TheInfoList

Magnetism is a class of physical attributes that are mediated by
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

s.
Electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
s and the
magnetic moment The magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, a ...

s of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Magnetism is one aspect of the combined phenomenon of
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

. The most familiar effects occur in
ferromagnetic Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first tra ...
materials, which are strongly attracted by magnetic fields and can be magnetized to become permanent
magnet Magnetic field#Magnetic field lines, Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings A magnet is a material or object that produces a magnetic field. This magnetic ...

s, producing magnetic fields themselves. Demagnetizing a magnet is also possible. Only a few substances are ferromagnetic; the most common ones are
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

,
cobalt Cobalt is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...
and
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

and their alloys. The rare-earth metals
neodymium Neodymium is a chemical element with the Symbol (chemistry), symbol Nd and atomic number 60. Neodymium belongs to the lanthanide series and is a rare-earth element. It is a hard, slightly malleable silvery metal that quickly tarnishes in air and mo ...

and
samarium Samarium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

are less common examples. The prefix ' refers to
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

, because permanent magnetism was first observed in
lodestone of the Smithsonian Image:Lodestone (black).jpg, Lodestone attracting small bits of iron A lodestone is a naturally magnetized piece of the mineral magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and min ...
, a form of natural iron ore called
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

, Fe3O4. All substances exhibit some type of magnetism. Magnetic materials are classified according to their bulk susceptibility. Ferromagnetism is responsible for most of the effects of magnetism encountered in everyday life, but there are actually several types of magnetism.
Paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, a ...
substances, such as
aluminum Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the Unit ...
and
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

, are weakly attracted to an applied magnetic field;
diamagnetic Diamagnetic materials are repelled by a magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experie ...
substances, such as
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

and
carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making four electrons available to form covalent bond, covalent chemical bonds. ...
, are weakly repelled; while
antiferromagnetic Antiferromagnetic ordering In materials that exhibit antiferromagnetism, the magnetic moment The magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field A magnetic field is a vec ...
materials, such as
chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

and
spin glass In condensed matter physics, a spin glass is a magnetic state characterized by randomness, besides cooperative behavior in freezing of spins at a temperature called 'freezing temperature' ''Tf''. Magnetic spins are, roughly speaking, the orien ...
es, have a more complex relationship with a magnetic field. The force of a magnet on paramagnetic, diamagnetic, and antiferromagnetic materials is usually too weak to be felt and can be detected only by laboratory instruments, so in everyday life, these substances are often described as non-magnetic. The magnetic state (or magnetic phase) of a material depends on temperature, pressure, and the applied magnetic field. A material may exhibit more than one form of magnetism as these variables change. The strength of a
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

almost always decreases with distance, though the exact mathematical relationship between strength and distance varies. Different configurations of magnetic moments and electric currents can result in complicated magnetic fields. Only
magnetic dipole A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the Electric dipole moment, electric ...
s have been observed, although some theories predict the existence of
magnetic monopole In particle physics, a magnetic monopole is a hypothetical elementary particle that is an isolated magnet Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings A ...
s.

# History

upDrawing of a medical treatment using magnetic brushes. Charles Jacque 1843, France. Magnetism was first discovered in the ancient world, when people noticed that
lodestone of the Smithsonian Image:Lodestone (black).jpg, Lodestone attracting small bits of iron A lodestone is a naturally magnetized piece of the mineral magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and min ...
s, naturally magnetized pieces of the mineral
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

, could attract iron. The word ''magnet'' comes from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of 2018; Athens is ...
term μαγνῆτις λίθος ''magnētis lithos'', "the Magnesian stone, lodestone." In ancient Greece,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit ...

attributed the first of what could be called a scientific discussion of magnetism to the philosopher
Thales Thales of Miletus ( ; el, wikt:Θαλῆς, Θαλῆς (ὁ Μιλήσιος), ''Thalēs''; ) was a Greeks, Greek Greek Mathematics, mathematician, astronomer and Pre-Socratic philosophy, pre-Socratic philosopher from Miletus in Ionia, Asia Min ...

