Magnetometers

All of us probably are aware about the fact that the magnets create magnetic fields; earth has a magnetic field; current flowing in a wire also generate magnetic field. But have we ever realized that the fields are generated also by our heart and brain. However, what differs between the magnetic field generated by a magnet and that generated by brain and heart is the magnitude of magnetic field. Following figure shows the magnitude of magnetic fields generated by various sources.

Fig. 1: A figure Explaining the Magnitude of Magnetic Fields Generated by Various Sources

We all are surrounded by magnetic fields. Magnetic fields are generated by flowing electrical current in various electrical/electronic appliances; TV, computers, power transmission lines, etc. Earth also has its own magnetic field, though relatively small. Earth magnetic field is largest at the poles (~ 60 000 nT) and smallest as the equator (~ 30 000 nT). The earth’s magnetic field strength is proportional to 1/r3 (until the influence from the solar wind gets noticeable).

Fig. 2: A Figure Demonstrating the Proportion of Magnetic Field Strength of the Earth to 1/r3r

Measurement of the magnetic fields is of interest for various scientific purposes, navigation, etc. Measurement of these fields is done by sensing devices called magnetometers.

MAGNETOMETER – WHAT IS IT?

Fig. 3: A Representational Image of a Magnetometer

Magnetometers are devices that measure magnetic fields. A magnetometer is an instrument with a sensor that measures magnetic flux density B (in units of Tesla or As/ m2). Magnetometers refer to sensors used for sensing magnetic fields OR to systems which measure magnetic field using one or more sensors.

Since magnetic flux density in air is directly proportional to magnetic field strength, a magnetometer is capable of detecting fluctuations in the Earth’s field.

Materials that distort magnetic flux lines are known as magnetic, and include materials such as magnetite that possess magnetic fields of their own, as well as very high magnetic conductivity. Such materials create distortions in the Earth’s magnetic flux that is flowing around them. Magnetometers detect these distortions.

A magnetometer measures magnetic flux density at the point in space where the sensor is located. A magnetic field drops in intensity with the cube of the distance from the object. Therefore, the maximum distance that a given magnetometer can detect the object is directly proportional to the cube root of the magnetometer’s sensitivity. The sensitivity is commonly measured in Tesla.

Types of Magnetometers

Magnetometers are classified into two categories:

• Vector magnetometers that measure the flux density value in a specific direction in 3 dimensional space. An example is a fluxgate magnetometer that can measure the strength of any component of the Earth’s field by orienting the sensor in the direction of the desired component.

• Scalar magnetometers that measure only the magnitude of the vector passing through the sensor regardless of the direction. Quantum magnetometers are an example of this type of magnetometer.

Various magnetometers used to measure magnetic fields are discussed in following sections.

Coil Magnetometer

Based on Faraday’s law, coil is the fundamental method of magnetic field sensing. Faraday’s law states that the induced emf in any closed conducting coil is equal to the time rate of change of magnetic flux through the circuit.

Fig. 4: A Diagram Demonstrating a Coil Magnetometer

With a coil having N turns wounded around a magnetic material with magnetic permeability m_r & and a flux f through it, the emf induced in the coil is:

Fig. 5: An Equation Showing Relation Between emf, Flux Density, Area of the Coil, and Angle Between the Two

If magnetic field has harmonic variation with time, induced voltage is proportional to the frequency of the magnetic field.

Thus, induced emf depends upon coil’s area. Sensitivity of the small coil magnetometer depends on the size and number of turns. This is used to detect only variations in the field (due to motion or due to the AC nature of the field). Since usually B needs to be measured instead of dB/dt, an integrator is usually used to obtain signal proportional to magnetic field B.

This is useful in such areas as mine detection or buried object detectors (pipe detection, “treasure” hunting, etc.). It is often used because of its simplicity.

Hall Sensors

Hall Sensors

The Hall Effect principle states that when a current carrying conductor is placed in a magnetic field, a voltage will be generated perpendicular to the direction of the field and the flow of current.

When a constant current is passed through a thin sheet of semiconducting material, there is no potential difference at the output contacts if the magnetic field is zero. However, when a perpendicular magnetic field is present, the current flow is distorted. The uneven distribution of electron density creates a potential difference across the output terminals. This voltage is called the Hall voltage. If the input current is held constant the Hall voltage will be directly proportional to the strength of the magnetic field.The Hall Effect sensor is polarity dependent. If the current changes direction or the magnetic field changes direction, the polarity of the Hall voltage flips.

Fig. 6: A Diagram Representing Hall Sensors

The Hall voltage is a low level signal of the order of 20 – 30 mvolts in a magnetic field of one gauss and requires amplification. Hall voltage is linear with respect to the field for given current and dimensions. Hall coefficient is temperature dependent and this must be compensated if accurate sensing is needed.

