Magnetoresistive (MR) sensors are finding applications in everything from hard disk drives to space exploration to biomedicine and diagnostics. Despite their wide range, most have one of two goals: to detect an object or determine its position in space.
MR sensors capitalize on the fact that the resistance of many materials changes in the presence of a magnetic field. Calculating the desired output value requires signal processing—a reason why MR sensors present special design challenges. But they compensate by providing accurate, reliable data without physical contact. They can also survive in very harsh industrial environments, and their results are stable over a wide temperature range.
Three kinds of MR sensors are most frequently used by design engineers:
- Anisotropic magnetoresistive effect (AMR)
- Giant magnetoresistive effect (GMR)
- Tunnel magnetoresistive effect (TMR)
The materials and fabrication of each type differ, but one important difference is the signal strength that the sensor produces. AMR produces changes in resistance of about 4 percent, GMR greater than 15 percent and TMR by greater than 400 percent.
AMR technology involves the straightforward measurement of change in resistance of a ferromagnetic material in the presence of a magnetic field. To achieve the best results, however, modern applications use microfabrication and thin-film technology to create AMR chips.
GMR also uses ferromagnetic material but is somewhat more complicated. It utilizes the dependence of electron scattering on spin orientation. It occurs in materials fabricated of very thin alternating ferromagnetic and non-magnetic conductive layers. The effect depends on whether the magnetizations of adjacent ferromagnetic layers are in parallel or antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment.
TMR also depends on microfabrication technologies and ferromagnetic material. A magnetic tunnel junction (MTJ) consists of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (a few nanometers), electrons can tunnel from one ferromagnet into the other. The direction of magnetization of the ferromagnetic films can be switched individually by an external magnetic field.
Figure 1 illustrates the relative simplicity of fabricating an AMR sensor compared to GMR and TMR.
Figure 1: Fabrication structures of three types of MR devices.
Regardless of their type, MR sensors are solid-state switches with no mechanical parts. They activate when the magnetic field of either the north or south poles of a magnetic field are in the sensing range. The sensing direction for an MR device is in the parallel plane of the IC, which differs from Hall sensors that sense a magnetic field perpendicular to the IC. When space is very tight, the sensor chip can be integrated directly on the system’s circuit board. Most MR sensors, however, are still sold in standard packages with leads.
To achieve the most precise results, four MR resistors are usually configured as a Wheatstone bridge. Two of the resistors are sensing resistors and the other two are reference resistors. In the presence of an external magnetic field, the resistance of the sensing resistors decreases while the reference resistors remain unchanged, causing a voltage at the bridge output. This topology provides a voltage output proportional to the magnetic field that has been applied but is also insensitive to variations in the absolute resistance of the MR device.
Applications
The ability of MR sensors to collect information over an air gap is responsible for one of their first widespread uses in information processing. An AMR sensor embedded in the read head of a hard disk drive enables the head (which is positioned over the spinning disc) to determine the polarity of the magnetic data on the disk—basically, to say whether it is a 0 or 1. Introduced by IBM in 1990, AMR read heads enabled rapid increases in data density—about 100 percent per year. In 1997, GMR heads started to replace AMR heads, and in 2004, the first drives to use TMR heads were introduced.
In the consumer world, smartphones use MR technology to sense the earth’s magnetic field in three ways:
- In mapping/driving directions applications, an MR-based magnetometer works with the accelerometer and GPS chip to determine the car’s location and direction.
- When an iPhone or Android is in compass mode, the magnetometer finds magnetic north and orients the screen accordingly.
- Not surprisingly, if a metal-detector app is installed, it’s the magnetometer that makes it work.
MR sensors are used extensively in automotive applications to monitor steering angle, wheel speed and electric motor commutation. The same applications are common in industrial applications.
Figure 2: Wheel speed sensor for automotive and industrial applications
MR sensor robustness is the reason for their extensive use in the space industry. Environmental conditions encountered during space exploration include a temperature range of –130°C to 85°C, high-G forces at launch and solar and cosmic radiation, all of which present real challenges to silicon-based position-sensing chips.
MR sensors function flawlessly at temperatures far above 200°C. Due to their small size and weight, mechanical shock has no significant impact on MR sensors. Only very high radiation affects them. As a result, more than 40 MR sensors were on the Mars Science Laboratory rover, Curiosity, when it landed in 2012.
MR sensor medical applications include uses as simple as position sensing and as complex as medical diagnosis. Applications include positioning hospital beds and measuring the position of cartridges in infusion pumps. AMR sensors are replacing traditional reed switches and Hall-effect sensors in these applications because they perform just as well and offer smaller package size and lower power consumption. Their sensitivity allows smaller magnets to be used, which further reduces overall system costs.
Due to their high sensitivity, less complex instrumentation, compact size and integration flexibility, MR biosensors are showing promise as diagnostic platforms. The idea is to replace traditional fluorescence labeling with MR labeling. In fluorescence labeling, the patient ingests a fluorescent material that can be observed with X-ray or other equipment. With MR labeling, the patient ingests superparamagnetic markers (e.g., magnetic micro-nanoparticles or magnetic nanostructures). Apart from the increased sensitivity, MR biosensors exhibit the unique ability of controlling and modulating the superparamagnetic markers by an externally applied magnetic force as well as the capability of compact integration of their electronics on a single chip.
Conclusion
Although the widespread use of magnetoresistive technology is just several decades old, new forms with improved detection capabilities are always emerging from the labs. Applications are proliferating as well largely because MR sensors are robust, sensitive, compact and operate over an air gap. In many applications, they significantly outperform their silicon competitors.