Giant Magnetoresistance (GMR) Sensors

This introduction to spintronic phenomena deals with the three major physical effects of this research field: giant magnetoresistance, tunnel magnetoresistance and spin transfer torque.This presentation aims at describing the concepts in the simplest way by recalling their historical development. The correlated technical improvements mostly concerning material issues are also described showing their evolution with time.

GMR sensors are fabricated by following a sort of techniques in a similar fashion to those related to standard CMOS processes. Deposition, patterning and encapsulation steps are found in both parts. However, characteristics related to the specific materials involved in the GMR technology recommend the use of some particular techniques. In this chapter, we focus on these specific methods, while keeping in mind the interest in merging standard CMOS with GMR technologies.

Giant Magnetoresistances (GMR) and Tunnel Magnetoresistances (TMR) take an increasing part in many applications like current sensing, magnetometry or position sensing, thanks to their high magnetoresistance at room temperature, which leads to a large output signal variation. But the real performances of such sensors can only be estimated with respect to the sources of noise. In this chapter, we give first some bases on noise theory and data treatment. Fluctuations, ergodicity and volume considerations will be discussed. A second part will detail noise measurement techniques and data analysis of typical noise power spectra. Sources of noise will be discussed in a third part. In the end of the chapter, specific cases of GMR and TMR magnetic noise and non magnetic noise will be discussed with their physical origin and their analytical or phenomenological expression. We will then present ways to design GMR and TMR sensors for noise reduction, depending on the applications targeted.

This chapter has the aim to give a complete overview on the first analog front-ends, describing some circuit and system solutions for the design of electronic interfaces suitable for resistive sensors showing different variation ranges: small, as in dedicated-application GMR sensors; wide, especially referred to GMR sensing devices whose baseline is unknown. After a description of the main interface parameters, the authors present several solutions, most of which do not require any calibration. These solutions are different according to the entity of resistive sensor variations, can utilize either AC or DC excitation voltages for the employed sensor and are developed in Voltage-Mode (VM, which considers the use of either the Operational Amplifier (OA) or the Operational Transconductance Amplifier (OTA) as main active block) as well as in Current-Mode (CM) approach (being in this case the Second Generation Current Conveyor (CCII) the active device). The described interfaces can be easily fabricated both as prototype boards, for a fast characterization, and as integrated circuits, also using modern microelectronics design techniques, in a standard CMOS technology with Low Voltage (LV) and Low Power (LP) characteristics, especially when designed for portable applications and instrumentation. Moreover, thanks to their reduced sizes in terms of chip area, the proposed solutions are suitable for being used for sensor arrays applications, where a number of sensors is employed, as in portable systems, to detect different environmental parameters.

The Giant MagnetoResistance (GMR) effect is a magnetic coupling mechanism that can be obtained in multilayer structures of few nanometers thick. In these devices, and at room temperature, the resistance is a function of the external magnetic field, at optimal levels for being used as sensors. Since the GMR effect was reported, scientists and engineers have dedicated their effort to this topic. This way, after two decades, a a very good knowledge of the GMR underlying physics together with notable designs of GMR based devices are nowadays available. They were initially used in the read heads of hard drives, but the constant evolution that this technology has experienced has open new fields of application, mainly related to the measurement of small magnetic fields using miniaturized devices, such as biotechnology and microelectronics.

Regarding the microelectronics case, these sensors can be potentially used in those scenarios that require a detection or measurement of nonintrusive power by the indirect measurement of the magnetic field.

In this chapter, an overview to the current research regarding the application of GMR sensors in the measurement of electrical currents at the integrated circuit (IC) level is drawn. In this particular case is important to take account of particular parameters of the GMR devices such as the sensing structures, the geometric arrangement and implementation of the sensors, the considered linear range, undesired couplings, biasing, hysteresis, temperature drifts, ... We have also described some cases of success describin particular applications of these devices. Finally, some aspects related to the monolithic integration of GMR devices onto standard CMOS engineered chips are also considered in this chapter.

In this chapter, the various automotive applications for GMR based sensors are presented. The different applications have different requirements which are reflected in the individual sensor concept and features. The advantage of GMR based magnetic sensors over conventional silicon based Hall sensors is shown.

The use of giant magnetoresistors, or GMR, for compass magnetometers is a recent trend in Earth field sensing. Thin film compass devices are often used in applications in global positioning systems (GPS) to aid in navigation. A GPS system can only tell a user about the direction of travel as long as the user is in motion. A compass indicates static orientation of the user which, with the addition of gyroscopic information along with GPS data can give precise location and orientation to users. Modern digital devices often incorporate compass devices with location applications to better aid consumers in locating services.

Space is an environment of extreme parameters. Wide temperature swings, very low pressures (vacuum), moderate to high radiation, mechanical vibrations and impacts, etc. Thus, components for space applications, which need to stand these hard conditions, are normally very expensive and it often takes a while to include emerging technologies in the space market. This means that space components are not always that innovative.

The case of vector magnetometers is not an exception. Since the beginning of the space exploration mainly fluxgate magnetometers have been used for magnetic mapping [1]. Fluxgates are robust sensors and massive core fluxgates present very good performances for geomagnetic mapping and further exploration in the solar system. Besides, they are normally combined with a scalar absolute sensor for calibration of the vector magnetometer.

In an attempt to be able to get ready as fast as possible to use emerging magnetic sensing technologies for space applications, INTA has devoted some effort in the qualification for flight use, of Commercial Off-The-Shelf (COTS) solid state magnetic sensors, as AMR and GMR sensors [2-4].

In this chapter we describe the chain of testing and adaptation of the available commercial GMR sensors for an experimental payload in a picosatellite (OPTOS, 3 kg). We present the calibration tests results and the expectations we have for the in-orbit measurements.

In this chapter, we report the utilization of spin-valve type giant magnetoresistance (GMR) sensors in non-destructive evaluation (NDE). The NDE application is the inspection of high-density printed circuit boards (PCBs) based on the eddy-current testing (ECT) technique. An ECT probe with a GMR sensor is presented for the inspection of high-density double-layer PCB models. The utilization of a GMR sensor as a magnetic sensor showed that PCB inspection could be performed with high-spatial resolution and sensitivity, over a large frequency range.

Magnetic fluid based hyperthermia therapy for treating cancerous tumors can be performed with a high success rate and minimal error given the possibility of detecting and estimating magnetic fluid weight density in vivo. In this chapter, a uniquely designed GMR needle probe is presented for the detection and estimation of magnetic fluid content density inside tumors. Experimental results showed that the proposed technique has a good potential to be implemented in hyperthermia therapy in the future.

This chapter provides an overview on several techniques used for surface imaging, including SQUIDs, Hall-effect sensors, Giant magnetoimpedance sensors, and magnetoresistive (MR) sensors. Among all magnetic field sensors, only SQUIDs and MR devices have the potential to localize buried and non-visual field sources (such as defects in integrated circuits or magnetic field sources in biological environments. In particular, we describe how MR sensors have been used with advantage for integrated circuit (IC) mapping, with resolution below 500 nm and sensitivity to detect currents as low as 50 nA and have been used for many applications requiring low magnetic field detection. Challenges and experimental considerations on integration of MR sensors on a commercial analysis tool are provided here. Examples obtained with real devices demonstrate how Scanning Magnetic Microscopy has become an established failure analysis technique for visualizing current paths in microelectronic devices.