  We report on the performance of a Magnetically Shielded Room (MSR) intended
for next level $^3$He/$^{129}$Xe co-magnetometer experiments which require
improved magnetic conditions. The MSR consists of three layers of Mu-metal with
a thickness of 3 mm each, and one additional highly conductive copper-coated
aluminum layer with a thickness of 10 mm. It has a cubical shape with an
walk-in interior volume with an edge length of 2560 mm. An optimized degaussing
(magnetic equilibration) procedure using a frequency sweep with constant
amplitude followed by an exponential decay of the amplitude will be presented.
The procedure for the whole MSR takes 21 minutes and measurements of the
residual magnetic field at the center of the MSR show that $|B|<1$ nT can be
reached reliably. The chosen degaussing procedure will be motivated by online
hysteresis measurements of the assembled MSR and by Eddy current simulations
showing that saturation at the center of the Mu-metal layer is reached.
Shielding Factors can be improved by a factor $\approx 4$ in all directions by
low frequency (0.2 Hz), low current (1 A) shaking of the outermost Mu-metal
layer.
