Magnetic field sensing using a fluxgate magnetometer

Introduction to magnetic field sensing

Magnetic field sensing plays a crucial role in both scientific research and practical applications, ranging from searching for fundamental physics, geophysical nagigations, to industrial and biomedical uses. Magnetic sensors are used to detect, map, and measure the intensity and direction of magnetic fields. There are several methods for sensing magnetic fields, and the choice of sensor depends on the application requirements, such as sensitivity, bandwidth, and environmental robustness.

Different types of magnetometers

• Fluxgate magnetometers use ferromagnetic cores and detect magnetic fields by sensing nonlinear changes in core permeability. They are valued for their robustness and precision in low-field measurements, making them ideal for space missions and geological studies.

• SQUID magnetometers (Superconducting Quantum Interference Devices) are among the most sensitive magnetic field detectors. Operating at cryogenic temperatures, SQUIDs exploit quantum interference in superconducting loops to detect fields as small as femtoteslas. They are widely used in biomagnetism (e.g., MEG – magnetoencephalography) and in fundamental physics experiments.

• Optically pumped magnetometers rely on the interaction between light and spin-polarized atoms, typically alkali metals like rubidium or cesium. These sensors can achieve extreme sensitivity while operating at room temperature <ref>Template:Cite journal</ref>. The most sensitive optical magnetometers, such as spin-exchange relaxation-free (SERF) devices, can reach sensitivities below 1 fT/√Hz, rivaling superconducting magnetometers. More compact or portable designs, while less sensitive, still operate effectively in the picotesla to nanotesla range, making them attractive for biomedical, navigation, and remote sensing applications.

• Hall effect sensors are solid-state devices that measure the voltage generated by a magnetic field perpendicular to an electric current. They are simple, low-cost, and widely used in consumer electronics and industrial systems where high sensitivity is not critical.

• Magnetoresistive sensors (AMR, GMR, TMR) rely on changes in resistance due to the presence of a magnetic field. They offer high bandwidth and miniaturization potential, useful in automotive and robotics applications.

• Proton precession and Overhauser magnetometers use nuclear magnetic resonance techniques to measure absolute magnetic field strength with high accuracy, commonly applied in geophysical surveys.

Each of these technologies offers a unique combination of sensitivity, size, cost, and operational constraints.

Applications

Magnetic sensors find applications across a wide spectrum of fields:

• In fundamental physics, they are used in searches for exotic particles and precision measurements of fundamental constants.
• In geophysics, magnetometers are employed for mineral exploration, archaeology, and mapping Earth's magnetic field.
• In navigation, especially in aerospace and underwater contexts, magnetic sensing provides orientation and heading information where GPS is unavailable.
• In the biomedical field, ultra-sensitive magnetometers (e.g., SQUID and optical) enable non-invasive detection of weak magnetic signals from the brain and heart.
• In industry, they are used for non-destructive testing, monitoring electric motors, and detecting ferrous materials in quality control.

Modern advances in materials science, quantum optics, and electronics continue to expand the reach and capability of magnetic field sensing technologies.

Basic principles of a fluxgate

How does a fluxgate magnetometer work?

A fluxgate magnetometer works by exploiting the magnetic response of a ferromagnetic core to detect an external magnetic field. It detects changes in magnetic permeability $\mu$ due to the presence of an external field, typically producing a signal at 2nd harmonic of the excitation frequency. In a fluxgate sensor, the core material has very high magnetic permeability, meaning the core material can concentrate magnetic fields very well. Besides, it reacts strongly to changes in external fields. Its permeability changes nonlinearly as the material saturates, which is crucial for generating detectable signals (like the 2nd harmonic) when there's an external field.

Illustrations of a fluxgate

(a) Illustration of a fluxgate with two parallel ferromagnetic coils

Introduction to the fluxgate magenotometer

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Modelling and measurements of a permanent magnet

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References

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