of
Miletus Miletus (; gr, Μῑ́λητος, Mīlētos; Hittite Hittite may refer to: * Hittites, ancient Anatolian people ** Hittite language, the earliest-attested Indo-European language ** Hittite grammar ** Hittite phonology ** Hittite cuneiform ** ...
, who lived from about 625 BC to about 545 BC. The ancient Indian medical text ''
Sushruta Samhita The ''Sushruta Samhita'' (सुश्रुतसंहिता, IAST The International Alphabet of Sanskrit Transliteration (IAST) is a transliteration scheme that allows the lossless romanisation of Brahmic family, Indic scripts as employ ...
'' describes using magnetite to remove arrows embedded in a person's body. In
ancient China The earliest known written records of the history of China date from as early as 1250 BC, from the Shang dynasty (c. 1600–1046 BC), during the king Wu Ding's reign, who was mentioned as the twenty-first Shang king by the same. Ancient his ...
, the earliest literary reference to magnetism lies in a 4th-century BC book named after its author, ''
Guiguzi Guiguzi () is a collection of ancient Chinese texts compiled between the late Warring States period and the end of the Han Dynasty The Han dynasty () was the second Dynasties in Chinese history, imperial dynasty of China (202 BC – 220 ...
''. The 2nd-century BC annals, ''
Lüshi Chunqiu The ''Lüshi Chunqiu'', also known in English as ''Master Lü's Spring and Autumn Annals'', is an encyclopedic Chinese classic text compiled around 239 BC under the patronage of the Qin Dynasty Chancellor Lü Buwei. In the evaluation of Michae ...
'', also notes: "The
lodestone of the Smithsonian Image:Lodestone (black).jpg, Lodestone attracting small bits of iron A lodestone is a naturally magnetized piece of the mineral magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and min ...
makes iron approach; some (force) is attracting it."
From the section "''Jingtong''" () of the "Almanac of the Last Autumn Month" (): "]"
The earliest mention of the attraction of a needle is in a 1st-century work ''
Lunheng The ''Lunheng'', also known by numerous English translations, is a wide-ranging Chinese classic text by Wang Chong (27- ). First published in 80, it contains critical essays on natural science Natural science is a branch of science ...
'' (''Balanced Inquiries''): "A lodestone attracts a needle." The 11th-century Chinese scientist
Shen Kuo Shen Kuo (; 1031–1095) or Shen Gua, courtesy name A courtesy name (), also known as a style name, is a name bestowed upon one at adulthood in addition to one's given name. This practice is a tradition in the Sinosphere, including China, Jap ...
was the first person to write—in the ''
Dream Pool Essays Image:Shen Kua sketch.jpg, Shen Kuo (沈括) (1031–1095 AD) ''The Dream Pool Essays'' or Dream Torrent Essays was an extensive book written by the Chinese polymath and statesman Shen Kuo (1031–1095) by AD 1088, during the Song dynasty (960– ...
''—of the magnetic needle compass and that it improved the accuracy of navigation by employing the
astronomical Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics Mathematics (from Ancient Greek, Gr ...
concept of
true north True north (also called geodetic north or geographic north) is the direction Direction may refer to: *Relative direction, for instance left, right, forward, backwards, up, and down ** Anatomical terms of location for those used in anatomy *Car ...
. By the 12th century, the Chinese were known to use the lodestone
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
for navigation. They sculpted a directional spoon from lodestone in such a way that the handle of the spoon always pointed south.
Alexander Neckam Alexander Neckam (8 September 115731 March 1217) was an English scholar, teacher, theologian and abbot of Cirencester Abbey from 1213 until his death. Early life Born on 8 September 1157 in St Albans, Alexander shared his birthday with Richard I ...
, by 1187, was the first in Europe to describe the compass and its use for navigation. In 1269, Peter Peregrinus de Maricourt wrote the ''Epistola de magnete'', the first extant treatise describing the properties of magnets. In 1282, the properties of magnets and the dry compasses were discussed by Al-Ashraf Umar II, a Yemeni physicist,
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, ga ...
, and
geographer A geographer is a physical scientist, social scientist and humanist whose area of study is geography, the study of Earth's natural environment and human society, including how society and nature interacts. The Greek prefix "geo" means "earth" ...
. Leonardo Garzoni's only extant work, the ''Due trattati sopra la natura, e le qualità della calamita'', is the first known example of a modern treatment of magnetic phenomena. Written in years near 1580 and never published, the treatise had a wide diffusion. In particular, Garzoni is referred to as an expert in magnetism by Niccolò Cabeo, whose Philosophia Magnetica (1629) is just a re-adjustment of Garzoni's work. Garzoni's treatise was known also to
Giovanni Battista Della Porta Giambattista della Porta (; 1535? – 4 February 1615), also known as Giovanni Battista Della Porta, was an Italian scholar, polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical la ...

