A multilayered structures consist of two or more magnetic layers of a Fe–Co–Ni alloy, as can be permalloy, separated by a very thin non magnetic conductive layer, as can be Cu [7]. A general scheme is shown in Figure 1(a). With magnetic films of about 4–6 nm width and a conductor layer of about 3–5 nm, magnetic coupling between layers is slightly small. With this configurations, MR levels of about 4%–9% are achieved, with linear ranges of about 50 Oe. The figures of merit of these devices can be improved by continuously repeating the basic structure.

Multilayered copper–permalloy (Ni₈₀Fe₂₀) [8] and copper–cobalt [9] structures have been developed by Mujika et al., following the early works of Piraux et al. [10] and Blondel et al. [11]. The films were deposited by DC sputtering onto thermally oxidized silicon wafers. The use of CMOS compatible substrates allowed the lithographic definition of meander-like magnetoresistances, deposition of platinum contacts and passivation with Si₃N₄. A final annealing step at 300 ^°C for two hours in a controlled formingas (N₂–H₂) atmosphere conferred the samples a higher thermal stability and better magnetoresistance levels (1.1% at room temperature, B = 10 kG).

Successful applications of multilayered structures in magnetic field sensing include bio-electronics [8, 9] and angle detectors [12].

The origin of spin valves are a particular case of multilayered structure [13]. In spin valves, an additional antiferromagnetic (pinning) layer is added to the top or bottom part of the structure, as shown in Figure 1(b). In this sort of structures, there is no need of an external excitation to get the antiparallel alignment. In spite of this, the pinned direction (easy axis) is usually fixed by raising the temperature above the knee temperature (at which the antiferromagnetic coupling disappears) and then cooling it within a fixing magnetic field. Obviously, so obtained devices have a temperature limitation below the knee temperature. Typical values displayed by spin valves are a MR of 4%-20% with saturation fields of 0.8-6 kA/m [14].

For linear applications, and without excitation, pinned (easy axis) and free layers are arranged in a crossed axis configuration (at 90^°). The response this structure is given by [15]: (4) Δ R = 1 2 ( Δ R R ) R □ i W h cos ( Θ p − Θ f )where (ΔR/R) is the maximum MR level (5%-20%), Rn is the sensor sheet resistance (15-20 Ω/n), L is the length of the element, W is its width, h is the thickness, i is the sensor current, and Θ_p and Θf are the angle of the magnetization angle of pinned and free layers, respectively. Assuming uniform magnetization for the free and pinned layers, for a linearized output, Θ_p = π/2 and Θ_f = 0.

As a practical example, in [16], the spin valve structure was deposited by ion beam sputtering (IBD) onto 3″ Si/SiO₂ 1500 Å(1) substrates with a base pressure of 1.0×10⁻⁸−5.0×10⁻⁸ Torr. For IBD deposition, a Xe flow was used for a deposition pressure of 4.110 5Torr. The spin valve structure was Ta(20 A) / NiFe(30 Å) / CoFe(20 Å) / Cu(22 Å) / CoFe(25 Å) / MnIr(60 Å) / Ta(40 Å). This structure has demonstrated to give magnetoresistance responses of about 6%−7%, linear ranges of about 20 Oe and sheet resistivities of about (10-15 Ω/□) [16]. Deposition rates ranged from 0.3 Å/s to 0.6 Å/s. A 40 Oe field was applied to the substrates during the deposition step in order to state the easy axis in the pinned and free layers. The wafer was 90 ^° rotated between both depositions to ensure a crossed-axis spin valve configuration.

Nano-oxide layers (NOL) inserted in the pinned layer and above the free layer have been found to increase the magnetoresistance ratio [17]. The enhancement of GMR is attributed to the specular scattering effect of the conduction electrons at the metal/insulator interfaces.

In [2], the specular spin valve structure was Ta(3 nm) / NiFe(3 nm) / MnIr(6 nm) / CoFe(1.6 nm) // NOL // CoFe(2.5 nm) / Cu(2.5 nm) / CoFe(1.5 nm) / NiFe(2.5 nm) // NOL // CoFe(2.0 nm) / Ta(0.5 nm). NOL layers were formed in a 15 minutes natural oxidation step at atmospheric pressure in the deposition tool load lock. The natural oxidation process, keeping its simplicity, has proven to be well effective. Finally, the samples were annealed at 270 ^°C under vacuum and cooled under a 3 kOe magnetic field applied parallel to the pinned and free layer easy axis.

