Working Principle and Development of Magnetic Sensors

Catalog

Ⅰ Introduction

Magnetic sensors are widely used in modern industrial and electronic products to measure physical parameters such as current, position, and direction through the intensity of the induced magnetic field. In current technology, there are many different types of sensors for measuring magnetic fields and other parameters. They have the characteristics of non-contact measurement, durability, and long service life. The detection signal is almost unaffected by the measured object. They are resistant to pollution and strong noise and can work reliably even in very harsh environmental conditions. Because of these advantages, such sensors are used in various sectors from defense and aerospace to the national economy, from medical care to everyday life.

The history of magnetic sensors begins with the use of magnets as compasses for navigation. Later, as components for sensing magnetic fields and magnetic fluxes, detection coils, fluxgate magnetometers, semiconductor Hall elements, and magnetoresistive elements were developed. This evolution included ferromagnetic thin film anisotropic magnetoresistive (AMR) components, stress sensors using bulk ferrite cores, temperature sensors using thermosensitive ferrite cores, fiber optic current sensors using ferromagnetic garnet magneto-optic effect, high sensitivity superconducting quantum interference devices (SQUID), and many more. In short, there are many types of magnetic sensors with continuous technological advancement.

compass

Magnetic sensors are usually assembled and used inside machines and equipment. Modern complete machines are rapidly developing towards smaller, lighter, multi-functional, and intelligent designs. They require sensors to respond with high sensitivity and high speed even when detecting physical quantities in small spaces. That is, while the sensor itself needs to be small and lightweight, there is also a demand to improve its working speed, detection resolution, and sensitivity.

The promotion and application of semiconductor large-scale integrated circuit manufacturing technology, micro-electromechanical system (MEMS) manufacturing technology, micro-assembly technology, and new materials such as magnetic thin films, multilayer films, and nano-magnetic wires have laid the foundation for miniaturization of magnetic sensors. Many new high-performance, miniaturized magnetic sensors that utilize various novel effects are being continuously introduced to the market. The AMR thin-film sensitive components and sensors that were launched in the early days, the GMI sensors, SI sensors, SV-GMR sensors, TMR (Tunnel Magnetoresistance) sensors, and thin-film fluxgate magnetometers are typical examples of this evolution.

Magnetic sensors are ubiquitous, compact, and affordable. They can be easily integrated with other circuits on a chip. Therefore, magnetic sensors are widely used in various applications including automotive systems, consumer electronics, industrial automation, and IoT devices. The advantages of magnetic sensing technology in robotics and factory automation are particularly significant. Since magnetic sensors offer higher reliability and precision in detecting component position and speed, they are crucial for motion control. This advantage allows robot arms and other components to move smoothly and accurately, thereby ensuring high-quality and high-security manufacturing processes.

Ⅱ Working principle of magnetic sensors

The working principle of magnetic sensors involves converting magnetic field information into voltage or current signals. Because the internal operation of the sensor and external components do not require physical contact, magnetic sensors are ideal for reducing environmental pollution in automotive and industrial environments. At the same time, they can also reduce wear caused by friction between components, thereby reducing equipment maintenance costs.

Magnetic sensors have a variety of functions, but it should be particularly noted that there are several types of sensors that can be widely integrated into other circuits, including Hall effect sensors, fluxgate sensors, magnetoresistive sensors (AMR, GMR, TMR), and magneto-inductive sensors.

Hall effect sensors can reliably operate in various electromagnetic environments and are highly respected by manufacturers. Common applications include fuel level gauges, which are mainly used to detect floats in fuel tanks. They can also be used in brushless motors to detect rotor position and control current timing. When Hall sensors are integrated with other circuits, the manufacturing process does not require special materials or special processing, which helps reduce design costs. Modern Hall sensors offer improved sensitivity and can operate at higher temperatures, making them suitable for automotive and industrial applications.

