Editing Magnetometer (section)

===Vector magnetometers===
Vector magnetometers measure one or more components of the magnetic field electronically. Using three orthogonal magnetometers, both azimuth and dip (inclination) can be measured. By taking the square root of the sum of the squares of the components the total magnetic field strength (also called total magnetic intensity, TMI) can be calculated by the [[Pythagorean theorem]].

Vector magnetometers are subject to temperature drift and the dimensional instability of the ferrite cores. They also require leveling to obtain component information, unlike total field (scalar) instruments. For these reasons they are no longer used for mineral exploration.

====Rotating coil magnetometer====
The magnetic field induces a sine wave in a rotating [[electromagnetic coil|coil]]. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the [[sine]] of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete.

====Hall effect magnetometer====
{{Main|Hall effect sensor}}
The most common magnetic sensing devices are [[solid state electronics|solid-state]] [[Hall effect]] sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity. They are used in applications where the magnetic field strength is relatively large, such as in [[anti-lock braking system]]s in cars, which sense wheel rotation speed via slots in the wheel disks.

====Magnetoresistive devices====
{{Main|Magnetoresistance}}
These are made of thin strips of [[Permalloy]], a high [[magnetic permeability]], nickel-iron alloy, whose electrical resistance varies with a change in magnetic field. They have a well-defined axis of sensitivity, can be produced in 3-D versions and can be mass-produced as an integrated circuit. They have a response time of less than 1 microsecond and can be sampled in moving vehicles up to 1,000 times/second. They can be used in compasses that read within 1°, for which the underlying sensor must reliably resolve 0.1°.<ref>{{citation|title=Applications of Magnetoresistive Sensors in Navigation Systems|author=Michael J. Caruso|publisher=Honeywell Inc.|url=http://www.ssec.honeywell.com/position-sensors/datasheets/sae.pdf|archive-url=https://web.archive.org/web/20100705002122/http://www.ssec.honeywell.com/position-sensors/datasheets/sae.pdf|url-status=dead|archive-date=5 July 2010|access-date=21 October 2012}}</ref>

====Fluxgate magnetometer====
{{See also|Gradiometer}}
[[File:Magnetometr transduktorowy by Zureks.jpg|thumb|A uniaxial fluxgate magnetometer]]
[[File:Floating core fluxgate inclinometer compass autonnic.jpg|thumb|A [[fluxgate compass]]/inclinometer]]
[[File:Fluxgate Magnetometers.ogv|thumb|Basic principles of a fluxgate magnetometer]]

A fluxgate magnetometer consists of a small magnetically susceptible core wrapped by two coils of wire. An alternating electric current is passed through one coil, driving the core through an alternating cycle of [[saturation (magnetic)|magnetic saturation]]; i.e., magnetised, unmagnetised, inversely magnetised, unmagnetised, magnetised, and so forth. This constantly changing field induces a voltage in the second coil which is measured by a detector. In a magnetically neutral background, the input and output signals match. However, when the core is exposed to a background field, it is more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field and the induced output voltage, are out of step with the input current. The extent to which this is the case depends on the strength of the background magnetic field. Often, the signal in the output coil is integrated, yielding an output analog voltage proportional to the magnetic field.

The fluxgate magnetometer was invented by H. Aschenbrenner and G. Goubau in 1936.<ref name="snare">{{cite book |last1=Snare |first1=Robert C. |chapter=A History of Vector Magnetometry in Space |chapter-url=https://faculty.epss.ucla.edu/~ctrussell/ESS265/History.html |pages=[https://books.google.com/books?id=hHC5FTZMSRkC&pg=PA101 101–114] |editor1-first=Robert F. |editor1-last=Pfaff |editor2-first=Joseph E. |editor2-last=Borovsky |editor3-first=David T. |editor3-last=Young|title=Measurement Techniques in Space Plasmas: Fields |date=1998 |publisher=[[American Geophysical Union]] |publication-place=Washington, D.C. |isbn=0-87590-086-0 |issn=0065-8448 |series=Geophysical Monograph |volume=103 |bibcode=1998GMS...103..101S }}</ref><ref>{{cite book |last1=Musmann |first1=Günter Dr. |title=Fluxgate Magnetometers for Space Research |date=2010 |publisher=Books on Demand |location=Norderstedt |isbn=9783839137024}}</ref>{{rp|4}} A team at Gulf Research Laboratories led by [[Victor Vacquier]] developed airborne fluxgate magnetometers to detect submarines during [[World War II]] and after the war confirmed the theory of [[plate tectonics]] by using them to measure shifts in the magnetic patterns on the sea floor.<ref name="lat">{{cite journal |author=Thomas H. Maugh II |date=24 January 2009 |title=Victor Vacquier Sr. dies at 101; geophysicist was a master of magnetics |url=http://www.latimes.com/news/science/la-me-vacquier24-2009jan24,0,3328591.story |journal=[[The Los Angeles Times]]}}</ref>