This case, the magnetic layers are separated not by a conductive layer but a very thin isolating one, following a CPP configuration (See Figure 1(c)). Electrons can surpass this thin film by means of the quantum tunnel effect [18]. As deducted from quantum mechanics arguments, the crossing probability is higher when both magnetic moments are aligned in parallel and lower when both magnetic moments are not aligned in parallel. This devices usually make use of the spin-valve principle in order to fix the easy axis by means of a pinning antiferromagnetic layer. Typical MR levels of MTJ are above 40%, with Al₂O₃ as isolating layer [19]. More recently, MR levels about 200% have been reported for MgO based structures [20]. Saturation fields are in the order of 1-100 Oe.

The basis of linear magnetic tunnel junctions is analogous to that of linear spin valve. When configured in a crossed axis configuration, linear ranges suitable for sensor applications can be achieved [15]. Nevertheless, the usage of linear MTJ is still in its initial stage and is demanding additional research efforts.

In [21], the MTJ structure was deposited by ion beam sputtering (IBD) onto 3″ Si/SiO₂ 1000 Å substrates. The final structure of the MTJ was: Al (600 Å) / Ta (90 Å) / NiFe (70 Å) / MnIr (250 Å) / CoFe (50 Å) / Al₂O₃ (12 Å) / CoFe (50 Å) / NiFe (25 Å) / Ta (60 Å) / TiW (300 Å). This structure has demonstrated to give magnetoresistance responses close to 40% whereas keeping linear ranges above 20 Oe and RAP of several hundreds of μm².

Granular films of Co–Cu and Co–Ag also exhibit a giant magnetoresistance effect [22]. In this case, the giant magnetoresistance effect is due to the spin-dependent scattering taking place at the boundaries of Co clusters embedded in the host lattice, as depicted in Figure 1(d). Because of these binary systems are not miscible, the characteristics of the devices are highly conditioned by the growth conditions and the post-deposition treatments. In fact, the amount of magnetoresistance is accepted to be associated to the size of the Co clusters [23].

Vergara et al. obtained granular films of Ag–Co and Cu–Co (10% Co) by pulsed laser ablated-deposition [24]. A pulsed Nd–YAG laser was used. The considered substrate was common glass. The process was carried out at room temperature (10–5 mbar), with deposition ratios of the different elements ranging from 0.04 nm/s to 0.08 nm/s. So obtained films (from 50 to 75 nm thick) needed a post-annealing process (10 min at 500 ^°C in Ar atmosphere) before their characterization. Scanning tunneling microscopy revealed the existence of 30 nm diameter and 10 nm high grains. Magnetoresistance levels up to 9% at 77 K in a 10 kOe magnetic field were reported.

Andrés et al. used RF sputtering for obtaining Co–Cu (25% Co) granular films onto unheated glass substrates. Deposition ratios ranged from 0.15 nm/s to 0.75 nm/s (4 × 10^-7 Torr). This case, the cluster radius were about 20 Å, and magnetoresistance values up to 9% at room temperature in a 15 kOe magnetic field were measured.

Arana et al. deposited Co–Cu (30% Co) granular films onto a polished alumina (Al₂O₃) substrate. This deposition was demonstrated to be compatible with some subsequent microfabrication processes (lithography, contact deposition and passivation) in order to obtain functional sensors [25, 26]. The same research group also studied the influence of the deposition temperatures on the performance of the films [9].

Giant magnetoresistance is also found in other structures. We collect three illustrative examples. Pena et al. [27] report on giant magnetoresistance in ferromagnet/superconductor superlattices. On the other hand, Pullini et al. [28] describe GMR in multilayered nanowires. Thirdly, Svalov et al. report on successful spin-valve structures with Co–Tb based multilayesr [29]. In any case, a magnetic/ non-magnetic interface is required in order to allow the spin-electron scattering producing the effect.

The first approach for implementing a GMR sensing device is to use a common (commercial) magnetic field sensor. We can also find Hall based electrical current sensors sharing this philosophy.

In [31], two AB001-02 GMR gradient sensors from NVE [30] are used in the design of a traffic speed monitoring system. The sensors are placed in a standard PCB with the necessary electronics. In this case, the separation between both sensors and the orientation of the sensors for the highest sensitivity are the key points in the design. On the other hand, for implementing a vibration detector based on GMR sensors, commercially available GMR magnetic sensors (SS501, from HuaXiaMag) were used in [32]. Three different sensors were geometrically arranged in order to cover the three spatial directions.