Hall effect sensor

Compared with Hall-effect sensors, fluxgate sensors are more sensitive and can detect very subtle changes in the magnetic field. Common applications for fluxgate sensors include electronic compasses for ships and aircraft, instruments used by geologists to detect underground structures, and space weather monitoring systems. Modern fluxgate sensors have achieved significant improvements in noise reduction and power consumption.

fluxgate sensor

Ⅲ Four stages of magnetic sensor development

There are many types of magnetic sensors with different performances and applications. Researchers have been pursuing magnetic sensors with superior performance. Among them, sensors with high performance, miniaturization, low power consumption, and low-cost development potential are of particular interest, such as superconducting quantum interference (SQUID) magnetometers, optical pump atomic magnetometers, TMR magnetic sensors, and fluxgate sensors.

1 High-temperature superconducting materials

SQUID can be used as a magnetic sensor based on the principle of magnetic flux passing through a superconducting ring to induce superconducting current. Its resolution can reach the femtotesla (fT) level, and the magnetic field resolution of commercial SQUID systems can reach the order of 10fT or better. It can be said that SQUID is currently one of the most sensitive magnetic sensors available. However, because the superconducting ring can only work at extremely low temperatures, SQUID must be equipped with a refrigeration device, which leads to a substantial increase in its volume, weight, and cost of use, which restricts the application range of SQUID. The emergence of high-temperature superconducting materials has brought new vitality to SQUID technology.

In 1987, M.K. Wu and colleagues reported a YBaCuO (Yttrium Barium Copper Oxide) high-temperature superconducting material system with a critical temperature of 93K, which means that the material system can enter the superconducting state under a liquid nitrogen environment (77K). Compared with superconducting materials working in a liquid helium (4K) environment, it has obvious advantages because the preparation difficulty and cost of liquid nitrogen are much lower than that of liquid helium. Therefore, SQUID based on high-temperature superconducting materials is now used in biomedicine, geophysical prospecting, material research, non-destructive testing, and other fields.

Manufacturing high-quality high-temperature superconducting thin films through micro-nano processing technology to create miniaturized SQUID probes has become an active area of research. SQUID magnetic sensors combining MEMS planar coils and YBaCuO thin films have been developed with resolutions reaching approximately 50fT/√Hz (@1Hz). Although electrothermal noise of the detection coil and temperature fluctuations of the superconducting ring in high-temperature SQUID systems result in key indicators that are lower than those of SQUID working in a liquid helium environment, ongoing efforts focus on miniaturization and high-temperature operation. Thin-film materials and noise reduction technology remain the main breakthrough directions for SQUID development.

2 MEMS technology

The optical pump atomic magnetometer is a scalar magnetic sensor. Its basic principle is to use optical pumping to excite alkali metal gaseous atoms in a closed cavity to achieve the same spin precession state, and the atomic spin precession frequency changes linearly with the external magnetic field. The measured magnetic field is obtained by detecting the spectral frequency shift through an optical detector. The resolution of optical pump atomic magnetometers can reach 1fT/√Hz (@1Hz), which is comparable to SQUID. The performance of optical pump atomic magnetometers mainly depends on the consistency of the spin states of the alkali metal atoms in the sealed cavity.

Historically, most optical pump atomic magnetometers were expensive and relatively large in volume and weight, which limited their applications. However, miniaturization of optical pump atomic magnetometers has become an attractive direction. The National Institute of Standards and Technology (NIST) successfully developed a millimeter-level optical pump atomic magnetometer using MEMS technology, effectively reducing size and cost. However, due to the small volume of the sealed cavity and the limited amount of alkali metal that can be contained, the magnetic field resolution was initially reduced to about 6pT/√Hz (in the frequency band of 10Hz-1kHz). Fortunately, a mechanism called "Spin Exchange Relaxation Free" (SERF) has been applied in MEMS optical pump atomic magnetometers to improve spin consistency and significantly increase the magnetic field resolution to tens of fT/√Hz (@1Hz). Although the SERF mechanism can only operate under extremely weak magnetic field conditions, its emergence is highly significant, opening up possibilities for realizing high-performance MEMS optical pump atomic magnetometers.

