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Magnetostrictive sensors are a well-known class of sensors that have been around for a long time and have found commercial use in many applications; ranging from industrial applications to everyday objects, such as laptops. These sensors utilize the magnetoresistive effect via a thin film, and this enables measurements to be taken without the need for physical contact. In this article, we look at these sensors, as well as their applications and the magnetoresistive effect.
Magnetoresistive sensors are based around the magnetoresistive effect, otherwise known as magnetoresistance. Magnetoresistance is the capability of a material to alter its electrical resistance under an applied magnetic field. In magnetostrictive sensors, this change in electrical resistivity can be detected, and many properties of a remote object/environment can be backed out as a result. The concept of magnetoresistance is an old one that dates back to 1857, but it wasn’t realized in any technical applications until 100 years later after being conceived.
There are two main types of magnetoresistive effects, depending on which types of materials are employed. The first, known as the Anisotropic Magnetoresistive (AMR) effect, is mostly exhibited by thin films of rare earth elements, transition metals, and ferromagnetic films. The second mechanism, known as the Giant Magnetoresistive (GMR) effect, is often seen with multi-layered films where a non-magnetic layer parts two magnetic layers. However, because most thin-film magnetostrictive sensors exhibit the AMR effect, we’re going to focus on that one.
The AMR effect specifically occurs in 3d transition metals, and the change in resistivity of these materials can be seen macroscopically. The AMR effect occurs within transition metal thin films because the 3d electron sub-shell is not filled, which causes the electrons in the 4s sub-shell to scatter into the 3d atomic orbitals under a magnetic field. The anisotropy is due to the asymmetric nature of the electrons’ orbits (due to spin-orbit coupling), and this causes the electrons to travel either parallel or perpendicular to the direction of magnetization.
Thin Film Magnetostrictive Sensors
Thin-film magnetostrictive sensors can sometimes be hard to use, as some signal processing is required to convert the signal into the desired output. However, they are a very useful tool because they can provide measurements without the need for physical contact. Overall, magnetostrictive sensors are a tool for measuring any remote disturbance due to an object or other event interfering with the local magnetic field. From a property perspective, they can provide information on direction, presence, rotation, angle, or electrical currents within these magnetic fields, but it is often held as an electrical current, hence the need for extra processing and conversion.
The main difference between thin-film and non-thin-film magnetostrictive sensors is in the active material component that undergoes magnetoresistance. Even though magnetoresistance can occur in some bulk materials and semiconductors, it is more often seen with thin films. The reason for this is that the thin films used in these sensors possess large saturation magnetostriction, high saturation magnetization, low anisotropy energies, and a low coercivity - which are some of the main practical requirements for these sensors.
In general, a Wheatstone bridge - an electrical circuit that measures an unknown electrical resistance using two legs of a bridge circuit - is usually employed to operate a magnetostrictive sensor. The so-called Hunt element (after the creator of the first magnetostrictive sensor) used to be used, but a Wheatstone bridge has a zero-reference point and doesn’t have any temperature dependence (unlike the Hunt element). The thin film within the sensor is often implemented with a curved geometry so that it induces a strong magnetic anisotropy and can provide a well-defined orientation of sensitivity. As mentioned, magnetostrictive sensors require the electrical output to be correlated. This takes the form of a quadratic trigonometric function, and this restricts many sensors to monitoring at an angle of 180°. However, magnetostrictive sensors have been designed to monitor a rotation 360° by using two Wheatstone bridges.
Applications of Magnetostrictive Sensors
The applications of magnetostrictive sensors are commercially widespread. These applications include:
- Mass sensors
- Compassing applications
- Stress sensors
- Magnetic tags in biomolecular detection for protein assays
- Magnetic bead manipulation in microfluidic systems
- High-frequency RF devices
- Transducers in fiber grating sensors (as well as other types of fiber optic sensor)
- Cylinder position sensors
- Blood analyzers
- Sensors for high-temperature environments (as the measurement values are not affected by temperature)
- Industrial proximity sensors
- Flow sensors
- Magnetic encoders
There are even some applications in everyday objects that many may not be aware of, such as in the lid of a laptop, in traffic lights to detect burnout, in handicapped vehicle lifts, in elevators and forklift trucks as a position sensor.
Sources and Further Reading
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