The possibility of SQUID sensitivity enhancement by the use of ferromagnetic antennae

The possibility of the of SQUID magnetometers sensitivity improving by application of ferromagnetic antenna has been studied. The antenna proposed is made of a "soft" magnetic material and is inserted into the pick-up coil of a magnetic flux transformer. It is shown that: (i) With the properly selected material and dimensions of antenna, it is possible to enhance the SQUID sensitivity to a uniform external magnetic field by adjusting the ratio (m) of the pick-up coil inductance (L-1) and the SQUID input coil inductance (L-2) of the transformer. (ii) When the conventional condition m = L-1/L-2 = 1 is met, the sensitivity of the SQUID magnetometer with an antenna is (W-1(F)/W-1(0))mu times higher than in the case without an antenna, the pick-up coil areas S-1 being equal. Here W-1(F) and W-1(0) are the numbers of turns on the pick-up coils in the transformers with and without an antenna, respectively; mu is the effective relative magnetic permeability of the antenna. This sensitivity can be twice as high if L-1 much greater than L-2. (iii) The magnetic noise of SQUIDs induced by 79 NM (79% Ni) magnetic permalloy antenna in the low frequency region (5-10 Hz) at T = 4.2 K does not exceed 10(-30) J/Hz. The noise contributed by a ferrite antenna is tens of times higher. (C) 2000 Elsevier Science Ltd. All rights reserved.