3 The miniaturization of fluxgate sensors

Fluxgate sensors were developed in the 1930s. They are vector magnetic sensors developed to overcome the inability of electromagnetic induction coils to measure DC magnetic fields. They use the principle that external magnetic fields affect magnetic core magnetization to achieve magnetic field measurement. They are mainly composed of a magnetic core, excitation coil, and detection coil, with the magnetic core being key to determining detection capability. When the high-frequency magnetic field generated by the excitation coil repeatedly magnetizes the magnetic core, the detection coil induces a distorted voltage signal. When the external magnetic field changes, the signal also changes. The measured magnetic field can be detected according to changes in the even-order harmonics of the distortion signal. By improving the soft magnetic performance of the magnetic core, the resolution of fluxgate sensors can reach the picotesla (pT) level.

With the development of micro-nano technology, the concept of micro-fluxgate has emerged. Researchers aim to reduce the volume and power consumption of high-performance fluxgate sensors through micro-nano processing technology. However, numerous studies have shown that as the size of miniature fluxgate sensors decreases, their sensitivity and resolution decline rapidly. The main reason is that the coils and magnetic cores of miniature fluxgate sensors are generally in thin-film form, and the thermal noise level is significantly higher than that of conventional coils and magnetic cores. In recent years, A. Baschirotto and colleagues have conducted systematic research on miniature fluxgate sensors, progressing from PCB fluxgates to IC fluxgates based on CMOS processes, with resolutions of approximately a few nT/√Hz (@1Hz). There is still a significant gap compared to traditional fluxgate sensors. Therefore, reducing the noise level of miniature fluxgate sensors through improved design and manufacturing processes remains a priority, though challenges persist. Recent advances in 2020-2025 have shown promise with new core materials and improved fabrication techniques achieving sub-nT resolution in compact form factors.

4 GMR (MTJ) magnetic sensor research

GMR (Giant Magnetoresistance) and TMR (Tunnel Magnetoresistance) magnetic sensors have the characteristics of small size, low power consumption, and high sensitivity. They are expected to be used in fields such as UAV anti-submarine warfare, micro-nano satellites, intelligent fuzes, automotive applications, and consumer electronics.

Since the discovery of the GMR effect in 1988, which earned Albert Fert and Peter Grünberg the 2007 Nobel Prize in Physics, the structure and theory of GMR have rapidly developed. From the initial sandwich structure to multilayer film structures, spin valve structures, and the newer magnetic tunnel junction (MTJ) structures, the magnetoresistance change rate and magnetic field sensitivity have continued to increase. In 2004, S. Yuasa and colleagues published results showing a large magnetoresistance change rate of over 200% in magnesium oxide-based MTJ at room temperature, which was later improved to 604% in 2008. In 2007, R.C. Chaves and colleagues obtained very high magnetic field sensitivity (87%/Oe) in a 26-micrometer-diameter columnar magnesium oxide-based MTJ (with magnetic bias and magnetic flux concentrator), but its low-frequency magnetic field resolution was still only 330pT/√Hz (@2.5Hz). After studying various GMR magnetoresistive materials, researchers found that with increases in magnetic field sensitivity, magnetic noise also increased, showing 1/f characteristic changes with frequency. In 2009, the National Institute of Standards and Technology, the University of Delaware, and the US Army Research Laboratory collaborated to develop an MTJ noise theoretical model. The model shows that increasing magnetic field sensitivity alone cannot suppress magnetic thermal noise and magnetic 1/f noise, and smaller volumes result in worse noise characteristics.

However, existing magnetic flux modulation methods have problems of low modulation efficiency and complicated structures. For this reason, scholars have proposed vertical magnetic flux modulation methods and developed prototypes of three-axis integrated magnetic sensors based on GMR. At the same time, new low-dimensional nanomaterials such as graphene are being used to develop a new generation of highly sensitive MTJ, which is expected to increase the resolution of magnetic sensors to approximately 1pT/√Hz (@1Hz). The tunnel magnetoresistance effect of graphene-based magnetic tunnel junctions has been experimentally verified, opening new possibilities for the development of high-performance magnetic sensors. Recent developments in TMR sensors have achieved commercial success, with many automotive and industrial applications now utilizing TMR technology for its superior sensitivity and low power consumption.

