A Detailed Explanation of the Working Principles of Four Types of Magnetic Field Sensors

The global market for magnetic field sensors is mainly divided into four categories based on technology types: Hall effect sensors, anisotropic magnetoresistive (AMR) sensors, giant magnetoresistive (GMR) sensors, and tunnel magnetoresistive (TMR) sensors. Among them, Hall effect sensors have the longest history and have been widely used. However, with ongoing research and development, various magnetoresistive sensors have emerged with superior performance and higher reliability. Let's see each magnetic field sensor working principle of them.

Hall magnetic field sensors

In 1879, American physicist Hall discovered the Hall effect while studying the conduction mechanism of metals. However, the Hall effect in metals is very weak and has not been practically applied until the discovery of the stronger Hall effect in semiconductors, which led to the development of Hall devices.

By passing a controlled current through a semiconductor thin film and applying a uniform magnetic field in the vertical direction of the film, electrons and holes in the semiconductor collect in different directions due to the Lorentz force. This phenomenon is called the Hall effect. In the direction perpendicular to the current and magnetic field, an intrinsic voltage difference, called the Hall voltage, is generated.

AMR magnetic field sensors

When certain metals or semiconductors encounter an external magnetic field, their resistance changes with the magnitude of the external magnetic field. This phenomenon is called magnetoresistive effect, and the change in resistance is called magnetoresistance.

For strongly magnetic metals with anisotropic properties, the change in magnetoresistance is related to the angle between the magnetic field and the current. Common metals with this property include iron, cobalt, nickel, and their alloys. When the external magnetic field is at zero degrees from the direction of the built-in magnetic field of the magnetic material, the resistance does not change with the external magnetic field. However, when the external magnetic field has a certain angle with the built-in magnetic field of the magnetic material, the internal magnetization vector of the material will shift, resulting in a decrease in film resistance. This characteristic is called anisotropic magnetoresistance (AMR) effect.

Anisotropic magnetoresistive magnetic field sensors have a measurement range centered around the distribution of the Earth's magnetic field, making them ideal for working in the Earth's magnetic field environment. They use common permalloy, have high accuracy, small size, and good stability. Moreover, they only require a single layer of magnetic film in the manufacturing process, making it simple and cost-effective without the need for expensive manufacturing equipment. The magnetic sensor uses make them suitable for mass production and more aligned with the demands of the consumer electronics market.

GMR magnetic field sensors

Compared to Hall sensors and anisotropic magnetoresistive (AMR) sensors, giant magnetoresistive (GMR) sensors are much younger! This is because the discovery of the GMR effect occurred more than 100 years later than the discovery of the Hall effect and AMR effect. As a result, the development and market share of GMR magnetoresistive sensors are relatively less than the first two.

In 1988, German scientist Grünberg discovered a phenomenon: a very small magnetic variation can cause a significant change in resistance in magnetic materials. At the same time, French scientist Fert discovered that weak magnetic field variations can cause a sharp change in resistance in multilayer film resistors made of iron and chromium, with a magnitude tens of times higher than usual. Fert and Grünberg jointly received the 2007 Nobel Prize in Physics for discovering the giant magnetoresistance effect.

Typical magnetic metals have resistance variations of 1% to 3% when a magnetic field is present or absent. However, for multilayer films composed of ferromagnetic metal/non-magnetic metal/ferromagnetic metal, the resistance change can reach 25% at room temperature, which becomes even more pronounced at low temperatures. This is why it is called the giant magnetoresistance effect.

TMR magnetic field sensors

Compared to other magnetic cylinder position sensor, TMR magnetic field sensors have excellent temperature stability, high sensitivity, low background noise, ultra-low power consumption, high resolution, larger dynamic range, and smaller size. They represent the new trend in the development of solid-state sensor technology. TMR magnetoresistive sensors have outstanding performance that cannot be compared to other magnetic field sensors. The exceptional performance of TMR magnetoresistive sensors ensures that this "rising star" will become the mainstream of future magnetic field sensors.