High-Tc rf SQUID for magnetic microscopy

The spectral density of magnetic flux noise S-Phi(1/2)(f ) is investigated for high-T-c rf SQUIDs with a pumping frequency of 390-457 MHz, placed inside three-layer Permalloy and superconducting shields. A superconducting interferometer with inner dimensions of the pickup loop of 100x100 mum is prepared by thin-film technology with a YBaCuO-PrBaCuO-YBaCuO Josephson junction of the ramp-edge type. It is shown that with the use of a cooled preamplifier, the energy sensitivity of SQUIDs in the "white" noise region (at frequencies above 1 kHz) is 4x10(-30) J/Hz and is mainly determined by the intrinsic noise of the high-T-c superconducting interferometer and the shields. At low frequencies the predominant noise is from external fields penetrating directly into the shields. The addition of a ferromagnetic antenna to the SQUID microscope increases the intrinsic noise of the magnetometer to 8x10(-30) J/Hz at frequencies above 1 kHz. (C) 2003 American Institute of Physics.