For specific electric current sensing, soldering the magnetic sensor onto a simple current PCB strip is the common way to implement a measuring system. This straightforward approach is followed in [33], where a AC004-01 from NVE is used for managing the charge and discharge process of a battery. A similar scheme is used in [34] for the conductance control of switching regulators. Being a simple way to construct a sensor, this kind of configuration displays some disadvantages. The first one is that the system is very sensitive to fabrication tolerances. A calibration procedure, not always easy to carry out, needs to be done after the installation. In addition, open designed electrical current sensors are very sensitive to external interferences. Because a typical magnetic sensor is the basic sensing element, any external magnetic field will be added to the current signal and disturb the measurement. When the external magnetic sources are known, they can be eliminated by geometrical arrangements, taking advantage of the directionality of the sensors. If, like in the most of the cases, this sources are unknown, other solutions must be considered. An additional magnetic probe can be placed close to the sensor, but out of the scope of the magnetic field generated by the current. We can also use to identical sensors placed oppositely onto the current track and then to perform a differential measurement. A good review of this sort of approaches can be found in [35].

As an intermediate approach, and if we have access to the sensor microfabrication process, we can consider the design as a mixed microelectronic/mechanical design procedure.

Bio-chip applications need to be specifically considered. Microfluidic devices require a multilayer technology compatible with CMOS-MEMS structures in order to achieve the integration of different functions and materials in a Lab On Chip. The sensing elements need to be placed close to the container containing the fluid to be analysed. The use of microchannels, magnetic markers and auxiliary external magnets are commonly used in this sort of devices. The appearance of epoxy-resin photoresists used jointly with polyimide films have allowed the fabrication of high-aspect-ratio microstructures by standard UV lithography [8, 9].

Regarding electric current sensors, if the current paths are considered as a sensor design step, we can include it in the encapsulation process. This way, a better control can be applied to the overall fabrication process and lower manufacturing tolerances are expected. So the obtained sensors then include two new terminals (input and output) for the current. The simplest way to do it is shown in Figure 2(a), assuming a half bridge operation. By geometrical considerations, a full bridge response can be obtained, as detailed in Figure 2(b)–(d).

When specifically dealing with electrical current sensors, we can directly incorporate the current strips into the integrated circuit during the microfabrication process. As an illustrative example, the cross section of a GMR low current sensor, as reported in [2], is shown in Figure 3. Even though the process is described for a spin-valve based device, it can be directly translated to MTJ or any other GMR structure. In addition, any of the geometrical arrangements shown in Figure 2 can be reproduced here.

The design of this sort of sensors is pretty delicate. The particular encapsulation of the device is on the origin the a number of undesired effects as can be: poor isolation due to an incorrect dielectric selection, thermal limitations originated by Joule heating, mutual couplings as a result of the closeness of the current paths, … Each effect should be separately considered and analyzed in order to obtain an optimal design.

As continuously suggested in this paper, the more straightforward application of GMR based electrical current sensors is the final implementation of an ammeter.

In [38], a specific spin valve sensor for industrial applications is designed, characterized, implemented and tested. The sensor was soldered onto a PCB current track and encapsulated chip-on-board, following the partially compact scheme previously presented. A full bridge (active in pairs) with crossed axis configuration was utilized. The sensor displayed a linear range up to 10 A.

In [16], a novel design principle is presented. The principle of operation is depicted in Figure 2(c). With this configuration, the current flows from-left-to-right above R₁ and R₃, and from-right-to-left above R₂ and R₄. Consequently, when a current is driven (from A to B), and depending on the sign, resistances R₁ and R₃ increase/decrease their values and resistances R₂ and R₄ decrease/increase their values, thus obtaining a full Wheatstone behavior. The sensor is fed through terminals a and b, and the output is taken between terminals c and d. Due to the particular arrangement of the magnetoresistors, this sensor is theoretically insensitive to external magnetic field, therefore minimizing the possibility of interferences (below 1% [16]). This particular approach has been successfully applied to PCB-IC mixed technology based moderate current sensors, with sensitivities close to 1 mV/(V·A).

