In today’s electronic age, magnetic sensors have a wide range of applications in motors, power electronics technology, the automotive industry, industrial automation, robotics, office automation, household appliances, and various security systems.

Magnetic sensors are devices that convert changes in magnetic properties caused by external factors such as magnetic fields, current, stress, strain, temperature, and light into electrical signals, thereby detecting corresponding physical quantities. They are used to sense speed, movement, and direction, with applications in automotive, wireless and consumer electronics, military, energy, medical, and data processing.

Diagram of the Hall Effect

Working Principle of Switch Type Hall Sensors

They use the potential difference generated by the Hall Effect to measure the magnitude and polarity of external magnetic fields. Magnetometers are planar devices using CMOS technology. The process is relatively simpler than that of general ICs, usually formed by a P-type substrate with N-well to create sensor devices, connected to other circuits (such as amplifiers, processing regulators, etc.) through metal electrodes.

However, Hall sensors designed this way can only sense magnetic field changes perpendicular to the surface of the chip, thus a magnetic flux concentrator is added, which is essentially a layer of permalloy added to the original chip to detect magnetic fields parallel to the chip’s direction. This allows Hall sensors to achieve a leap from uniaxial to triaxial magnetometers.

In 1857, Thomson discovered the anisotropic magnetoresistance effect of permalloy. For ferromagnetic metals with anisotropic characteristics, the change in magnetoresistance is related to the angle between the magnetic field and the current. Common examples of such metals include iron, cobalt, nickel, and their alloys.

When the external magnetic field is at zero degrees to the built-in magnetic field of the magnet, the resistance does not change with the external magnetic field; however, when there is a certain angle between the external magnetic field and the built-in magnetic field, the internal magnetization vector of the magnet will shift, reducing the film’s resistance. This characteristic is referred to as the anisotropic magnetoresistive effect (Anisotropic Magnetoresistive Sensor, abbreviated as AMR). The effect of the magnetic field is shown in the diagram below.

Relationship Between Magnetoresistance Change and Angle Change

GMR and AMR Resistivity Changes Under External Magnetic Field

The term “giant” to describe this type of magnetoresistance effect comes not only from its apparent characteristics but also from its formation mechanism being different. Conventional magnetoresistance arises from the direct action of the magnetic field on the motion of electrons, presenting anisotropic magnetoresistance, which is related to the relative orientation of magnetization strength and current. In contrast, GMR magnetoresistance is isotropic and is fundamentally unrelated to the relative orientation of magnetization strength and current.

The Giant Magnetoresistance effect depends solely on the relative orientation of the magnetic moments of adjacent magnetic layers; the external magnetic field merely serves to alter the relative orientation of the magnetic moments of adjacent ferromagnetic layers. Furthermore, the GMR effect has significant implications for further exploration of new physics—such as tunneling magnetoresistance effect (TMR: Tunneling Magnetoresistance), spintronics, and new sensor technologies.

Basic Structure of Spin Valve GMR Sensors

Equivalent Circuit Diagrams Under Parallel and Anti-Parallel Magnetic Fields

As early as 1975, Julliere observed the TMR (Tunnel Magneto-Resistance) effect in Co/Ge/Fe magnetic tunnel junctions (MTJs). However, this discovery did not attract much attention at the time. In the subsequent decades, research progress on the TMR effect was slow. It was not until in-depth studies of the GMR effect that the TMR effect, also part of spintronics, began to gain attention. In 2000, the discovery of MgO as a tunnel insulator provided an opportunity for the development of TMR magnetic sensors.

In 2001, Butler and Mathon made theoretical predictions that using iron as the ferromagnetic material and MgO as the insulator, the tunneling magnetoresistance could reach several percent. In the same year, Bowen et al. first experimentally demonstrated the TMR effect in magnetic tunnel junctions (Fe/MgO/FeCo). In 2008, the team of S. Ikeda and H. Ohno from Tohoku University found that the resistivity change in the magnetic tunnel junction CoFeB/MgO/CoFeB reached 604% at room temperature and exceeded 1100% at 4.2K. The TMR effect’s significant resistivity change has led to increased interest in its research and product development.

Comparison of Technical Parameters of Hall Devices, AMR Devices, GMR Devices, and TMR Devices

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