Magnetism is a class of physical phenomena
that are mediated by magnetic fields. Electric
currents and the magnetic moments of elementary
particles give rise to a magnetic field, which
acts on other currents and magnetic moments.
The most familiar effects occur in ferromagnetic
materials, which are strongly attracted by
magnetic fields and can be magnetized to become
permanent magnets, producing magnetic fields
themselves. Only a few substances are ferromagnetic;
the most common ones are iron, nickel and
cobalt and their alloys. The prefix ferro-
refers to iron, because permanent magnetism
was first observed in lodestone, a form of
natural iron ore called magnetite, Fe3O4.
Although ferromagnetism is responsible for
most of the effects of magnetism encountered
in everyday life, all other materials are
influenced to some extent by a magnetic field,
by several other types of magnetism. Paramagnetic
substances such as aluminum and oxygen are
weakly attracted to an applied magnetic field;
diamagnetic substances such as copper and
carbon are weakly repelled; while antiferromagnetic
materials such as chromium and spin glasses
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 and other
variables such as pressure and the applied
magnetic field. A material may exhibit more
than one form of magnetism as these variables
change.
== History ==
Magnetism was first discovered in the ancient
world, when people noticed that lodestones,
naturally magnetized pieces of the mineral
magnetite, could attract iron. The word magnet
comes from the Greek term μαγνῆτις
λίθος magnētis lithos, "the Magnesian
stone, lodestone." In ancient Greece, Aristotle
attributed the first of what could be called
a scientific discussion of magnetism to the
philosopher Thales of Miletus, who lived from
about 625 BC to about 545 BC. The ancient
Indian medical text Sushruta Samhita describes
using magnetite to remove arrows embedded
in a person's body.In ancient China, the earliest
literary reference to magnetism lies in a
4th-century BC book named after its author,
The Sage of Ghost Valley.
The 2nd-century BC annals, Lüshi Chunqiu,
also notes:
"The lodestone makes iron approach, or it
attracts it."
The earliest mention of the attraction of
a needle is in a 1st-century work Lunheng
(Balanced Inquiries): "A lodestone attracts
a needle."
The 11th-century Chinese scientist Shen Kuo
was the first person to write – in the Dream
Pool Essays – of the magnetic needle compass
and that it improved the accuracy of navigation
by employing the astronomical concept of true
north.
By the 12th century the Chinese were known
to use the lodestone compass for navigation.
They sculpted a directional spoon from lodestone
in such a way that the handle of the spoon
always pointed south.
Alexander Neckam, 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,
a Yemeni physicist, astronomer, and geographer.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 terrella.
From his experiments, he concluded that the
Earth was itself magnetic and that this was
the reason compasses pointed north (previously,
some believed that it was the pole star (Polaris)
or a large magnetic island on the north pole
that attracted the compass).
An understanding of the relationship between
electricity and magnetism began in 1819 with
work by Hans Christian Ørsted, 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, who in 1820 discovered that the magnetic
field circulating in a closed-path was related
to the current flowing through the perimeter
of the path; Carl Friedrich Gauss; Jean-Baptiste
Biot and Félix Savart, both of whom in 1820
came up with the Biot–Savart law giving
an equation for the magnetic field from a
current-carrying wire; Michael Faraday, 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 synthesized
and expanded these insights into Maxwell's
equations, unifying electricity, magnetism,
and optics into the field of electromagnetism.
In 1905, Einstein used these laws in motivating
his theory of special relativity, requiring
that the laws held true in all inertial reference
frames.
Electromagnetism has continued to develop
into the 21st century, being incorporated
into the more fundamental theories of gauge
theory, quantum electrodynamics, electroweak
theory, and finally the standard model.
== Sources of magnetism ==
Magnetism, at its root, arises from two sources:
Electric current.
Spin magnetic moments of elementary particles.The
magnetic properties of materials are mainly
due to the magnetic moments of their atoms'
orbiting electrons. 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 (NMR) and magnetic resonance
imaging (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 (see electron
configuration), 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 cancelled by the opposite
moment of another electron. Moreover, even
when the electron configuration 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, for the reasons mentioned above,
and also on the temperature. At high temperatures,
random thermal motion makes it more difficult
for the electrons to maintain alignment.
== Materials ==
=== 
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:
When a material is put in a magnetic field,
the electrons circling the nucleus will experience,
in addition to their Coulomb attraction to
the nucleus, a Lorentz force from the magnetic
field. Depending on which direction the electron
is orbiting, this force may increase the centripetal
force on the electrons, pulling them in towards
the nucleus, or it may decrease the force,
pulling them away from the nucleus. This effect
systematically increases the orbital magnetic
moments that were aligned opposite the field,
and decreases the ones aligned parallel to
the field (in accordance with Lenz's law).
This results in a small bulk magnetic moment,
with an opposite direction to the applied
field.
Note that this description is meant only as
a heuristic; the Bohr-van Leeuwen theorem
shows that diamagnetism is impossible according
to classical physics, and that a proper understanding
requires a quantum-mechanical description.
Note that 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 orbitals
with exactly one electron in them. While paired
electrons are required by the Pauli exclusion
principle 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,
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, nickel, cobalt, their
alloys, and some alloys of rare-earth metals.
==== Magnetic domains ====
The magnetic moments of atoms in a ferromagnetic
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 or Weiss domains. Magnetic domains
can be observed with a magnetic force microscope
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 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
into a crystalline solid.