. In 1600, William Gilbert published his '' De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure'' (''On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth''). In this work he describes many of his experiments with his model earth called the
terrellae. out-of-print, full text onlineSection 2, Chapter VI, page 678 A terrella (Latin for "little earth") is a small magnet Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron ...
. From his experiments, he concluded that the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...
was itself magnetic and that this was the reason compasses pointed north (previously, some believed that it was the pole star (
Polaris Polaris ( ), designated α Ursae Minoris ( Latinized to Alpha Ursae Minoris, abbreviated Alpha UMi, α UMi), commonly the North Star or Pole Star, is the brightest star of the constellation Ursa Minor. It is very close to the nort ...

) or a large magnetic island on the north pole that attracted the compass). An understanding of the relationship between
electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon in which an object changes its positio ...

and magnetism began in 1819 with work by
Hans Christian Ørsted Hans Christian Ørsted ( , ; often rendered Oersted in English; 14 August 17779 March 1851) was a Danish physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge ...

, a professor at the University of Copenhagen, who discovered by the accidental twitching of a compass needle near a wire that an electric current could create a magnetic field. This landmark experiment is known as Ørsted's Experiment. Several other experiments followed, with
André-Marie Ampère André-Marie Ampère (, ; ; 20 January 177510 June 1836) was a French physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of ...
, who in 1820 discovered that the magnetic field circulating in a closed-path was related to the current flowing through a surface enclosed by the path;
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes referr ...

;
Jean-Baptiste Biot Jean-Baptiste Biot (; ; 21 April 1774 – 3 February 1862) was a French physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of ...
and Félix Savart, both of whom in 1820 came up with the
Biot–Savart law In physics, specifically electromagnetism, the Biot–Savart law ( or ) is an equation describing the magnetic field generated by a constant electric current. It relates the magnetic field to the magnitude, direction, length, and proximity of the ...
giving an equation for the magnetic field from a current-carrying wire;
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of interest. In clas ...

, who in 1831 found that a time-varying magnetic flux through a loop of wire induced a voltage, and others finding further links between magnetism and electricity.
James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of interest. In classica ...

synthesized and expanded these insights into
Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematic ...
, unifying electricity, magnetism, and
optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion a ...

into the field of
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

. In 1905,
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the theory of relativity The theory ...
used these laws in motivating his theory of
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between Spacetime, space and time. In Albert Einstein's original treatment, the theory is based on two Postulates of ...
,A. Einstein: "On the Electrodynamics of Moving Bodies"
June 30, 1905.
requiring that the laws held true in all
inertial reference frame In classical physics Classical physics is a group of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...
s. Electromagnetism has continued to develop into the 21st century, being incorporated into the more fundamental theories of
gauge theory In physics, a gauge theory is a type of Field theory (physics), field theory in which the Lagrangian (field theory), Lagrangian (and hence the dynamics of the system itself) does not change (is Invariant (physics), invariant) under local symmetry, ...
,
quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity and quantum mecha ...
,
electroweak theory In particle physics, the electroweak interaction or electroweak force is the unified field theory, unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces ...
, and finally the
standard model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis ...