Ⅳ Recent Advances and Applications (2020-2025)

The period from 2020 to 2025 has witnessed significant advances in magnetic sensor technology:

Automotive Applications: TMR sensors have become standard in modern electric vehicles (EVs) for current sensing, position detection, and motor control. The automotive magnetic sensor market has grown substantially, driven by the electrification of vehicles and advanced driver-assistance systems (ADAS).

IoT and Wearables: Ultra-low-power magnetic sensors have enabled new applications in wearable devices, smart home systems, and industrial IoT. These sensors consume less than 10μA in active mode, making them ideal for battery-powered devices.

Quantum Sensing: Diamond nitrogen-vacancy (NV) center magnetometers have emerged as a promising technology, offering nanoscale spatial resolution and high sensitivity at room temperature. These sensors are finding applications in biological imaging and materials science.

AI Integration: Machine learning algorithms are being integrated with magnetic sensor systems to improve signal processing, noise reduction, and anomaly detection. This has enhanced the performance of magnetic sensors in complex environments.

5G and Beyond: Magnetic sensors are playing a crucial role in 5G infrastructure and next-generation communication systems, particularly in antenna positioning and beam steering applications.

Ⅴ Frequently Asked Questions (FAQs)

Q1: What is the difference between Hall effect sensors and magnetoresistive sensors?

A: Hall effect sensors detect magnetic fields using the Hall effect, where a voltage is generated perpendicular to both the current flow and magnetic field. Magnetoresistive sensors (AMR, GMR, TMR) detect magnetic fields through changes in electrical resistance. Generally, magnetoresistive sensors offer higher sensitivity and lower power consumption, while Hall sensors are more cost-effective and robust.

Q2: What are the typical applications of TMR sensors?

A: TMR (Tunnel Magnetoresistance) sensors are widely used in automotive applications (wheel speed sensing, gear tooth detection), industrial automation (position sensing, current measurement), consumer electronics (electronic compasses, proximity detection), and data storage (hard disk drive read heads). Their high sensitivity and low power consumption make them ideal for battery-powered and precision applications.

Q3: How do I choose the right magnetic sensor for my application?

A: Consider the following factors: required sensitivity and measurement range, operating temperature range, power consumption requirements, response time, size constraints, cost, and environmental conditions (humidity, vibration, electromagnetic interference). For high-precision applications, consider fluxgate or SQUID sensors. For cost-sensitive applications with moderate requirements, Hall sensors are suitable. For high-sensitivity, low-power applications, TMR sensors are excellent choices.

Q4: What is the resolution limit of modern magnetic sensors?

A: The resolution varies by sensor type: SQUID magnetometers can achieve sub-femtotesla (fT) resolution, optical pump atomic magnetometers reach 1-10 fT/√Hz, fluxgate sensors achieve picotesla (pT) levels, TMR sensors reach hundreds of pT/√Hz, and Hall sensors typically offer nanotesla (nT) resolution. The choice depends on the specific application requirements and constraints.

Q5: Can magnetic sensors work in extreme temperatures?

A: Yes, but the temperature range depends on the sensor type. Hall sensors can typically operate from -40°C to +150°C, with some specialized versions reaching +200°C. TMR and GMR sensors generally work from -40°C to +125°C. For extreme high-temperature applications (above 200°C), specialized fluxgate sensors or high-temperature Hall sensors are available. SQUID sensors require cryogenic temperatures for operation.

Q6: What is the future outlook for magnetic sensor technology?

A: The future of magnetic sensor technology is promising, with trends toward further miniaturization, improved sensitivity, lower power consumption, and integration with AI and IoT systems. Emerging technologies like quantum sensors, spintronic devices, and 2D material-based sensors (graphene, MoS2) are expected to revolutionize the field. The market is projected to grow significantly, driven by automotive electrification, industrial automation, consumer electronics, and healthcare applications.

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Article Update Information:

This article was originally published in 2020 and has been updated in October 2025 to reflect the latest developments in magnetic sensor technology, including recent advances in TMR sensors, quantum sensing, AI integration, and emerging applications in electric vehicles and IoT devices. Technical specifications, market trends, and application examples have been revised to reflect current industry standards and practices. A comprehensive FAQ section has been added to address common questions about magnetic sensor selection and implementation.