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A magnetometer is a basic instrument for measuring the magnetic field in geophysical surveys and space research. Although there are countless methods for detecting magnetic fields, most of them do not give valuable results. When we talk about magnetometers, we are referring to methods with high sensitivity.

Magnetometers are also built into smartphones, such as the iPhone, as digital compasses, but the data they provide is only indicative and, when combined with GPS and map data, can help users find their way around. Hall sensors have been in use for a long time and are manufactured in large quantities for industry, for the detection of permanent magnets, electromagnets and the analogue version for DC current transformers. Their low sensitivity does not allow their use as magnetometers. Their typical sensitivity is given in miliTesla (mT). By comparison, the Earth's magnetic field in Budapest is 48,800 nT (nanoTesla), which varies by about 50 nT per year, and the field lines are strongly downward, with a horizontal component of 21,250 nT and a vertical component of 43,950 nT. The sensitivity of magnetoresistive sensors made with modern MEMS technology is already sufficient for rough measurements of the Earth's magnetic field, so they can easily be used to make low-cost electronic compasses. The main concern regarding the production of telephones is to build them from cheap components. So they are not expected to replace the precision electronic compasses used in weapons, navigation systems, satellites and instruments any time soon.

Before the spread of electronics, electrodynamic magnetometers of a similar design to analogue electrical instruments were still in use, using a sufficiently small conducting frame with a current path that can easily turn in any direction. The magnetic field exerts a torque (M) on the magnetometer, which then adjusts to an equilibrium position. The torque is at its maximum when the plane of the magnetometer is perpendicular to this equilibrium position. The magnitude of the maximum torque depends on the strength of the magnetic field.

In space research, magnetometers are used to measure the magnetic fields created by celestial bodies and the space between them.

There are two types of magnetometers:

• Scalar magnetometers. These measure the total magnetic induction regardless of the direction of the lines of force. Typical applications include geophysical mapping of a given area or monitoring the Earth's magnetism at a given location.
• Vector magnetometers. These instruments are mainly used as part of navigation systems, but are also used to orient and measure the inclination of exploratory boreholes. It is also possible to determine the total magnetic induction by using vector algebra to calculate it from the result of a measurement with the sensors aligned along the axes of the XYZ coordinate system.

Proton precession magnetometer

Proton magnetometer from 1967 

The proton precession magnetometer, or proton magnetometer for short, is specifically designed to study the Earth's magnetic field. A typical application is the detection of ferromagnetic materials under the surface or deep under the sea. It can also detect frozen magnetic fields. This means that the magnetic field of the time is preserved in lava solidified during volcanic activity. Local anomalies in the magnetic field are often caused by mineral indicators in the natural case and man-made objects in the artificial case. Thus proton magnetometers are often used as instruments in ore exploration and archaeological excavations.

The principle of operation is that a proton (hydrogen ion) rich liquid is excited by an electromagnet, i.e. polarised. The protons are then aligned in the direction of the excitation magnetic field. When the excitation is removed, the protons are returned to the direction of the external magnetic field. However, this alignment should be thought of as a pendulum, i.e. a decaying oscillation begins. This oscillation can be measured using a receiver coil. The frequency of the oscillation is proportional to the strength of the magnetic field.

Since the sensor has to be held still during the measurement and a measurement can take 4~10 seconds, mapping a large area is definitely a time-consuming task.

SQUID magnetometer

 

Schematic of the SQUID sensor. The current I in the two branches is divided in two ways, Ia and Ib. The two superconducting pieces are separated by a Josephson junction. The symbolizes the magnetic flux.

The SQUID sensor is a superconductor-based device that uses the principle of quantum interference. It is a highly sensitive magnetic sensor operating over a wide dynamic range. Its sensitivity can reach 5×10-18 T.

 

Left: Relationship between the excitation current and output voltage of the SQUID sensor. The upper curve shows the Phi n- 0 state of the magnetic field, while the lower curve an+*0 shows its state. Right: Periodic variation of the SQUID sensor output voltage as a function of flux. A period is the quantum flux 0.

Its structure is a superconducting ring interrupted by a Josephson junction. The Josephson junction is a thin insulating layer between the two superconductors, through which the charge carriers pass with a tunnelling effect. In the case of the use of a niobium superconductor, the junction is typically made of Al2O3 and has a thickness of ~10Å.

When a constant current is passed through the SQUID sensor, the voltage measured across the gate is a function of the magnetic flux measured inside. The relationship between the flux and the output voltage is not linear, but the voltage periodically increases and decreases as the flux increases. Hence its sensitivity, because a small change in flux causes a large change in output signal. This is also a disadvantage if you want to measure an absolute value rather than a change, but there are measurement tricks for this. The difference in flux between two identical output voltages is called quantum flux.

Despite the fact that superconductors require considerable cooling, there are field versions of SQUID magnetometers as well as laboratory versions. Serious cooling means that the niobium used for the sensor becomes superconducting below 9.26K, i.e. they need liquid helium cooling, and even so-called high-temperature sensors need to be cooled with liquid nitrogen.