Direct measurements of the magnetic field induced by optically polarized /sup 3/He atoms

An alternative magnetic field induced by the standard cell of an optically pumped /sup 3/He magnetometer has been directly measured by the SQUID-based second-order gradiometer with a signal-to-noise ratio higher than six. A measured field of 5*10/sup -13/ T at a 5-cm distance from the cell axis and a transverse relaxation time of 7 min have been found. The measured transverse decay time was equal to 7 min; thus, the resolution in the frequency domain is limited at a level on the order of 10/sup -3/ Hz, which corresponds to an uncertainty of approximately 5*10/sup -11/ T. The results imply that the direct measurement of the induced field in the optically pumped /sup 3/He magnetometers makes it possible to introduce a suitable metrological scale in the region of the weak magnetic field 5*10/sup -8 /T