Slotted high-T/sub c/ dc SQUID magnetometers

Recently, it was observed that the low frequency noise of slotted SQUIDs is not affected when cooling them down in magnetic fields up to 0.05 mT. This behavior is ascribed to the fact that magnetic vortices are expelled out of the narrow strips into the slots. Thus 1/f noise by hopping of vortices in the superconducting structure is eliminated. Here we present a systematic investigation on a series of slotted high-T/sub c/ dc SQUIDs. The number of slots varies from zero to eight. A model is developed that provides values for the effective area and inductance of the slotted washers and the current distribution within the slotted SQUIDs. The model can explain the experimentally observed maximum in the effective area for four slots very well. The observed agreement with respect to the modulation depth supports the theoretical values for the inductance. The white noise of the slotted SQUIDs is higher than expected, while the expected rise of low frequency noise for field cooled solid washer SQUIDs was not observed.