Less than thirty years after the giant magnetoresistance (GMR) effect was described, GMR sensors are the preferred choice in many applications demanding the measurement of low magnetic fields in small volumes. This rapid deployment from theoretical basis to market and state-of-the-art applications can be explained by the combination of excellent inherent properties with the feasibility of fabrication, allowing the real integration with many other standard technologies. In this paper, we present a review focusing on how this capability of integration has allowed the improvement of the inherent capabilities and, therefore, the range of application of GMR sensors. After briefly describing the phenomenological basis, we deal on the benefits of low temperature deposition techniques regarding the integration of GMR sensors with flexible (plastic) substrates and pre-processed CMOS chips. In this way, the limit of detection can be improved by means of bettering the sensitivity or reducing the noise. We also report on novel fields of application of GMR sensors by the recapitulation of a number of cases of success of their integration with different heterogeneous complementary elements. We finally describe three fully functional systems, two of them in the bio-technology world, as the proof of how the integrability has been instrumental in the meteoric development of GMR sensors and their applications.

A novel fully-analog lock-in amplifier for the accurate measurements of low quantities of gas in sensor applications is here presented. When compared with commercial lock-in and other solutions available in the literature, the proposed system implements an automatic alignment of the relative phase between input and reference signals, both at power-on and for any variation of the input signal phase and amplitude during the working time, so allowing to detect, accurately and in a continuous way, the mean value of the input signal coming from sensors (typically buried into noise). The circuit has been optimized to operate at a specified reference frequency, in particular 77 Hz; this value is suitable for gas sensor applications, since it avoids also any kind of interferences at 50 Hz net frequency and its harmonics. Experimental results have confirmed the correct functionality of the system, also tested for both carbon monoxide (CO) and ethylene glycol detections. With respect to the simpler resistive gas sensor interface, implemented by a resistive voltage divider, the improvement experimentally given by the proposed automatic lock-in amplifier, in terms of resolution, has been of a factor of about 100 for the CO and 230 for the ethylene glycol measurements, so allowing a theoretical gas detection in the order of ppb.