A wide variety of sensors are currently available and used to measure magnetic fields. [[Fluxgate compass]]es and [[gradiometer]]s measure the direction and magnitude of magnetic fields. Fluxgates are affordable, rugged and compact with miniaturization recently advancing to the point of complete sensor solutions in the form of IC chips, including examples from both academia <ref>{{cite journal |doi=10.3390/s140813815  |doi-access=free|title=High-Sensitivity Low-Noise Miniature Fluxgate Magnetometers Using a Flip Chip Conceptual Design|year=2014|last1=Lu|first1=Chih-Cheng|last2=Huang|first2=Jeff|last3=Chiu|first3=Po-Kai|last4=Chiu|first4=Shih-Liang|last5=Jeng|first5=Jen-Tzong|journal=Sensors|volume=14|issue=8|pages=13815–13829|pmid=25196107|pmc=4179035|bibcode=2014Senso..1413815L}}</ref> and industry.<ref>http://www.ti.com/lit/gpn/drv425 {{Bare URL PDF|date=March 2022}}</ref> This, plus their typically low power consumption makes them ideal for a variety of sensing applications. Gradiometers are commonly used for archaeological prospecting, and [[unexploded ordnance]] (UXO) detection such as the German military's popular ''Foerster''.<ref>{{cite web |url=http://pdf.directindustry.com/pdf/foerster-instruments/landmine-and-uxo-detection-brochure/16605-113277.html |title=Landmine and UXO detection brochure – Foerster Instruments |access-date=25 October 2012}}</ref> Utility location specialists also use gradiometers for locating underground utilities such as pipeline valves, septic tanks, and manhole covers.<ref>{{Cite web |last=Foster |first=Ryan |date=2024-10-01 |title=The Ultimate Guide to Magnetic Locators |url=https://precisionoutdoortech.com/blogs/information-articles/the-ultimate-guide-to-magnetic-locators |access-date=2024-11-10 |website=Precision Outdoor Tech |language=en}}</ref>

The typical fluxgate magnetometer consists of a "sense" (secondary) coil surrounding an inner "drive" (primary) coil that is closely wound around a highly permeable core material, such as [[mu-metal]] or [[permalloy]]. An alternating current is applied to the drive winding, which drives the core in a continuous repeating cycle of saturation and unsaturation. To an external field, the core is alternately weakly permeable and highly permeable. The core is often a toroidally wrapped ring or a pair of linear elements whose drive windings are each wound in opposing directions. Such closed flux paths minimise coupling between the drive and sense windings. In the presence of an external magnetic field, with the core in a highly permeable state, such a field is locally attracted or gated (hence the name fluxgate) through the sense winding. When the core is weakly permeable, the external field is less attracted. This continuous gating of the external field in and out of the sense winding induces a signal in the sense winding, whose principal frequency is twice that of the drive frequency, and whose strength and phase orientation vary directly with the external-field magnitude and polarity.

There are additional factors that affect the size of the resultant signal. These factors include the number of turns in the sense winding, magnetic permeability of the core, sensor geometry, and the gated flux rate of change with respect to time.

Phase synchronous detection is used to extract these harmonic signals from the sense winding and convert them into a DC voltage proportional to the external magnetic field. Active current feedback may also be employed, such that the sense winding is driven to counteract the external field. In such cases, the feedback current varies linearly with the external magnetic field and is used as the basis for measurement. This helps to counter inherent non-linearity between the applied external field strength and the flux gated through the sense winding.

====SQUID magnetometer====
{{Main|SQUID}}
[[SQUID]]s, or superconducting quantum interference devices, measure extremely small changes in magnetic fields. They are very sensitive vector magnetometers, with noise levels as low as 3&nbsp;fT&nbsp;Hz<sup>−½</sup> in commercial instruments and 0.4&nbsp;fT&nbsp;Hz<sup>−½</sup> in experimental devices. Many liquid-helium-cooled commercial SQUIDs achieve a flat noise spectrum from near DC (less than 1&nbsp;Hz) to tens of kilohertz, making such devices ideal for time-domain biomagnetic signal measurements. SERF atomic magnetometers demonstrated in laboratories so far reach competitive noise floor but in relatively small frequency ranges.

