Modulation technique for magneto -optic imaging of weak magnetic fields with rare-earth iron garnet films.

摘要

Magneto-optic imaging technology has been applied to nondestructive evaluation of aging aircraft as well as imaging of eddy currents in superconductors, but so far has been limited to imaging fields in the millitesla range. The bismuth-doped rare-earth iron garnet films studied here exhibit hysteresis, which can be overcome by a kilohertz-frequency modulation scheme. The result is a sensitivity to external magnetic fields orders of magnitude higher than for other similar magneto-optic materials. This approach enables the demonstration of a room-temperature magnetic field detection system capable of measuring fields as low as 1 nT and with 500 mum spatial resolution. Such an instrument is of interest for biomagnetic imaging, including magnetocardiography for diagnosis of cardiac anomalies and imaging of magnetic nanoparticles for cancer diagnosis and studies of drug delivery mechanisms. Dynamic detection of ferromagnetic nanoparticles is also demonstrated. Studies of system noise reveal that the detection floor of the system is limited by intrinsic film noise due to domain wall motion.