Detection of weak (∼0.5–300nT), low frequency (5–100Hz) magnetic fields at room temperature by kilohertz modulation of the magneto-optical hysteresis in rare earth–iron garnet films

Abstract

Periodic magnetic fields with frequencies in the 5–100Hz range and peak strengths as low as 0.5nT have been detected at 300K by modulating at kHz rates the magneto-optical response of epitaxial (Tm,Bi)3(Ga,Fe)5O12 rare earth–iron garnet films at λ=532nm. By exploiting the ∼1°∕μT slope of the magneto-optical transition region between the two magnetization states of these low coercivity (<0.5mT) films, sub-nT field strengths can be measured by upconverting the detection process into the kHz domain. The domain wall velocities are measured to be 3–30m∕s for modulation frequencies in the 0.3–10kHz interval, and the noise generated by wall motion appears to be the primary barrier to further reductions in the detection sensitivity floor. Films with thicknesses of 2–4μm and mean domain widths of 3–6mm exhibit figures of merit (Θ∕α) of ∼3°–8.5° at 532nm. Imaging of mesoscopic magnetic particles is demonstrated with a spatial resolution of <300μm.