=== Antiferromagnetism ===
In an antiferromagnet, 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 and is an example of geometrical
frustration.
=== 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 ferrites are ferrimagnetic. The first
discovered magnetic substance, magnetite,
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
Molecular magnet
Spin glass
== 
Electromagnet ==
An electromagnet is a type of magnet in which
the magnetic field 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
or ferrimagnetic material such as iron; 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 motors,
generators, relays, solenoids, loudspeakers,
hard disks, 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)
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, analogous
to how relativity "mixes" space and time into
spacetime.
All observations on electromagnetism 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,
B
=
μ
0
H
,
{\displaystyle \mathbf {B} \ =\ \mu _{0}\mathbf
{H} ,}
where μ0 is the vacuum permeability.
In a material,
B
=
μ
0
(
H
+
M
)
.
{\displaystyle \mathbf {B} \ =\ \mu _{0}(\mathbf
{H} +\mathbf {M} ).\ }
The quantity μ0M is called magnetic polarization.
If the field H is small, the response of the
magnetization M in a diamagnet or paramagnet
is approximately linear:
M
=
χ
H
,
{\displaystyle \mathbf {M} =\chi \mathbf {H}
,}
the constant of proportionality being called
the magnetic susceptibility. If so,
μ
0
(
H
+
M
)
=
μ
0
(
1
+
χ
)
H
=
μ
r
μ
0
H
=
μ
H
.
{\displaystyle \mu _{0}(\mathbf {H} +\mathbf
{M} )\ =\ \mu _{0}(1+\chi )\mathbf {H} \ =\ \mu
_{r}\mu _{0}\mathbf {H} \ =\ \mu \mathbf {H}
.}
In a hard magnet such as a ferromagnet, M
is not proportional to the field and is generally
nonzero even when H is zero (see Remanence).
== Magnetic force ==
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 the case of steady currents,
describe the origin and behavior of the fields
that govern these forces. Therefore, magnetism
is seen whenever electrically charged particles
are in 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 dipoles arising
from quantum-mechanical 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, it feels a Lorentz force F given
by the cross product:
F
=
q
(
v
×
B
)
{\displaystyle \mathbf {F} =q(\mathbf {v}
\times \mathbf {B} )}
where
q
{\displaystyle q}
is the electric charge of the particle, and
v is the velocity vector of the particleBecause
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 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
=
q
v
B
sin
⁡
θ
{\displaystyle F=qvB\sin \theta \,}
where
θ
{\displaystyle \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, as of 2010,
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 theory
predicts that individual positive or 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 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 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.
== Quantum-mechanical origin of magnetism
==
While heuristic explanations based on classical
physics can be formulated, diamagnetism, paramagnetism
and ferromagnetism can only be fully explained
using quantum theory.
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
{\displaystyle u_{A}}
and
u
B
{\displaystyle 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
σ
{\displaystyle \sigma }
-orbitals are formed, namely the resulting
orbital is:
ψ
(
r
1
,
r
2
)
=
1
2
(
u
A
(
r
1
)
u
B
(
r
2
)
+
u
B
(
r
1
)
u
A
(
r
2
)
)
{\displaystyle \psi (\mathbf {r} _{1},\,\,\mathbf
{r} _{2})={\frac {1}{\sqrt {2}}}\,\,\left(u_{A}(\mathbf
{r} _{1})u_{B}(\mathbf {r} _{2})+u_{B}(\mathbf
{r} _{1})u_{A}(\mathbf {r} _{2})\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 as one will see, also
for magnetism, i.e. 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
χ
(
s
1
,
s
2
)
{\displaystyle \chi (s_{1},s_{2})}
, 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:
χ
(
s
1
,
s
2
)
=
1
2
(
α
(
s
1
)
β
(
s
2
)
−
β
(
s
1
)
α
(
s
2
)
)
{\displaystyle \chi (s_{1},\,\,s_{2})={\frac
{1}{\sqrt {2}}}\,\,\left(\alpha (s_{1})\beta
(s_{2})-\beta (s_{1})\alpha (s_{2})\right)}
,I.e., not only
u
A
{\displaystyle u_{A}}
and
u
B
{\displaystyle 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
si (= ±½); thereby we have
α
(
+
1
/
2
)
=
β
(
−
1
/
2
)
=
1
{\displaystyle \alpha (+1/2)=\beta (-1/2)=1}
and
α
(
−
1
/
2
)
=
β
(
+
1
/
2
)
=
0
{\displaystyle \alpha (-1/2)=\beta (+1/2)=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 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, cobalt and nickel, 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 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.
== Units ==
=== SI ===
=== Other ===
gauss – the centimeter-gram-second (CGS)
unit of magnetic field (denoted B).
oersted – the CGS unit of magnetizing field
(denoted H)
maxwell – the CGS unit for magnetic flux
gamma – a unit of magnetic flux density
that was commonly used before the tesla came
into use (1.0 gamma = 1.0 nanotesla)
μ0 – common symbol for the permeability
of free space (4π × 10−7 newton/(ampere-turn)2)
== Living things ==
Some organisms can detect magnetic fields,
a phenomenon known as magnetoception. In addition
to detection, biomagnetic phenomena are utilized
by organisms in a number of ways. For instance,
chitons, a type of marine mollusk, produce
magnetite to harden their teeth, and even
humans produce magnetite in bodily tissue.
Magnetobiology studies magnetic fields as
a medical treatment; fields naturally produced
by an organism are known as biomagnetism.
== See also
