Degaussing procedure and performance enhancement by low-frequency shaking of a 3-layer magnetically shielded room

We report on the performance of a Magnetically Shielded Room (MSR) intended for next level He-3/Xe-129 co-magnetometer experiments that 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 8 mm. It has a cubical shape with a walk-in interior volume with an edge length of 2560 mm. An optimized degaussing (magnetic equilibration) procedure using a frequency sweep with a constant amplitude followed by an exponential decay of the amplitude will be presented. The procedure for the whole MSR takes 21 min, 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 approximate to 4 in all directions by low frequency (0.2 Hz), low current (1 A) shaking of the outermost Mu-metal layer.