SV based sensors have been successfully applied to low current measurement, in different scenarios [43], in particular some compatible with CMOS technology. As examples, we can consider the inclusion of these milli-Ammeters in GMR based systems-on-chip (SoC) for biological applications or the need of built-in current sensors (BICS) for built-in self testing of different families of integrated circuits (IC). Some work has been previously reported regarding the application of GMR based sensors to the electrical current measurement at the IC level. In fact, We recently demonstrated the applicability of spin-valve structures to the measurement of low electric currents [2, 44]. In these works, we introduced the concept and some fabrication parameters were established. In [45], the potentiality of SV based full Wheatstone bridges for low current monitoring at the IC level is demonstrated. A number of prototypes are specifically designed, fabricated and tested. The current lines are incorporated in the chip during the microfabrication process which reduces the separation to the sensing elements, leading to improved sensitivity. Therefore, in order to get a balanced bridge, the current paths need to be properly designed. The characteristics and geometry of these current paths have been considered as basic design parameters. Current ranges from 10 μA to 100 mA can be covered with these sensors with excellent linearity and sensitivities above 1 mV/(VmA). AC characteristics have also been analyzed and bandwidths exceeding 100 kHz are demonstrated. In order to highlight the design properties, dependence of the sensor's performance with external magnetic perturbations and self-heating have also been measured and quantified. The associated errors are in the range of 1%–2% of the full scale.

The most of the applications developed with GMR magnetic field sensing is related to the measurement of the Earth's magnetic field perturbations produced by specifically considered ferreous body. This way, a position detecting scheme is always present.

For example, it is possible to use GMR sensors to locally measure the small magnetic perturbations caused by the iron of the car's body over the Earth's magnetic field. Moreover, if we use GMR gradient type sensors, the output signal is only dependent on the magnitude of the magnetic field variation, and no additional external magnetic field compensation is required. This way, a voltage ″signature″ is obtained from the differential output of such a sensor when a car is running close to it. Within this scheme, it is easy to incorporate another sensor, placed to a well known distance in order to also measure the car speed. This proposal has been successfully developed by Pelegrí et al. [31].

The same physical principle can be directly translated to the measurement of vibrations in industrial machines. The small magnetic variations over the Earth's field produced by the vibration of the ferromagnetic pieces in industrial installations can be converted into resistance variations by the use of GMR magnetic field gradient devices. By using three sensors with the appropriated XYZ arrangement, a complete description of the vibration can be obtained. A prototype was developed by Pelegrí et al. [32] and successfully tested with a drilling machine.

For linear magnetic position, in addition to the measurement of the Earth's field variations produced by magnetic materials, we can also use, if possible, permanent magnets associated to the moving part of the system. This way, the measurement of the absolute magnetic field is considered. Arana et al. [26] reported on the design of a high sensitivity linear position sensor using granular GMR devices. Sensitivities above 10 mV/V/mm are demonstrated by the utilization of Nd-Fe-B (0.4 T) magnets.

Angle and circular position detectors are also demanded by the industry: automotive applications, rotational machinery, … This kind of sensors are usually designed as contact-less systems in which a magnetic sensor (GMR in our case) detects the relative angular position of a rotationally moving magnet. This is the case presented in [25] and [12]. In the first case the authors focus on their specifically designed sensor, based on a granular MR. Because of the independence on the magnetic field direction, this technology is optimal for cylindrical symmetry problems. When a NdFeB is used, sensitivities about 0.25 mV/V/ ^° are achieved.

With the rapid development of microfabrication techniques, together with the finding of compatible devices, the concept of Lab-on-a-Chip has become more and more important in the last years. Portable devices have been recently developed which are capable of driving a fluid through microchannels close to a detecting region, with additional conditioning and acquiring electronics. The usual scheme is the detection of the magnetic fringe field of a magnetically labeled biomolecule interacting with a complementary biomolecule bound to a magnetic field sensor. In this context, magnetoelectronics has emerged as a promising new platform technology for biosensor and biochip development [46].

Mujika et al. [8, 9] report on the detection of Escherichia coli O157:H7, an infectious agent that is present in several cook categories, such as meat and milk. Even though no field tests are reported, the developed system is fully described and characterized.

The conservative aerospace sector traditionally used old and well experimented components in its developments. The utilization of brand new technologies in commercial of the shelf (COTS) for space missions is nowadays only in the early stage. COTS are cheaper, faster in delivering and with wider reliability. Michelena et al. [47–49] introduce the possibility of using GMR commercial sensors in space applications. GMR sensors have not been flown yet but INTA, the Spanish National Institute of Aerospace Technology is working on the adaptation of a miniaturized GMR three axis sensor (HMC2003, from Honeywell) to the attitude control system in the frame of the OPTOS project, which is a 10 × 10 × 10 cm³ Picosat devoted to be technological test bed. The circuitry consist of conditioning and biasing electronics blocks.