Fig. 7: A Diagram Explaining Hall Coefficient as a Dependent on Temperature

Magnetoresistive Sensors

Magnetoresistive Sensors

Magneto-resistive sensors are based on two principles.

First principle involves those which have basic structure similar to Hall elements but with no Hall voltage electrodes. The electrons are affected by the magnetic field as in the hall element Because of the magnetic force on them; they will flow in an arc.

Fig. 8: A Figure Illustrating Magentostrictive Sensors

The larger the magnetic field, the larger the arc radius. It forces electrons to take a longer path. The resistance to their flow increases. A relationship between magnetic field and current is established and therefore, the resistance of the device becomes a measure of field. The relation between field and current is proportional to B² for most configurations. It is dependent on carrier mobility in the material used (usually a semiconductor). The exact relationship is rather complicated and depends on the geometry of the device.

Another principle used by magnetoresistive sensors is the property of some materials to change their resistance in the presence of a magnetic field (caused by the Lorentz force). Most conductors have a positive magnetoresistivity; their resistance increase in the presence of a magnetic field.

AMR (anisotropic magnetoresistance are metals with highly anisotropic properties and they change their magnetization direction due to application of the field.

Magnetostrictive Sensors

Magnetostrictive Sensors

Magnetostriction involves two effects: the contraction or expansion of a material under the influence of the magnetic field (Joule effect) and the inverse effect of changes in susceptibility of the material when subjected to mechanical stress (Villari Effect). This bi-directional effect between the magnetic and mechanical states of a magnetostrictive material is a transduction capability that is used for both actuation and sensing. The magnetostrictive effect is quite small and requires indirect methods for its measurement.

Operation is explained in the following figure. Change in length is proportional to the magnetic field.

Fig. 9: A Diagram Explaining the Operation of a Magentostrictive Sensors

Fluxgate Magnetometer

Fig. 10: A Figure Demonstrating Fluxgate Magnetometer

Originally designed to detect submarines in WWII, Fluxgate Magnetometers sense the intensity and orientation of magnetic fields. They are used to measure fluctuations in earth’s magnetic field due to solar winds and tectonic plate shifts.Many space applications have used fluxgate magnetometers to detect planet’s and moon’s gravitational fields and their orientation.

Fluxgate sensors are much more sensitive than coil magnetometers though a bit more complex. It can be used as a general purpose magnetic sensor. It is used in electronic compasses, detection of fields produced by the human heart, fields in space.

Underlying principle of magnetometer is explained below:

An external magnetic field H applied to a ferromagnetic core, induce a magnetic flux in the core; B = ?H (? is the permeability of the material). For high H values the material saturates, and the magnetic flux B cannot be further increased.

When the core is not saturated, the flux lines in the vicinity are drawn into the core. When the core is saturated, the magnetic flux lines are no more affected by the core

Fig. 10: A Diagram Explaining the Magnetic Field Strength in Saturated and Unsaturated Core

Fig. 11: A Figure Explaining Two Solid Cores and Ring Core

Two coils are wounded on highly ferromagnetic cores, a driver coil and a sense coil. The driver coil drives the core into and out of saturation, by applying an excitation current through the coil. Whenever magnetic flux lines are drawn out of the core, they induce a positive current spike in the sense coil. When they are drawn inside the core, they generate a negative current in the sense coil (Lenz law). The induced signal in the sense coil is proportional to dB/dt.

The two cores are placed in close proximity to each other, symmetrically opposed. Each core is wound with a primary coil; the primary windings are reversed of each other. An alternating current is applied to the primary windings producing a variable magnetic field in both cores.

When no external magnetic field is present, the induced magnetic fields in the cores are equal and opposite cancelling each other out. When an external magnetic field is present, the core generating a field opposite to its direction will come out of saturation sooner while the core in line with its direction will come out of saturation later. During this time the fields do not cancel out and there is a net change in flux. This change in flux induces a voltage across the secondary windings which can be measured

The magnetization curve for most ferromagnetic materials is highly nonlinear. Almost any ferromagnetic material is suitable as a core for fluxgate sensors. In practice, the coil is driven with an ac source (sinusoidal or square). Under no external field, the magnetization is identical along the magnetic path. Hence the sense coil will produce zero output.

If an external magnetic field perpendicular to the sense coil exists, this condition changes and, in effect, the core becomes nonuniformly magnetized. It produces an emf in the sensing coil of the order of a few mV/mT. The reason for the name fluxgate is this switching of the flux in the core to opposite directions.

Magnetostrictive materials are highly nonlinear. The sensors so produced are extremely sensitive – with sensitivities of 10^–⁶ to 10^–⁹ T quite common.

