Magnetometer

A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, one that measures the direction of an ambient magnetic field, in this case, the Earth's magnetic field. Other magnetometers measure the magnetic dipole moment of a magnetic material such as a ferromagnet, for example by recording the effect of this magnetic dipole on the induced current in a coil.

The first magnetometer capable of measuring the absolute magnetic intensity at a point in space was invented by Carl Friedrich Gauss in 1833 and notable developments in the 19th century included the Hall effect, which is still widely used.

Magnetometers are widely used for measuring the Earth's magnetic field, in geophysical surveys, to detect magnetic anomalies of various types, and to determine the dipole moment of magnetic materials. In an aircraft's attitude and heading reference system, they are commonly used as a heading reference. Magnetometers are also used by the military as a triggering mechanism in magnetic mines to detect submarines. Consequently, some countries, such as the United States, Canada and Australia, classify the more sensitive magnetometers as military technology, and control their distribution.

Magnetometers can be used as metal detectors: they can detect only magnetic ( ferrous) metals, but can detect such metals at a much greater distance than conventional metal detectors, which rely on conductivity. Magnetometers are capable of detecting large objects, such as cars, at over , while a conventional metal detector's range is rarely more than .

In recent years, magnetometers have been miniaturized to the extent that they can be incorporated in integrated circuits at very low cost and are finding increasing use as miniaturized compasses ( MEMS magnetic field sensor).

Introduction

Magnetic fields

Magnetic fields are vector quantities characterized by both strength and direction. The strength of a magnetic field is measured in units of tesla in the SI units, and in gauss in the cgs system of units. 10,000 gauss are equal to one tesla. Measurements of the Earth's magnetic field are often quoted in units of nanotesla (nT), also called a gamma. The Earth's magnetic field can vary from 20,000 to 80,000 nT depending on location, fluctuations in the Earth's magnetic field are on the order of 100 nT, and magnetic field variations due to magnetic anomalies can be in the picotesla (pT) range. ''Gaussmeters'' and ''teslameters'' are magnetometers that measure in units of gauss or tesla, respectively. In some contexts, magnetometer is the term used for an instrument that measures fields of less than 1 millitesla (mT) and gaussmeter is used for those measuring greater than 1 mT.

Types of magnetometer

There are two basic types of magnetometer measurement. ''Vector magnetometers'' measure the vector components of a magnetic field. ''Total field magnetometers'' or ''scalar magnetometers'' measure the magnitude of the vector magnetic field. Magnetometers used to study the Earth's magnetic field may express the vector components of the field in terms of ''declination'' (the angle between the horizontal component of the field vector and true, or geographic, north) and the ''inclination'' (the angle between the field vector and the horizontal surface).

''Absolute magnetometers'' measure the absolute magnitude or vector magnetic field, using an internal calibration or known physical constants of the magnetic sensor. ''Relative magnetometers'' measure magnitude or vector magnetic field relative to a fixed but uncalibrated baseline. Also called ''variometers'', relative magnetometers are used to measure variations in magnetic field.

Magnetometers may also be classified by their situation or intended use. ''Stationary magnetometers'' are installed to a fixed position and measurements are taken while the magnetometer is stationary. ''Portable'' or ''mobile magnetometers'' are meant to be used while in motion and may be manually carried or transported in a moving vehicle. ''Laboratory magnetometers'' are used to measure the magnetic field of materials placed within them and are typically stationary. ''Survey magnetometers'' are used to measure magnetic fields in geomagnetic surveys; they may be fixed base stations, as in the INTERMAGNET network, or mobile magnetometers used to scan a geographic region.

Performance and capabilities

The performance and capabilities of magnetometers are described through their technical specifications. Major specifications include
*''Sample rate'' is the number of readings given per second. The inverse is the ''cycle time'' in seconds per reading. Sample rate is important in mobile magnetometers; the sample rate and the vehicle speed determine the distance between measurements.
*''Bandwidth'' or ''bandpass'' characterizes how well a magnetometer tracks rapid changes in magnetic field. For magnetometers with no onboard signal processing, bandwidth is determined by the Nyquist limit set by sample rate. Modern magnetometers may perform smoothing or averaging over sequential samples, achieving a lower noise in exchange for lower bandwidth.
*''Resolution'' is the smallest change in a magnetic field the magnetometer can resolve. A magnetometer should have a resolution a good deal smaller than the smallest change one wishes to observe. This includes quantization error which is caused by recording roundoff and truncation of digital expressions of the data.
*''Absolute error'' is the difference between the readings of a magnetometer true magnetic field.
*''Drift'' is the change in absolute error over time.
*''Thermal stability'' is the dependence of the measurement on temperature. It is given as a temperature coefficient in units of nT per degree Celsius.
*''Noise'' is the random fluctuations generated by the magnetometer sensor or electronics. Noise is given in units of $\rm/\sqrt$, where frequency component refers to the bandwidth.
*''Sensitivity'' is the larger of the noise or the resolution.
*''Heading error'' is the change in the measurement due to a change in orientation of the instrument in a constant magnetic field.
*The ''dead zone'' is the angular region of magnetometer orientation in which the instrument produces poor or no measurements. All optically pumped, proton-free precession, and Overhauser magnetometers experience some dead zone effects.
*''Gradient tolerance'' is the ability of a magnetometer to obtain a reliable measurement in the presence of a magnetic field gradient. In surveys of unexploded ordnance or landfills, gradients can be large.

Early magnetometers

The compass, consisting of a magnetized needle whose orientation changes in response to the ambient magnetic field, is a simple type of magnetometer, one that measures the direction of the field. The oscillation frequency of a magnetized needle is proportional to the square-root of the strength of the ambient magnetic field; so, for example, the oscillation frequency of the needle of a horizontally situated compass is proportional to the square-root of the horizontal intensity of the ambient field.

In 1833, Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field. It described a new instrument that consisted of a permanent bar magnet suspended horizontally from a gold fibre. The difference in the oscillations when the bar was magnetised and when it was demagnetised allowed Gauss to calculate an absolute value for the strength of the Earth's magnetic field.

The gauss, the CGS unit of magnetic flux density was named in his honour, defined as one maxwell per square centimeter; it equals 1×10⁻⁴ tesla (the SI unit).

Francis Ronalds and Charles Brooke independently invented magnetographs in 1846 that continuously recorded the magnet's movements using photography, thus easing the load on observers. They were quickly utilised by Edward Sabine and others in a global magnetic survey and updated machines were in use well into the 20th century.

Laboratory magnetometers

Laboratory magnetometers measure the magnetization, also known as the magnetic moment of a sample material. Unlike survey magnetometers, laboratory magnetometers require the sample to be placed inside the magnetometer, and often the temperature, magnetic field, and other parameters of the sample can be controlled. A sample's magnetization, is primarily dependent on the ordering of unpaired electrons within its atoms, with smaller contributions from nuclear magnetic moments, Larmor diamagnetism, among others. Ordering of magnetic moments are primarily classified as diamagnetic, paramagnetic, ferromagnetic, or antiferromagnetic (although the zoology of magnetic ordering also includes ferrimagnetic, helimagnetic, toroidal, spin glass, etc.). Measuring the magnetization as a function of temperature and magnetic field can give clues as to the type of magnetic ordering, as well as any phase transitions between different types of magnetic orders that occur at critical temperatures or magnetic fields. This type of magnetometry measurement is very important to understand the magnetic properties of materials in physics, chemistry, geophysics and geology, as well as sometimes biology.

SQUID (superconducting quantum interference device)

SQUIDs are a type of magnetometer used both as survey