SQUID magnetometers require cooling with liquid [[helium]] ({{Val|4.2|ul=K}}) or [[liquid nitrogen]] ({{Val|77|u=K}}) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint. SQUID magnetometers are most commonly used to measure the magnetic fields produced by laboratory samples, also for brain or heart activity ([[magnetoencephalography]] and [[magnetocardiography]], respectively). Geophysical surveys use SQUIDs from time to time, but the logistics of cooling the SQUID are much more complicated than other magnetometers that operate at room temperature.

====Zero-field optically-pumped magnetometers====
Magnetometers based on atomic gasses can perform vector measurements of the magnetic field in the low field regime, where the decay of the atomic coherence becomes faster than the [[Larmor frequency]]. The physics of such magnetometers is based on the [[Hanle effect]]. Such zero-field optically pumped magnetometers have been tested in various configurations and with different atomic species, notably [[Alkali metal|alkali]] (potassium, rubidium and cesium), [[helium]] and [[Mercury (element)|mercury]]. For the case of alkali, the coherence times were greatly limited due to spin-exchange relaxation. A major breakthrough happened at the beginning of the 2000 decade, Romalis group in Princeton demonstrated that in such a low field regime, alkali coherence times can be greatly enhanced if a high enough density can be reached by high temperature heating, this is the so-called [[SERF|SERF effect]].

The main interest of optically-pumped magnetometers is to replace SQUID magnetometers in applications where cryogenic cooling is a drawback. This is notably the case of medical imaging where such cooling imposes a thick thermal insulation, strongly affecting the amplitude of the recorded biomagnetic signals. Several startup companies are currently developing optically pumped magnetometers for biomedical applications: those of TwinLeaf,<ref>{{Cite web |title=MicroSERF Twinleaf magnetometers |url=https://twinleaf.com/vector/microSERF/}}</ref> quSpin<ref>{{Cite web |title=quSpin QZFM magnetometers |url=https://quspin.com/products-qzfm/}}</ref> and FieldLine<ref>{{Cite web |title=FieldLine website |url=https://fieldlineinc.com/}}</ref> being based on alkali vapors, and those of Mag4Health on metastable helium-4.<ref>{{Cite web |title=Mag4Health website |url=https://www.mag4health.com/technology/mag4health-quantum-sensors/}}</ref>

=====Spin-exchange relaxation-free (SERF) atomic magnetometers=====
{{Main|SERF}}
At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free ([[SERF]]) atomic magnetometers containing [[potassium]], [[caesium]], or [[rubidium]] vapor operate similarly to the caesium magnetometers described above, yet can reach sensitivities lower than 1 fT Hz<sup>−{{1/2}}</sup>. The SERF magnetometers only operate in small magnetic fields. The Earth's field is about 50 [[tesla (unit)|μT]]; SERF magnetometers operate in fields less than 0.5 μT.

Large volume detectors have achieved a sensitivity of 200 aT Hz<sup>−{{1/2}}</sup>.<ref>{{cite journal|last1=Kominis |first1=I.K. |last2=Kornack |first2=T.W. |last3=Allred |first3=J.C. |last4=Romalis |first4=M.V.|doi=10.1038/nature01484 |date=4 February 2003|title=A subfemtotesla multichannel atomic magnetometer|bibcode= 2003Natur.422..596K |volume=422 |issue=6932 |pmid=12686995| journal=Nature| pages=596–9|s2cid=4204465 }}</ref> This technology has greater sensitivity per unit volume than SQUID detectors.<ref>{{cite journal |last1=Budker |first1=D. |last2=Romalis |first2=M.V. |date=2006 |title=Optical Magnetometry |arxiv=physics/0611246 |doi=10.1038/nphys566 |volume=3 |issue=4 |journal=Nature Physics |pages=227–234| bibcode=2007NatPh...3..227B |s2cid=96446612 }}</ref> The technology can also produce very small magnetometers that may in the future replace coils for detecting radio-frequency magnetic fields.{{Citation needed|date=January 2012}} This technology may produce a magnetic sensor that has all of its input and output signals in the form of light on fiber-optic cables.<ref>{{Cite book |last1= Kitching |first1= J. |last2= Knappe |first2= S. |last3= Shah |first3= V. |last4= Schwindt |first4= P. |last5= Griffith |first5= C. |last6= Jimenez |first6= R. |last7= Preusser |first7= J. |last8= Liew |first8= L. -A. |last9= Moreland |first9= J. |chapter= Microfabricated atomic magnetometers and applications |doi= 10.1109/FREQ.2008.4623107 |title= 2008 IEEE International Frequency Control Symposium |pages= 789 |year= 2008 |isbn= 978-1-4244-1794-0 |s2cid= 46471890 |chapter-url= https://zenodo.org/record/1232197 }}</ref> This lets the magnetic measurement be made near high electrical voltages.