Focusing the Axion Wind with Ferrite Flux Concentrators

Axion dark matter may couple to fermion spins through an effective oscillating magnetic field, the ``axion wind". Existing approaches for detecting axion wind via axion-electron coupling gₐₑ face sensitivity limitations due to the extremely weak pseudomagnetic field generated, typically a few times 10⁻¹⁸ tesla for gₐₑ ~ 10⁻¹⁰. We propose to use ferrimagnetic flux concentrators to amplify the pseudomagnetic field into a genuine magnetic field, which can then be measured using general precision magnetometry techniques. This greatly expands the experimental possibilities beyond conventional methods that rely on detecting small transverse magnetic fields from spin precession induced by axion wind. Here we utilize the libration mode of a levitated ferromagnet, which can achieve sub-femtotesla sensitivity, enabling detection of axion-electron coupling strengths gₐₑ 10⁻¹¹ in the frequency range $1-500$ Hz.