.

# Sources

Magnetism, at its root, arises from two sources: #
Electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
. #
Spin magnetic moment In physics, mainly quantum mechanics and particle physics, a spin magnetic moment is the magnetic moment caused by the spin (physics), spin of elementary particles. For example, the electron is an elementary spin-1/2 fermion. Quantum electrodyna ...
s of
elementary particles In particle physics, an elementary particle or fundamental particle is a subatomic particle with no substructure, i.e. it is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, l ...
. The magnetic properties of materials are mainly due to the magnetic moments of their
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

s' orbiting
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s. The magnetic moments of the nuclei of atoms are typically thousands of times smaller than the electrons' magnetic moments, so they are negligible in the context of the magnetization of materials. Nuclear magnetic moments are nevertheless very important in other contexts, particularly in
nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing an electromagne ...
(NMR) and
magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging Medical imaging is the technique and process of imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of a ...
(MRI). Ordinarily, the enormous number of electrons in a material are arranged such that their magnetic moments (both orbital and intrinsic) cancel out. This is due, to some extent, to electrons combining into pairs with opposite intrinsic magnetic moments as a result of the
Pauli exclusion principle The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermion In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc ...
(see ''
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance tha ...
''), and combining into filled subshells with zero net orbital motion. In both cases, the electrons preferentially adopt arrangements in which the magnetic moment of each electron is canceled by the opposite moment of another electron. Moreover, even when the
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance tha ...
''is'' such that there are unpaired electrons and/or non-filled subshells, it is often the case that the various electrons in the solid will contribute magnetic moments that point in different, random directions so that the material will not be magnetic. Sometimes, either spontaneously, or owing to an applied external magnetic field—each of the electron magnetic moments will be, on average, lined up. A suitable material can then produce a strong net magnetic field. The magnetic behavior of a material depends on its structure, particularly its
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance tha ...
, for the reasons mentioned above, and also on the temperature. At high temperatures, random
thermal motion of the ideal gas is proportional to the average kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as the work (physics), work needed to accelerate a bod ...
makes it more difficult for the electrons to maintain alignment.

# Types of magnetism

## Diamagnetism

Diamagnetism appears in all materials and is the tendency of a material to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior dominates. Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely diamagnetic material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises from the electrons' orbital motions, which can be understood classically as follows: This description is meant only as a
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving Problem solving consists of using generic or ad hoc Ad hoc is a List of Latin phrases, Latin phrase meaning literally 'to this'. In English, it generally signifi ...
; the Bohr–Van Leeuwen theorem shows that diamagnetism is impossible according to classical physics, and that a proper understanding requires a quantum-mechanical description. All materials undergo this orbital response. However, in paramagnetic and ferromagnetic substances, the diamagnetic effect is overwhelmed by the much stronger effects caused by the unpaired electrons.

## Paramagnetism

In a paramagnetic material there are ''unpaired electrons''; i.e., atomic or
molecular orbital In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...
s with exactly one electron in them. While paired electrons are required by the
Pauli exclusion principle The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermion In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc ...
to have their intrinsic ('spin') magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it.

## Ferromagnetism

A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moment's tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered-energy state. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another. Every ferromagnetic substance has its own individual temperature, called the
Curie temperature In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throug ...
, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order. Ferromagnetism only occurs in a few substances; common ones are
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

,
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

,
cobalt Cobalt is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...
, their
alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts Elec ...
s, and some alloys of
rare-earth The rare-earth elements, also called the rare-earth metals or (in context) rare-earth oxides, or the lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical element Image:Simple Period ...
metals.

