A magnetoresistive sensor, also known as a magnetic field sensor or magnetoresistor, is a type of sensor that measures changes in the magnetic field. It relies on the magnetoresistive effect, which is the change in electrical resistance of certain materials when exposed to a magnetic field. This effect was first observed in the 1850s by William Thomson (Lord Kelvin), and later became the basis for the development of magnetoresistive sensors.
The working principle of a magnetoresistive sensor can be explained as follows:
Magnetoresistive materials: The sensor is typically constructed using materials that exhibit the magnetoresistive effect. These materials include certain metals and semiconductors like permalloy, amorphous alloys, or giant magnetoresistive (GMR) materials. These substances have specific properties that make their electrical resistance change in response to varying magnetic fields.
Electrical resistance: Electrical resistance is a property that opposes the flow of electrical current through a material. In a magnetoresistive sensor, when there is no external magnetic field, the electrons flow through the material with minimal resistance.
Application of a magnetic field: When a magnetic field is applied to the magnetoresistive sensor, the orientation of the magnetic domains within the material aligns with the direction of the magnetic field. This causes a change in the electrical resistance of the material.
Magnetoresistive effect: There are two main types of magnetoresistive effects observed in these sensors:
a. Anisotropic Magnetoresistance (AMR): In materials exhibiting AMR, the electrical resistance changes with the angle between the direction of the current flowing through the material and the direction of the magnetic field. The resistance is minimized when the current and magnetic field are parallel and maximized when they are perpendicular.
b. Giant Magnetoresistance (GMR): GMR materials have multiple layers of ferromagnetic and non-magnetic materials stacked together. When a magnetic field is applied, it causes a change in the alignment of the magnetic moments in the ferromagnetic layers, affecting the electrical resistance of the entire structure. This change can be much more significant than in AMR materials, leading to higher sensitivity in GMR-based sensors.
Measurement: To measure the change in resistance due to the magnetic field, the magnetoresistive sensor is connected to an electrical circuit. A current is passed through the sensor, and the resulting voltage drop across it is measured. By monitoring the voltage or current changes, the sensor can detect and quantify variations in the surrounding magnetic field.
Magnetoresistive sensors find numerous applications, including magnetic field measurement, position sensing, automotive applications (e.g., wheel speed sensors), and magnetic storage devices (e.g., hard disk drives). They offer advantages such as high sensitivity, low power consumption, and compatibility with integrated circuit technology.