  We discuss the design and performance of a very sensitive low-field
magnetometer based on the detection of free spin precession of gaseous, nuclear
polarized 3He or 129Xe samples with a SQUID as magnetic flux detector. The
device will be employed to control fluctuating magnetic fields and gradients in
a new experiment searching for a permanent electric dipole moment of the
neutron as well as in a new type of 3He/129Xe clock comparison experiment which
should be sensitive to a sidereal variation of the relative spin precession
frequency. Characteristic spin precession times T_2 of up to 60h could be
measured. In combination with a signal-to-noise ratio of > 5000:1, this leads
to a sensitivity level of deltaB= 1fT after an integration time of 220s and to
deltaB= 10^(-4)fT after one day. Even in that sensitivity range, the
magnetometer performance is statistically limited, and noise sources inherent
to the magnetometer are not limiting. The reason is that free precessing 3He
(129Xe) nuclear spins are almost completely decoupled from the environment.
That makes this type of magnetometer in particular attractive for precision
field measurements where a long-term stability is required.