### Magnetic domains

The magnetic moments of atoms in a
ferromagnetic Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first tra ...
material cause them to behave something like tiny permanent magnets. They stick together and align themselves into small regions of more or less uniform alignment called
magnetic domains A magnetic domain is a region within a magnetic material in which the magnetization In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density The density (more precisely, the vol ...
or Weiss domains. Magnetic domains can be observed with a
magnetic force microscope Magnetic force microscopy (MFM) is a variety of atomic force microscopy, in which a sharp magnetized tip scans a magnetic sample; the tip-sample magnetic interactions are detected and used to reconstruct the magnetic structure of the sample surf ...
to reveal magnetic domain boundaries that resemble white lines in the sketch. There are many scientific experiments that can physically show magnetic fields. When a domain contains too many molecules, it becomes unstable and divides into two domains aligned in opposite directions, so that they stick together more stably, as shown at the right. When exposed to a magnetic field, the domain boundaries move, so that the domains aligned with the magnetic field grow and dominate the structure (dotted yellow area), as shown at the left. When the magnetizing field is removed, the domains may not return to an unmagnetized state. This results in the ferromagnetic material's being magnetized, forming a permanent magnet. When magnetized strongly enough that the prevailing domain overruns all others to result in only one single domain, the material is magnetically saturated. When a magnetized ferromagnetic material is heated to the
Curie point In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throug ...
temperature, the molecules are agitated to the point that the magnetic domains lose the organization, and the magnetic properties they cause cease. When the material is cooled, this domain alignment structure spontaneously returns, in a manner roughly analogous to how a liquid can
freeze Freeze may refer to: Liquids turning to solids *Freezing, the physical process of a liquid turning into a solid Cessation of movement or change *Freeze (b-boy move), the halting of all movement in a clever position *Freeze (command), freez ...

into a crystalline solid.

## Antiferromagnetism

In an
antiferromagnet Antiferromagnetic ordering In materials that exhibit antiferromagnetism, the magnetic moments of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and ...
, unlike a ferromagnet, there is a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in ''opposite'' directions. When all atoms are arranged in a substance so that each neighbor is anti-parallel, the substance is antiferromagnetic. Antiferromagnets have a zero net magnetic moment, meaning that no field is produced by them. Antiferromagnets are less common compared to the other types of behaviors and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferromagnetic properties. In some materials, neighboring electrons prefer to point in opposite directions, but there is no geometrical arrangement in which ''each'' pair of neighbors is anti-aligned. This is called a
spin glass In condensed matter physics, a spin glass is a magnetic state characterized by randomness, besides cooperative behavior in freezing of spins at a temperature called 'freezing temperature' ''Tf''. Magnetic spins are, roughly speaking, the orien ...
and is an example of
geometrical frustrationIn condensed matter physics, the term geometrical frustration (or in short: frustration) refers to a phenomenon, where atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substanc ...
.

## Ferrimagnetism

Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins tend to point in opposite directions. These two properties are not contradictory, because in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons that point in one direction, than from the sublattice that points in the opposite direction. Most Ferrite (magnet), ferrites are ferrimagnetic. The first discovered magnetic substance,
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

, is a ferrite and was originally believed to be a ferromagnet; Louis Néel disproved this, however, after discovering ferrimagnetism.

## Superparamagnetism

When a ferromagnet or ferrimagnet is sufficiently small, it acts like a single magnetic spin that is subject to Brownian motion. Its response to a magnetic field is qualitatively similar to the response of a paramagnet, but much larger.

## Other types of magnetism

* Metamagnetism * Molecule-based magnets * Single-molecule magnet * Spin glass

# Electromagnet

An electromagnet is a type of
magnet Magnetic field#Magnetic field lines, Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings A magnet is a material or object that produces a magnetic field. This magnetic ...

in which the
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

is produced by an electric current. The magnetic field disappears when the current is turned off. Electromagnets usually consist of a large number of closely spaced turns of wire that create the magnetic field. The wire turns are often wound around a magnetic core made from a
ferromagnetic Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first tra ...
or ferrimagnetic material such as
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

; the magnetic core concentrates the magnetic flux and makes a more powerful magnet. The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field. Electromagnets are widely used as components of other electrical devices, such as electric motor, motors, Electric generator, generators, relays, solenoids, loudspeakers, hard disks, Magnetic resonance imaging, MRI machines, scientific instruments, and magnetic separation equipment. Electromagnets are also employed in industry for picking up and moving heavy iron objects such as scrap iron and steel. Electromagnetism was discovered in 1820.

