetic PSD $S_B(f)$ measured around the resonance point and the fitted magnetic noise baseline. The vertical line highlights the resonance frequency $f_0\approx 5\text{Hz}$ of the torsional mode.
• Figure 3: The 90% confidence level limits on the axion-electron coupling strength $g_{aee}$ with axion mass $m_a$ (bottom axis) and Compton frequency $f_c$ (top axis). Shown are the data-driven limits derived from the measurements presented in Fig. \ref{['fig:psd']}, as well as the median limit and $1\sigma/2\sigma$ bands derived from our simulations. Also shown are the existing limits from the old comagnetometers Bloch:2019lcy. "Shielding attenuated (Sh. att.)" denotes the inclusion of a potential attenuation factor on the axion-induced field $B_a$ due to the innermost magnetic shielding layer.
• Figure 4: Projected sensitivity of the levitated FMTO to the axion–electron coupling $g_{aee}$ versus axion mass $m_a$, assuming a one-month integration time and the sidereal-day–averaged factor $\cos^2\psi = 0.22$. The star marks the most stringent limit from the current room-temperature, vibration-limited experiment ($\cos^2\psi = 0.061$). Curves show projections for optimized FMTO configurations at various temperatures and quality factors $Q$, including improved room-temperature designs with/without conservative shielding-attenuation corrections, and cryogenic setups featuring enhanced vibration isolation, higher $Q$, and superconducting shielding. Existing limits from comagnetometers Bloch:2019lcy, XENONnT solar-axion searches XENON:2022ltv, and red-giant cooling Capozzi:2020cbu are also shown, highlighting that upgraded FMTO experiments can approach or compete with current laboratory and astrophysical constraints.
• Figure S1: Variation of $\cos^2\psi$ over a sidereal day, with the red segment indicating the time window corresponding to the analyzed data.