The fluxgate sensors can be designed with two or three axes. Fluxgate sensors are available in integrated circuits where Permalloy is the choice material since it can be deposited in thin films and its saturation field is low. Nevertheless, current integrated fluxgate sensors have lower sensitivities – of the order of 100 mT – but still higher than other magnetic field sensors.

Chinese Mars exploration satellite YH-1 used fluxgate magnetometers for Space magnetic field measurements

Proton Magnetometer

Proton Magnetometer

Also known as Proton precession magnetometers, PPM’s, measure the resonance frequency of protons or hydrogen nuclei in the magnetic field to be measured. As the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer can reach 1 ppm.

A polarizing DC current is passed through a coil wound around a liquid sample (water, kerosene, or similar), thereby creating auxiliary magnetic field and also causing protons to polarize themselves to stronger net magnetization. When the auxiliary flux is terminated, the “polarized” protons precess to re-align them to the normal flux density. Frequency of precession is directly proportional to magnetic flux density.

Fig. 12: A Figure Explaining the Process Flow of Proton Magnetometer

Proton precession measurements are necessarily sequential. This means that there is an initial polarization, followed by a frequency measurement – after which, the cycle is repeated. This differs from continuous measurements where the nuclei are polarized and frequency measurements are made simultaneously.

Overhauser Magnetometers

Overhauser Magnetometers

High sensitivity; superior omnidirectional sensors; no dead zones; no heading errors; or warm-up time; wide operating temperature range; rugged and reliable design; low maintenance; high absolute accuracy, rapid speed of operation; exceptionally low power consumption are some of the characteristics of Overhauser Magnetometers.

In Overhauser magnetometers, a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field.

Due to exposure to RF energy, the free electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

RF magnetic fields are ideal for use in magnetic devices because they are “transparent” to the Earth’s “DC” magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons.

Compared to proton precession magnetometers, Overhauser magnetometers

1. Have very sensitivity, matching with cesium magnetometers.

2. Offers continual or sequential operation.

3. Have higher Sampling rate.

Optically Pumped Magnetometers

Optically Pumped Magnetometers

Optically pumped magnetometers include 1 nuclear magnetometer (Helium 3) and four electron resonance magnetometers (Helium 4, Rubidium, Cesium and Potassium).

Alkali vapor optically pumped magnetometers use gaseous alkali metals from the first column of the periodic table, such as Cesium, Potassium or Rubidium.

Fig. 13: A Diagram of Glass Vapor Cell Containing Gaseous Metal is Exposed by Light of Very Specific Wavelength

A glass vapor cell containing gaseous metal is exposed by light of very specific wavelength. The frequency of light is specifically selected and circularly polarized for each element to shift electrons from the ground level 2 to the excited state 3. Electrons at level 3 are not stable, and these electrons spontaneously decay to both energy levels 1 and 2. Eventually, the level 1 is fully populated. When this happens, the absorption of polarizing light stops and the vapour cell becomes more transparent.

Then, RF depolarization comes into play. RF power corresponding to the energy difference between levels 1 and 2 is applied to the cell to move electrons from level 1 back to level 2 (and the cell becomes opaque again). The frequency of the RF field required to repopulate level 2 varies with the ambient magnetic field and is called Larmor frequency. Depolarization by a circular magnetic field at the Larmor frequency will rebalance populations of the two ground levels and the vapour cell will start absorbing more of the polarizing light.

The effect of polarization and depolarization is that light intensity becomes modulated by the RF frequency. By detecting light modulation and measuring the frequency, we can find a value of the magnetic field.

SQUID

SQUID

SQUID stands for Superconducting QUantum Interference Device. They are the most sensitive of all magnetometers, they can sense down to 10^–¹⁵ T. They operate at very low temperatures – usually at 4.2 °K (liquid helium). Higher temperature SQUIDs and integrated SQUIDs also exist. Still, SQUIDs are not as common as other types of sensors; they can’t just be picked off the shelf and used.

SQUIDs are based on the so- called Josephson junction; a junction formed if two superconductors are separated by a small insulating gap (discovered in 1962 by B.D. Josephson). If the insulator between two superconductors is thin enough the superconducting electrons can tunnel through the insulator. The base material is usually Niobium or a lead (90%)-gold (10%) alloy.

There are two basic types of SQUIDs. RF (radio frequency) SQUIDs which have only one Josephson junction and DC SQUIDs which usually have two junctions. DC SQUIDs are more expensive to produce, but are much more sensitive.

The main difficulty with SQUIDs is the cooling requirement and the necessary mass. Nevertheless, it is a remarkably valuable sensor where the cost can be justified. It is exclusively used in applications such as magneto-encephalography. Measurements of very low magnetic fields are done in shielded room where the terrestrial magnetic field can be eliminated.