# Magnetism, electricity, and special relativity

As a consequence of Einstein's theory of special relativity, electricity and magnetism are fundamentally interlinked. Both magnetism lacking electricity, and electricity without magnetism, are inconsistent with special relativity, due to such effects as length contraction, time dilation, and the fact that the magnetic force is velocity-dependent. However, when both electricity and magnetism are taken into account, the resulting theory (
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

) is fully consistent with special relativity. In particular, a phenomenon that appears purely electric or purely magnetic to one observer may be a mix of both to another, or more generally the relative contributions of electricity and magnetism are dependent on the frame of reference. Thus, special relativity "mixes" electricity and magnetism into a single, inseparable phenomenon called
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

, analogous to how relativity "mixes" space and time into spacetime. All observations on
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

apply to what might be considered to be primarily magnetism, e.g. perturbations in the magnetic field are necessarily accompanied by a nonzero electric field, and propagate at the speed of light.

# Magnetic fields in a material

In a vacuum, :$\mathbf \ = \ \mu_0\mathbf,$ where is the vacuum permeability. In a material, :$\mathbf \ = \ \mu_0\left(\mathbf + \mathbf\right). \$ The quantity is called ''magnetic polarization''. If the field is small, the response of the magnetization in a diamagnet or paramagnet is approximately linear: :$\mathbf = \chi \mathbf,$ the constant of proportionality being called the magnetic susceptibility. If so, :$\mu_0\left(\mathbf + \mathbf\right) \ = \ \mu_0\left(1 + \chi\right) \mathbf \ = \ \mu_r\mu_0 \mathbf \ = \ \mu \mathbf.$ In a hard magnet such as a ferromagnet, is not proportional to the field and is generally nonzero even when is zero (see Remanence).

# Magnetic force

Image:Magnet0873.png, Magnetic lines of force of a bar magnet shown by iron filings on paper The phenomenon of magnetism is "mediated" by the magnetic field. An electric current or magnetic dipole creates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields. Maxwell's equations, which simplify to the
Biot–Savart law In physics, specifically electromagnetism, the Biot–Savart law ( or ) is an equation describing the magnetic field generated by a constant electric current. It relates the magnetic field to the magnitude, direction, length, and proximity of the ...
in the case of steady currents, describe the origin and behavior of the fields that govern these forces. Therefore, magnetism is seen whenever electrically electric charge, charged particles are in Motion (physics), motion—for example, from movement of electrons in an electric current, or in certain cases from the orbital motion of electrons around an atom's nucleus. They also arise from "intrinsic"
magnetic dipole A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the Electric dipole moment, electric ...
s arising from quantum-mechanical Spin (physics), spin. The same situations that create magnetic fields—charge moving in a current or in an atom, and intrinsic magnetic dipoles—are also the situations in which a magnetic field has an effect, creating a force. Following is the formula for moving charge; for the forces on an intrinsic dipole, see magnetic dipole. When a charged particle moves through a Magnetic field#B and H, magnetic field B, it feels a Lorentz force F given by the cross product: : $\mathbf = q \left(\mathbf \times \mathbf\right)$ where : $q$ is the electric charge of the particle, and : v is the velocity Vector (geometric), vector of the particle Because this is a cross product, the force is perpendicular to both the motion of the particle and the magnetic field. It follows that the magnetic force does no mechanical work, work on the particle; it may change the direction of the particle's movement, but it cannot cause it to speed up or slow down. The magnitude of the force is : $F=qvB\sin\theta\,$ where $\theta$ is the angle between v and B. One tool for determining the direction of the velocity vector of a moving charge, the magnetic field, and the force exerted is labeling the index finger "V", the middle finger "B", and the thumb "F" with your right hand. When making a gun-like configuration, with the middle finger crossing under the index finger, the fingers represent the velocity vector, magnetic field vector, and force vector, respectively. See also right-hand rule.

