A magnetometer is a sensory device for measuring magnetic flux densities. Magnetic flux densities are measured in the unit Tesla (T), and common measurement ranges of magnetometers range from about 10−15 T to 10 T. Because of the large range of values, different measurement methods are used under the term “magnetometer”.
Magnetometer sensor modules are sophisticated instruments/modules designed to measure the strength, direction, and variations of magnetic fields. They find extensive applications across diverse fields, including geophysics, navigation, aerospace, and consumer electronics.
Principles of Operation
Magnetometers are instruments that detect and measure magnetic fields. They operate based on various physical principles, including electromagnetic induction, Hall effect, and nuclear magnetic resonance (NMR). By analyzing the characteristics of magnetic fields, magnetometers provide valuable insights into geological formations, Earth’s magnetic field, navigation, and industrial processes. Magnetometers function based on different principles, each suited to specific applications:
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Electromagnetic Induction: Electromagnetic induction-based magnetometers utilize coils or antennas to detect changes in magnetic flux. When a magnetic field changes, it induces an electromotive force (EMF) in the coil, which can be measured and calibrated to determine the strength and direction of the magnetic field.
Hall Effect: Hall effect magnetometers rely on the Hall effect, where a voltage is generated across a conductor placed in a magnetic field perpendicular to the current flow. By measuring the Hall voltage, the strength and direction of the magnetic field can be determined. Hall effect magnetometers are commonly used in consumer electronics and industrial applications.
Nuclear Magnetic Resonance (NMR): NMR magnetometers exploit the principles of nuclear magnetic resonance to measure the magnetic properties of materials. They are used in scientific research, medical imaging (MRI), and environmental monitoring to study the composition and properties of substances.
Types of Magnetometers
Here is a list of the most common magnetometers:
- Hall Sensors. Hall effect magnetometers are compact, low-cost sensors commonly used in consumer electronics, smartphones, drones, and automotive applications for navigation, orientation sensing, and motion tracking.
- Fluxgate magnetometer or Förster probes or saturation core magnetometers. Fluxgate magnetometers utilize a core of ferromagnetic material that becomes magnetized in the presence of an external magnetic field. By measuring changes in the magnetic flux within the core, fluxgate magnetometers can detect weak magnetic fields with high sensitivity, making them ideal for geophysical surveys, archaeology, and navigation.
- Fixed and rotating coils (induction), e.g. vibrating sample magnetometer
- Optically pumped magnetometers, such as alkaline vapor magnetometers (e.g., with atomic rubidium or cesium vapor)
- SQUIDs – Superconducting Quantum Interference Device (SQUID) magnetometers are highly sensitive instruments capable of detecting extremely weak magnetic fields. They are used in scientific research, medical diagnostics, and materials characterization to study phenomena such as magnetic resonance imaging (MRI), biomagnetic signals, and magnetic properties of materials at the quantum level.
- Proton magnetometers such as Overhauser magnetometers. Proton precession magnetometers measure the magnetic field by detecting the precession of protons in a magnetic field. They are commonly used in geological exploration, mineral prospecting, and archaeology to map subsurface structures and detect anomalies in the Earth’s magnetic field.
- Magnetometer based on Bose-Einstein condensates (BEC magnetometer) with a sensitivity of 10−9 T and a spatial resolution of 3 μm.
- Kerr Magnetometer
- Faraday magnetometer (use of the Faraday effect)
In addition, there are other sensors for magnetic flux densities, but they are rarely used under the term magnetometer. They are not as sensitive and low-noise as the ones mentioned so far:
- XMR sensor; thin-film sensors that directly change their resistance under the influence of magnetic flux and are therefore called “X-MagnetoResistive”, such as GMR sensor (giant), AMR sensor (anisotropic) or CMR sensors (colossal).
- Field plates.
Although XMR and Hall sensors are not as sensitive as the aforementioned ones, they are widely used for simpler tasks due to their simple design (semiconductor technology) and the associated inexpensive production. These include compasses, proximity switches, hard drives, current sensors.
Application
One of the classic tasks of a magnetometer is to measure the Earth’s magnetic field with its fluctuations (geomagnetics). If local deviations of the magnetic field occur at the Earth’s surface, this indicates ferromagnetic materials underground. These disturbances can be used to detect objects as diverse as ore deposits, submarines, contaminated sites (e.g. unexploded ordnance) or archaeological structures.
Magnetometers are of great importance in space travel. They are used there as attitude sensors and are used to determine the position of a satellite. Magnetometers used by aircraft are also known by the abbreviation MAD (magnetic anomaly detector), especially in the military sector. In order to minimise influence by ferromagnetic components of the aircraft, such magnetometers are either towed behind the aircraft on a rope or, for example, housed in a spike-like extension of the fuselage tail.
Magnetometers are used in smartphones, navigation devices and VR headsets to create a magnetic compass or to reference the signals of the accelerometers for attitude determination.
Magnetometers have diverse applications across various industries, scientific disciplines and can be summarized as:
Geophysics and Exploration: Magnetometers are used in geophysical surveys, mineral exploration, and archaeological studies to map subsurface structures, detect ore deposits, and identify buried artifacts based on variations in the Earth’s magnetic field.
Navigation and Positioning: Magnetometers are integral components of navigation systems, compasses, and inertial measurement units (IMUs) used in aerospace, maritime, automotive, and consumer electronics for orientation sensing, heading determination, and navigation.
Industrial Monitoring: Magnetometers are employed in industrial processes, manufacturing, and quality control to detect magnetic anomalies, monitor magnetic fields in machinery and equipment, and ensure compliance with safety standards.
Medical Imaging: Magnetometers, particularly SQUID magnetometers and NMR-based instruments, are used in medical imaging modalities such as magnetic resonance imaging (MRI) and magnetoencephalography (MEG) for non-invasive diagnosis, brain mapping, and research.
