Sensing Spin Precession with Free Electrons

We present a method that combines spin resonance spectroscopy with transmission electron microscopy (TEM), enabling localized <i>in situ</i> detection of microwave (MW)-driven spin transitions in the specimen, utilizing the free-space electron beam of the TEM as a signal receiver. Spin state polarization is achieved via the magnetic field of the TEM's polepiece, while a custom-designed microresonator integrated into a TEM sample holder delivers continuous wave MW excitation at GHz frequencies. At resonance, the MW field drives spin transitions that produce a dynamic, precessional magnetic field around the specimen, inducing a deflection of the free-space electron beam. Phase-locked detection synchronized to the MW driving fields enables the isolation of spin precession contributions to the electron beam deflection. The presented technique offers a pathway for the <i>in situ</i> exploration of spin excitations at the nanoscale.