# Magnetic dipoles

A very common source of magnetic field found in nature is a dipole, with a "South pole" and a "North pole", terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. The Earth's North Magnetic Pole (currently in the Arctic Ocean, north of Canada) is physically a south pole, as it attracts the north pole of a compass. A magnetic field contains energy, and physical systems move toward configurations with lower energy. When diamagnetic material is placed in a magnetic field, a ''magnetic dipole'' tends to align itself in opposed polarity to that field, thereby lowering the net field strength. When ferromagnetic material is placed within a magnetic field, the magnetic dipoles align to the applied field, thus expanding the domain walls of the magnetic domains.

## Magnetic monopoles

Since a bar magnet gets its ferromagnetism from electrons distributed evenly throughout the bar, when a bar magnet is cut in half, each of the resulting pieces is a smaller bar magnet. Even though a magnet is said to have a north pole and a south pole, these two poles cannot be separated from each other. A monopole—if such a thing exists—would be a new and fundamentally different kind of magnetic object. It would act as an isolated north pole, not attached to a south pole, or vice versa. Monopoles would carry "magnetic charge" analogous to electric charge. Despite systematic searches since 1931, , they have never been observed, and could very well not exist. Nevertheless, some theoretical physics models predict the existence of these magnetic monopoles. Paul Dirac observed in 1931 that, because electricity and magnetism show a certain symmetry, just as Quantum electrodynamics, quantum theory predicts that individual positive charge, positive or negative charge, negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain the quantization of electric charge—that is, why the observed
elementary particles In particle physics, an elementary particle or fundamental particle is a subatomic particle with no substructure, i.e. it is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, l ...
carry charges that are multiples of the charge of the electron. Certain grand unified theories predict the existence of monopoles which, unlike elementary particles, are solitons (localized energy packets). The initial results of using these models to estimate the number of monopoles created in the Big Bang contradicted cosmological observations—the monopoles would have been so plentiful and massive that they would have long since halted the expansion of the universe. However, the idea of Cosmic inflation, inflation (for which this problem served as a partial motivation) was successful in solving this problem, creating models in which monopoles existed but were rare enough to be consistent with current observations.

# Units

## Other

* gauss (unit), gauss – the centimeter-gram-second (CGS) units of measurement, unit of magnetic field (denoted B). * oersted – the CGS unit of Magnetic field#B and H, magnetizing field (denoted H) * maxwell (unit), maxwell – the CGS unit for magnetic flux * gamma – a unit of ''magnetic flux density'' that was commonly used before the tesla (unit), tesla came into use (1.0 gamma = 1.0 nanotesla) * ''μ''0 – common symbol for the permeability (electromagnetism), permeability of free space ( newton (unit), newton/(ampere-turn)2)

# Living things

Some organisms can detect magnetic fields, a phenomenon known as magnetoception. Some materials in living things are ferromagnetic, though it is unclear if the magnetic properties serve a special function or are merely a byproduct of containing iron. For instance, chitons, a type of marine mollusk, produce magnetite to harden their teeth, and even humans produce
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

in bodily tissue. Magnetobiology studies the effects of magnetic fields on living organisms; fields naturally produced by an organism are known as biomagnetism. Many biological organisms are mostly made of water, and because water is diamagnetic, extremely strong magnetic fields can repel these living things.

