Highly sensitive magnetometers - a review

Conventional magnetic sensors, easy to use, are supposed to work mainly well over the nanotesla range, as due to the large magnetic environmental noise occurring in urban and industrial environments. On the other hand, a strange world exists, well below the nanotesla range, where the very efficient magnetic properties of superconducting materials have been used. It is the world investigated using cryogenic sensors, especially those of the SQUID's family. During a long time starting from the 1960s, SQUID people have refined their technologies, together with the use of advanced signal processing both analogue and digital, in order to input couple various external magnetic sources at room temperature, such the bio magnetic ones. State of the art of SQUID sensors is given. In the early 1990s, the dramatic improvement of the operating temperature led to the hope of lighter and lower costs systems with reduced cryogenic mount, designed to operate in open environment. An important target of multi SQUID systems using high critical temperature superconductors was and still is the magnetocardiography (MCG) mapping that could be daily used for improved diagnosis, as compared to conventional electrocardiography. It is known that such an important application is realistic only with noise spectral densities referred at the input lower than 100 fT/root Hz in a frequency bandwidth lying in between 1 Hz and 1 kHz and with a spatial resolution lower than I cm. The talk will review the recent advances in room temperature solid state sensors that could reach the above specified noise level. The review includes: magnetoresistive devices (AMR, GMR, spin valve, and spin dependent tunnelling device), Giant magneto-inductive devices. Non-solid, atomic vapor laser magnetometers, which have recently shown their ability to deliver very clear MCG signals, and start to be used to map the MCG signal above the chest just like SQUIDs systems, are reviewed. A simple, convenient energy resolution-volume is proposed, which allows a convenient way to compare high sensitivity magnetic sensors. (c) 2006 Elsevier B.V. All rights reserved.