In industry, magnetic field sensing is often used for control and measurement--linear and rotary position sensing, gear tooth sensing, and current sensing . In these applications, large magnetic fields are used to avoid confusion with background magnetic fields, such as the Earth's magnetic field, fields from ferromagnetic objects, and EMI. The fields detected are generated by permanent magnets or currents in coils, sometimes with soft magnetic cores. The size of the magnetic fields is usually tens to hundreds of oersteads (1 Oe = ~80 A/m.) Because magnetic field sources are inherently dipole in nature, they decrease with the inverse cube of distance. Therefore, the fields from these sources are localized.
|Figure 1. This illustration compares the power and cost of several low magnetic field sensor technologies. The size of the circle indicates the relative size of the device|
Despite the difficulty encountered when measuring low fields, magnetic fields of <1 Oe are attracting increasing attention in industry. Compassing applications detect the components of the Earth's magnetic field (<0.5 Oe) to determine direction relative to magnetic north. Sensitive instruments that measure magnetic fields or magnetic field gradients can detect at considerable distances the small magnetic fields of materials (e.g., motor vehicles and buried iron surveying stakes) magnetized by the Earth's magnetic fields.
Other objects produce small magnetic fields because they are small themselves. For example, the black ink in many currencies and other negotiable documents contains magnetic particles that act as dipoles. Denomination determination and currency validation can be based on the magnetic signature of a bill passed close to a magnetic sensor. The more sensitive the sensor, the larger the allowable head-to-sensor gap. Eddy current sensing to detect flaws in conducting materials or even differing conductivity in soil requires high-frequency, low-field sensors.
Solid-state magnetic field sensors have an inherent advantage in size and power consumption when compared with search coil, flux gate, and more complicated low-field sensing techniques (e.g., superconducting quantum interference detectors [SQUID] and spin resonance magnetometers). A solid-state magnetic sensor converts the magnetic field into a voltage or resistance. The sensing can be done in an extremely small, lithographically patterned area, further reducing size and power requirements. The small size of a solid-state element increases the resolution for fields that change over small distances and allows for packaging arrays of sensors in a small enclosure. Figure 1 compares the cost and power of several low-field sensors, all designed with the same minimum field resolution limited by thermal noise, 10–8 Oe/(check)Hz.
Recent developments in thin-film magnetic technology have resulted in films that exhibit a large change in resistance in response to a magnetic field. This phenomenon is known as giant magnetoresistance (GMR) to distinguish it from conventional anisotropic magnetoresistance (AMR). Where AMR resistors exhibit a change of resistance of <3%, various commercial GMR materials achieve a change of 10%–20%.
GMR films have two or more magnetic layers separated by a nonmagnetic layer. Because of spin-dependent scattering of the conduction electrons, the resistance is maximum when the magnetic moments of the layers are antiparallel [2,3,4], and minimum when they are parallel. The structures used in GMR sensors are unpinned sandwiches, antiferromagnetic pinned spin valves, and antiferromagnetic multilayers. Computer manufacturers are increasingly using spin valves as magnetic read heads in hard disks .
Spin-dependent tunneling (SDT) structures also exhibit GMR. In these structures, an insulating layer separates two magnetic layers. The conduction is due to quantum tunneling through the insulator. The size of the tunneling current between the two magnetic layers is modulated by the direction of the magnetization vectors in the two layers. The conduction path must be perpendicular to the plane of the GMR material because there is such a large difference between the conductivity of the tunneling path and that of any path in the plane.
SDT devices measuring several microns on a side with high resistance can be built using photolithography, allowing dense packing of magnetic sensors in small areas. These materials are a topic of considerable research. GMR values of 10%–30% have been regularly observed. The saturation fields depend on the composition of the magnetic layers and the method of achieving parallel and antiparallel alignment. Values of saturation field range from 0.1 kA/m to 10 kA/m (1.25–125 Oe), offering the possibility of extremely sensitive magnetic sensors. The insulating, tunneling layer provides inherently high-resistance sensors suitable for battery operation.
Magnetic Fields in
Industry and Medicine
There are many places in industry and in medicine in which magnetic fields smaller than the earth's field are of interest. The source of these fields can be magnetized objects, electrical currents, or the Earth's magnetic field. The low-field aspect of these applications can be due to the distance to a magnetic object or the size of the object itself.
All magnetic sources produce a magnetic dipole field if the observer is at a distance from the source. Dipole fields decrease as the inverse cube of the distance from the source. The fields are also proportional to the volume of the source and to the maximum magnetization at the source. A magnetized cylinder whose diameter and length are one-half those of a larger cylinder at any distance will have a magnetic field one-eighth as strong as the field from the larger cylinder. In addition, doubling the distance from a magnetized cylinder will decrease the field to one-eighth the field at the original position. Distance and miniaturization lead to low fields.