# Quantum-mechanical origin of magnetism

While heuristic explanations based on classical physics can be formulated, diamagnetism, paramagnetism and ferromagnetism can be fully explained only using quantum theory.Ferromagnetism, ''Feynman Lectures in Physics'' Ch 36
/ref> A successful model was developed already in 1927, by Walter Heitler and Fritz London, who derived, quantum-mechanically, how hydrogen molecules are formed from hydrogen atoms, i.e. from the atomic hydrogen orbitals $u_A$ and $u_B$ centered at the nuclei ''A'' and ''B'', see below. That this leads to magnetism is not at all obvious, but will be explained in the following. According to the Heitler–London theory, so-called two-body molecular $\sigma$-orbitals are formed, namely the resulting orbital is: :$\psi\left(\mathbf r_1,\,\,\mathbf r_2\right)=\frac\,\,\left \left(u_A\left(\mathbf r_1\right)u_B\left(\mathbf r_2\right)+u_B\left(\mathbf r_1\right)u_A\left(\mathbf r_2\right)\right \right)$ Here the last product means that a first electron, r1, is in an atomic hydrogen-orbital centered at the second nucleus, whereas the second electron runs around the first nucleus. This "exchange" phenomenon is an expression for the quantum-mechanical property that particles with identical properties cannot be distinguished. It is specific not only for the formation of chemical bonds, but also for magnetism. That is, in this connection the term exchange interaction arises, a term which is essential for the origin of magnetism, and which is stronger, roughly by factors 100 and even by 1000, than the energies arising from the electrodynamic dipole-dipole interaction. As for the ''spin function'' $\chi \left(s_1,s_2\right)$, which is responsible for the magnetism, we have the already mentioned Pauli's principle, namely that a symmetric orbital (i.e. with the + sign as above) must be multiplied with an antisymmetric spin function (i.e. with a − sign), and ''vice versa''. Thus: :$\chi \left(s_1,\,\,s_2\right)=\frac\,\,\left \left(\alpha \left(s_1\right)\beta \left(s_2\right)-\beta \left(s_1\right)\alpha \left(s_2\right)\right \right)$, I.e., not only $u_A$ and $u_B$ must be substituted by ''α'' and ''β'', respectively (the first entity means "spin up", the second one "spin down"), but also the sign + by the − sign, and finally ri by the discrete values ''s''i (= ±½); thereby we have $\alpha\left(+1/2\right)=\beta\left(-1/2\right)=1$ and $\alpha\left(-1/2\right)=\beta\left(+1/2\right)=0$. The "singlet state", i.e. the − sign, means: the spins are ''antiparallel'', i.e. for the solid we have antiferromagnetism, and for two-atomic molecules one has diamagnetism. The tendency to form a (homoeopolar) chemical bond (this means: the formation of a ''symmetric'' molecular orbital, i.e. with the + sign) results through the Pauli principle automatically in an ''antisymmetric'' spin state (i.e. with the − sign). In contrast, the Coulomb repulsion of the electrons, i.e. the tendency that they try to avoid each other by this repulsion, would lead to an ''antisymmetric'' orbital function (i.e. with the − sign) of these two particles, and complementary to a ''symmetric'' spin function (i.e. with the + sign, one of the so-called "triplet state, triplet functions"). Thus, now the spins would be ''parallel'' (ferromagnetism in a solid, paramagnetism in two-atomic gases). The last-mentioned tendency dominates in the metals
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

,
cobalt Cobalt is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...
and
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

, and in some rare earths, which are ''ferromagnetic''. Most of the other metals, where the first-mentioned tendency dominates, are ''nonmagnetic'' (e.g. sodium, aluminium, and magnesium) or ''antiferromagnetic'' (e.g. manganese). Diatomic gases are also almost exclusively diamagnetic, and not paramagnetic. However, the oxygen molecule, because of the involvement of π-orbitals, is an exception important for the life-sciences. The Heitler-London considerations can be generalized to the Heisenberg model (classical), Heisenberg model of magnetism (Heisenberg 1928). The explanation of the phenomena is thus essentially based on all subtleties of quantum mechanics, whereas the electrodynamics covers mainly the phenomenology.

* Coercivity * Gravitomagnetism * Magnetic hysteresis * Magnetar * Magnetic bearing * Magnetic circuit * Magnetic cooling * Magnetic field viewing film * Magnetic stirrer * Magnetic structure * Magnetism and temperature * Micromagnetism * Neodymium magnet * Plastic magnet * Rare-earth magnet * Spin wave * Spontaneous magnetization * Vibrating-sample magnetometer