ferromagnetism
, physical phenomenon in which certain electrically uncharged materials strongly attract others. Two materials found in nature, lodestone (or
magnetite
, an oxide of
iron
, Fe
3
O
4
) and iron, have the ability to
acquire
such attractive powers, and they are often called natural ferromagnets. They were discovered more than 2,000 years ago, and all early scientific studies of
magnetism
were conducted on these materials. Today, ferromagnetic materials are used in a wide variety of devices essential to everyday life?e.g.,
electric motors
and
generators
,
transformers
,
telephones
, and
loudspeakers
.
Study the phenomena of ferromagnetism, antiferromagnetism, and paramagnetism and also how temperature affects magnetic properties
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Ferromagnetism is a kind of magnetism that is associated with iron,
cobalt
,
nickel
, and some
alloys
or
compounds
containing one or more of these
elements
. It also occurs in
gadolinium
and a few other
rare-earth elements
. In contrast to other substances, ferromagnetic materials are magnetized easily, and in strong magnetic fields the magnetization approaches a definite limit called saturation. When a field is applied and then removed, the magnetization does not return to its original value; this phenomenon is referred to as
hysteresis
. When heated to a certain
temperature
called the
Curie point
, which is different for each substance, ferromagnetic materials lose their characteristic properties and cease to be magnetic; however, they become ferromagnetic again on cooling.
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magnetism: Ferromagnetism
The magnetism in ferromagnetic materials is caused by the alignment patterns of their
constituent
atoms, which act as elementary electromagnets. Some species of
atoms
possess a
magnetic moment
; that is, such an atom itself is an elementary
electromagnet
produced by the motion of
electrons
about its nucleus and by the
spin
of those electrons. Below the Curie point, atoms that behave as tiny magnets in ferromagnetic materials spontaneously align themselves. They become oriented in the same direction, so that their magnetic fields reinforce each other.
One requirement of a ferromagnetic material is that its atoms or
ions
have permanent magnetic moments. The magnetic moment of an atom comes from its electrons, since the nuclear contribution is
negligible
. Another requirement for ferromagnetism is that
quantummechanical
effects keep the magnetic moments of many atoms parallel to each other. According to the
Pauli exclusion principle
, two electrons in the same location cannot have their
spins
oriented in the same direction. Their spins must be antiparallel. In electrons in nearby atoms, if two electrons were antiparallel, that would mean the two electrons would have a strong repulsive force from being near each other. By having their spins and thus their magnetic moments be parallel, the material is in a lower energy configuration than that in which nearby electrons are exerting a strong repulsive force on each other. Without these
quantum
mechanical effects, the atoms would be disordered by thermal agitation, the moments of neighbouring atoms would neutralize each other, and the large magnetic moment characteristic of ferromagnetic materials would not exist.
There is ample evidence that some atoms or ions have a permanent magnetic moment that may be pictured as a
dipole
consisting of a positive, or north, pole separated from a negative, or south, pole. In ferromagnets, the large coupling between the atomic magnetic moments leads to some degree of dipole alignment and hence to a net magnetization.
The French physicist
Pierre-Ernest Weiss
postulated a large-scale type of magnetic order for ferromagnets called domain structure. According to his theory, a ferromagnetic solid consists of a large number of small regions, or domains, in each of which all of the atomic or ionic magnetic moments are aligned. If the resultant moments of these domains are randomly oriented, the object as a whole will not display magnetism, but an externally applied magnetizing field will, depending on its strength, rotate one after another of the domains into alignment with the external field and cause aligned domains to grow at the expense of nonaligned ones. In the limiting state called saturation, the entire object will
comprise
a single domain.
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Domain structure
can be observed directly. In one technique, a
colloidal
solution of small magnetic particles, usually magnetite, is placed on the surface of a ferromagnet. When surface poles are present, the particles tend to concentrate in certain regions to form a pattern that is readily observed with an optical
microscope
. Domain patterns have also been observed with polarized light, polarized
neutrons
,
electron beams
, and
X-rays
.
In many ferromagnets the dipole moments are aligned parallel by the strong coupling. This is the magnetic arrangement found for the elemental metals iron (Fe), nickel (Ni), and cobalt (Co) and for their alloys with one another and with some other elements. These materials still
constitute
the largest group of ferromagnets commonly used. The other elements that possess a collinear ordering are the rare-earth metals gadolinium (Gd),
terbium
(Tb), and
dysprosium
(Dy), but the last two become ferromagnets only well below room temperature. Some alloys, although not composed of any of the elements just mentioned, nevertheless have a parallel moment arrangement. An example of this is the
Heusler alloy
CuAlMn
3
, in which the
manganese
(Mn) atoms have magnetic moments, though manganese metal itself is not ferromagnetic.
Several ionically bound
compounds
have been discovered to be ferromagnetic. Some of these compounds are electrical
insulators
; others have a conductivity of magnitude typical of
semiconductors
. Such compounds include chalcogenides (compounds of
oxygen
,
sulfur
,
selenium
, or
tellurium
), halides (compounds of
fluorine
,
chlorine
,
bromine
, or
iodine
), and their combinations. The ions with permanent dipole moments in these materials are manganese,
chromium
(Cr), and
europium
(Eu); the others are
diamagnetic
. At low temperatures, the rare-earth metals
holmium
(Ho) and erbium (Er) have a nonparallel moment arrangement that gives rise to a substantial spontaneous magnetization. Some ionic compounds with the
spinel
crystal structure
also possess ferromagnetic ordering. A different structure leads to a
spontaneous
magnetization in
thulium
(Tm) below 32 kelvins (K).
Above the Curie point (also called the Curie temperature), the spontaneous magnetization of the ferromagnetic material vanishes, and it becomes
paramagnetic
(i.e., it remains weakly magnetic). This occurs because the
thermal energy
becomes sufficient to overcome the internal aligning forces of the material. The Curie temperatures for some important ferromagnets are: iron, 1,043 K; cobalt, 1,394 K; nickel, 631 K; and gadolinium, 293 K.