Objects made of soft magnetic materials are easily magnetized by relatively small magnetic fields. These objects can be as simple as the small iron pipes used as surveying markers or as complex as entire automobiles and trucks. In one application, the objective is to locate a buried object from a distance; in the other, it's to detect the presence or passage of a vehicle close by. In both cases, the smaller the field detected, the more useful the sensor. Also, the field detected must be separated from the Earth's magnetic field, which may be stronger than the field of interest.
Various methods are used to subtract the Earth's magnetic field. Because this field is constant, it can be subtracted in applications in which the sensor is stationary. In applications for which the field of interest is time varying, the Earth's field can be subtracted or filtered out.
When looking for a magnetic dipole that is fixed relative to the Earth, two sensors separated by a distance can be used as a field gradient sensor. The dipole field from the object sought will have a larger field gradient than that of the Earth, whose center is several thousand miles away. Low-field magnetic sensors and magnetic field gradient sensors can also be used to locate magnetized objects or even holes in ferromagnetic plates behind concealing nonmagnetic sheets. An extension of the same principle is the location of unexploded ordnance (UXO) using magnetic sensors and arrays of magnetic sensors.
|Figure 2. The graph plots voltage vs. the applied field for a 2 mm wide stripe of unpinned sandwich GMR material with 1.5 mA current. The GMR equals 5%.|
Magnetic sensors using eddy current technology can even detect nonmagnetic metals. An AC magnetic field generated by a current in a coil causes eddy currents in the conducting material that opposes the applied field. A magnetic field sensor can detect the difference between a field with and one without the conducting material present. The sensor can be oriented in such a way that its sensitivity does not lie in the direction of the field generated by the coil. The presence of a conducting material, or even the existence of a crack or flaw in the conducting material, can change the direction of the magnetic field enough that the sensor will detect a magnetic field.
You can use the same general principle for subsurface geophysical exploration . Eddy current techniques can detect changes in the conductivity of soil caused by water and can identify buried conducting pipes or nonconducting pipes containing water.
A financial application in which small magnetic fields are detected is magnetic ink character recognition (MICR). In this process, the sensor reads the magnetic numbers on the bottom of checks. The stylized MICR numbers produce a unique magnetic signature when the checks are sorted at high speeds.
A number of medical applications involve the detection of small magnetic fields, such as the monitoring of magnetic fields from physiological functions. Nerve impulses are electrical currents, and these currents create magnetic fields. Monitoring nerve signals by detecting the magnetic fields is less invasive and more reliable than implanting electrodes to pick up voltage signals.
|Figure 3. This graph plots resistance vs. applied field for a 2 mm wide stripe of antiferromagnetically coupled multilayer GMR material. The GMR equals 14%.|
Relatively large and cumbersome magnetometers (e.g., SQUIDs) are used for monitoring magnetic signals for such studies as magneto-encephalographs. Improved solid-state magnetic sensors will make it possible to manufacture smaller sensors that can be placed closer to the source of the magnetic fields, resulting in larger signals.
Monitoring the position of parts of the body, especially the head, is important to various medical studies. Position monitoring is also used in virtual reality and heads-up targeting. Three-axis magnetic sensors attached to the body part in question detect the components of the Earth's magnetic field. From this information, the orientation relative to the Earth's field can be calculated. By adding accelerometers, the actual position can be calculated .
Rapid, portable biosensors that use low-field magnetic sensors to measure the presence of DNA or antibodies are a recent area of research . Small magnetic beads coated with biological molecules are allowed to settle on a substrate with a substance that bonds to specific molecules of interest. After removing the beads that have not bonded to the substrate, the presence of the remaining magnetic microbeads is detected by magnetic sensors. Several bioassays can be simultaneously accomplished using an array of magnetic sensors, each with a substance that bonds to a different biological molecule. This application requires extremely small, low-power, low-field magnetic sensors.
As mentioned in an earlier section, GMR materials must have at least two separated ferromagnetic layers whose magnetization vectors can assume different, in-plane directions. Unpinned sandwich GMR materials consist of two soft magnetic layers of iron, nickel, and cobalt alloys separated by a layer of a nonmagnetic conductor (e.g., copper). With magnetic layers 4–6 nm (40–60 Å) thick separated by a conductor layer 3–5 nm thick, little magnetic coupling occurs between the layers.
|Figure 4. The plot shows sheet resistance vs. applied field for an antiferromagnetically pinned spin valve with the field applied parallel to the magnetization of the pinned layer. The GMR is 6%.|
In sensors, the sandwich material is usually patterned into narrow stripes a few microns wide. The magnetic field generated by a current of a few milliamperes per micron of stripe width flowing along the stripe is sufficient to rotate the magnetic layers into antiparallel or high-resistance alignment. An external magnetic field of 3–4 kA/m (35–50 Oe) applied along the length of the stripe can overcome the field from the current as well as any magnetic interaction between the layers and rotate the magnetic moments of both layers parallel to the external field, reducing the resistance. A positive or negative external field parallel to the stripe will produce the same change in resistance. An external field applied perpendicular to the stripe will have little effect due to the demagnetizing fields associated with the extremely narrow dimensions of the magnetic objects. Therefore, these stripes effectively respond to the component of magnetic field along their length.
