## 2 Potential magnetic sensors for space applications
This reason together with the lower price compared to rad-hardened components, and high functionality of the COTS sensors, motivate their use in space. Among the sensors in Table 1, AMR sensors are the ones with a longer commercial history after the fluxgates. They achieved a high maturity level at the end of the nineties, as was demonstrated in relevant environments like, for instance, the automotive and the mobile communications sectors. The effect, discovered in 1857 by [PERSON] ([PERSON]), consists in the change of electrical resistance (in the order of 3 %) in a magnetic material as a response of a variation in the environmental magnetic field [19-22]. To date AMRs have been used as magnetic field and current sensors, encoders, position and angular sensors, etc, in many on-ground applications. Commercial AMR sensors of the HMCxxxx series produced by Honeywell typically have dynamic ranges of hundreds of \(\mu\)T and resolutions in the order of 1 nT (detectivities in the order of nT/Hz\({}^{1/2}\) for frequencies higher than 10 Hz [23]), with sensitivities in the order of 10 mV/(mT V\({}_{\mathrm{bridge}}\)) with Set/Reset pulses [24]. They can measure fast variations in the magnetic field (bandwidth: 5 MHz) and they are silicon compatible, but they are highly temperature dependent. Thus, while they can operate over a huge temperature range (-55\({}^{\circ}\)C up to 155\({}^{\circ}\)C), their temperature sensitivity coefficients can be as high as 1 %o/oC. They also  
\begin{table}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|} \hline  
display some variation in the offset with the temperature, although this is much less significant in general terms [23, 25]. The variation of the behaviour with temperature often is solved using Pt resistors glued to the magnetometer body. This problem with the temperature is also a drawback in the relatively recently discovered (1988) GMR sensors. A GMR element consists of a trilayer or multilayer structure with two or more magnetic layers, each separated by a non-magnetic layer. Due to spin-dependent scattering of the conduction electrons, the resistance is maximum when the magnetic moments of the layers are antiparallel and minimum when they are parallel (the variation being as high as 20 %) [26-31]. In contrast to AMR (which has hysteresis in the order of 1 %o full scale), GMR devices present high hysteresis, up to 10 %, and thus their use for low magnetic field sensing is not trivial, and they were mainly used for reading heads in magnetic recording systems, i.e. hard disks [32]. However, GMR devices can also be used for sensing by means of a suitable resetting and biasing mechanism [33,34], achieving in the end good sensing properties and repeatability with the drawback of a higher power consumption (0.5 W). Since 1995 there have been commercial GMR devices devoted to other kinds of magnetic applications such as position and speed sensing and detection for medical and automotive solutions. GMR based sensors by NVE and Hitachi Metals have sensitivity values of 10-100 mV/(mT Vbridge), ranges of up to 1 mT and minimum detectable fields in the order of tenths of nT [35]. More recently structures in which an insulating layer (instead of a conductive layer) separates two magnetic layers have been studied [36-41]. In these structures, electrical conduction between the two magnetic layers is allowed by quantum tunneling through the insulator. The tunneling current is a function of the relative magnetization direction in the two magnetic layers (SDT-Spin-Dependent Tunnel). The rotation of the magnetization of one of the layers caused by an external magnetic field can lead to relative resistance changes as high as 40 %. Sensitivities of SDT devices can easily be ten times those presented by AMR devices, depending on the composition and method used for the antiparallel disposition. The VLSI developed for these structures makes it possible to use them in array for high resolution (50 \(\mu\)m) mapping [42].