The characteristic value usually associated with the GMR effect is the percent of change in resistance normalized by the saturated, or minimum, resistance. Typically, sandwich materials have GMR values of 4%–9% and saturate with 2.4–5 kA/m (30–60 Oe) applied field. Figure 2 shows a typical resistance vs. field plot for sandwich GMR material.
Antiferromagnetic multilayers of GMR material consist of repetitions of alternating conducting magnetic layers and conducting nonmagnetic layers. Because multilayers have more interfaces than do sandwiches, the GMR effect is greater. The thickness of the nonmagnetic layers is less than that of the sandwich material (which is typically 1.5–2.0 nm), and the thickness is critical. The polarized conduction electrons cause antiferromagnetic coupling between the magnetic layers only when the spacers are a certain thickness. In the absence of an external magnetic field, each magnetic layer has its magnetic moment antiparallel to the moments of the magnetic layers on each side--exactly the condition needed for maximum spin-dependent scattering. A large external field can overcome the coupling that causes this alignment and can align the moments so that all the layers are parallel--the low resistance state. If the conducting layer is not the proper thickness, the same coupling mechanism can cause ferromagnetic coupling between the magnetic layers resulting in no GMR effect.
|Figure 5. This plot shows sheet resistance vs. applied field for an antiferromagnetically pinned spin valve with the field applied perpendicular to the magnetization of the pinned layer. The GMR is 2.4%.|
A plot of resistance vs. applied field for a multilayer GMR material is shown in Figure 3. Note the higher GMR value, typically 12%–16%, and the much higher external field required to saturate the effect, typically 20 kA/m (250 Oe). Most multilayer GMR materials have better linearity and lower hysteresis than sandwich GMR material.
Spin valves, or antiferromagnetically pinned spin valves, are somewhat similar to unpinned spin valves or sandwich materials. An additional layer of antiferromagnetic material is added at the top or the bottom. The antiferromagnetic material (e.g., FeMn or NiO) couples with the adjacent magnetic layer and pins it in a fixed direction. The other magnetic layer is free to rotate. If the external magnetic field is applied in a direction parallel to the magnetization of the pinned layer, there is a change in sheet resistance from its high level for one field direction to a low level for the opposite field direction (see Figure 4,). If the field is applied perpendicular to the pinned layer, the sheet resistance is minimum at zero field and increases for both positive and negative applied fields (see Figure 5). The maximum change in sheet resistance in this configuration is only one half the total possible value. The free magnetic layer rotates from parallel to the pinned layer to perpendicular to it rather than from parallel to antiparallel.
These materials do not require the field from a current to achieve antiparallel alignment or a strong antiferromagnetic exchange coupling to adjacent layers. The direction of the pinning layer is usually fixed by elevating the temperature of the GMR structure above the blocking temperature. Above this temperature, the antiferromagnet is no longer coupled to the adjacent magnetic layer. The structure is then cooled in a strong magnetic field, which fixes the direction of the moment of the pinned layer. If the spin valve material is heated above its blocking temperature, it can lose its orientation. The operating temperature of a spin valve sensor is limited to below its blocking temperature. Because the change in magnetization in the free layer is due to rotation rather than domain wall motion, hysteresis is reduced. GMR values are 4%–20%, and the saturation fields are 0.8–6 kA/m (10–80 Oe).
To obtain significantly higher sensitivities to magnetic fields, a new type of magnetoresistive material is being adapted to use in magnetic field sensors. The material exhibits a phenomenon called spin-dependent tunneling (SDT), which results in a change in effective resistance caused by a change in the applied field . The resistance vs. field effects are similar to the usual GMR spin valve effect but larger. Sensors have been built from SDT material for use in low-field applications that presently require fluxgate magnetometers. As with other GMR sensors, the devices are small (SOIC-8 package), require little power, and are easily combined with other electronics.
Next month, Sensors will run the second and final part of this article. In it, we will discuss GMR sensors and circuit considerations relating to noise reduction.
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Carl H. Smith is Senior Physicist and Robert W. Schneider is Director of Marketing at Nonvolatile Electronics, Inc., 11409 Valley View Rd., Eden Prairie, MN 55344, 612-829-9217, fax 612-996-1600.