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Magnetism
Magnetism is a phenomenon by which materials assert an
attractive or repulsive force on other materials. Some well known materials that exhibit magnetic properties
are iron, some steels, and the naturally occurring mineral lodestone.
In reality all materials are influenced to one degree or another by the
presence of a magnetic field, although in some cases the influence is too small
to detect without special equipment.
Magnetic forces are fundamental forces that arise due to
the movement of electrically charged particles.
The origin and behavior of these forces are described by Maxwell's equations.
For the case of electric current moving through a wire, the
resulting force is directed according to the "right hand rule".
If the thumb of the right hand points along the wire from positive
towards the negative side, the magnetic forces will wrap around the wire in the
direction indicated by the fingers of the right hand.
If a loop is formed, such that the charged particles are traveling in a
circle then all of the forces in the center of the loop are directed in the same
direction. The result is called a magnetic
dipole. When placed in a
magnetic field, a magnetic dipole will tend to align itself with that field. For
the case of a loop, if the fingers of the right hand are directed in the
direction of current flow, the thumb will point in the direction corresponding
to the North pole of the dipole. In
the earth's magnetic field the North pole of the dipole will tend to point
north.
Magnetic dipoles or magnetic moments can often result on
the atomic scale due to the movements of electrons. Each electron has magnetic moments that originate from two
sources. The first is the orbital
motion of the electron around the nucleus.
In a sense this motion can be considered as a current loop, resulting in
a magnetic moment along its axis of rotation.
The second source of electronic magnetic moment is due to a quantum
mechanical property called spin.
In an atom the orbital magnetic moments of some electron
pairs cancel each other. The same
is true for the spin magnetic moments. The
overall magnetic moment of the atom is thus the sum of all of the magnetic
moments of the individual electrons, accounting for moment cancellation between
properly paired electrons. For the
case of a completely filled electron shell or subshell, the magnetic moments
completely cancel each other out. Thus
only atoms with partially filled electron shells have a magnetic moment.
The magnetic properties of materials are in large part determined by the
nature and magnitude of the atomic magnetic moments.
Several forms of magnetic behavior have been observed
including:
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Diamagnetism
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Paramagnetism
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Ferromagnetism
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Antiferromagnetism
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Ferrimagnetism
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Superparamagnetism
Diamagnetism
Diamagnetism is a very weak form of magnetism that is only
exhibited in the presence of an external magnetic field.
It is the result of changes in the orbital motion of electrons due to the
external magnetic field. The
induced magnetic moment is very small and in a direction opposite to that of the
applied field. When placed between
the poles of a strong electromagnet, diamagnetic materials are attracted towards
regions where the magnetic field is weak. Diamagnetism
is found in all materials, however because it is so weak it can only be observed
in materials that do not exhibit other forms of magnetism.
An exception to the "weak" nature of diamagnetism
occurs with the rather large number of materials that become superconducting,
something that usually happens at lowered temperatures. Superconductors are
perfect diamagnets and when placed in an external magnetic field expel the field
lines from their interiors (depending on field intensity and temperature).
Superconductors also have zero electrical resistance, a consequence of their
diamagnetism. Superconducting structures have been known to tear themselves
apart with astonishing force in their attempt to escape an external field.
Superconducting magnets are the major component of most MRI systems, perhaps the
only important application of diamagnetism.
A thin slice of pyrolitic graphite, which is an unusually
strongly diamagnetic material, can be stably floated on a magnetic field, such
as that from rare earth permanent magnets. This can be done with all components
at room temperature, making a visually effective demonstration of diamagnetism.
Paramagnetism
Paramagnetism refers to the tendency of the atomic magnetic
dipoles in a material that is otherwise non-magnetic to align with an external
magnetic field. The alignment of
the atomic dipoles with the magnetic field tends to strengthen it, resulting in
a relative magnetic permeability greater than one and a small positive magnetic
susceptibility.
In paramagnetism the field acts on each atomic dipole
independently and there are no interactions between individual atomic dipoles.
Paramagnetic behavior can also be observed in magnetic materials that
are above their Curie or Neel temperature.
Ferromagnetism
Ferromagnetism is one of the strongest forms of magnetism.
It is responsible for most of the magnetic behavior encountered in
everyday life. Most permanent
magnets are ferromagnetic, as are the metals that are attracted to them.
Some examples of ferromagnetic materials include iron, cobalt, nickel,
and gadolinium.
The strong magnetic forces in ferromagnetic materials arise
due to a combination of the properties of the individual atoms and the
properties of the crystal structure of the solid material.
At the atomic level, magnetic forces arise due to the movements of
electrons. Each electron has
magnetic moments that originate from two sources. The first is the orbital
motion of the electron around the nucleus. In a sense this motion can be
considered as a current loop, which like a tiny electromagnet results in a
magnetic moment along its axis of rotation. The second source of the electronic
magnetic moment is due to a Quantum Mechanical property called "spin",
this property is in some ways analogous to the picture of an electron spinning
about an axis and is related to the electron's angular momentum. However, it
should be remembered that the Quantum Mechanical "spin" is actually a
unique phenomenon from spinning in a macroscopic sense, so the analogy doesn't
always hold. The spin magnetic moments may be in one of two directions, either
the "up" direction or the "down" direction.
In an atom the orbital magnetic moments of electron pairs
point in opposite directions canceling each other. The same is true for the spin
magnetic moments. The overall magnetic moment of the atom is thus the sum of all
of the magnetic moments of the individual electrons, accounting for moment
cancellation between properly paired electrons. For the case of a completely
filled electron shell or subshell, the magnetic moments completely cancel each
other out. Thus only atoms with partially filled electron shells have a magnetic
moment. All ferromagnetic materials
have partially filled electron shells and thus posses an atomic magnetic moment.
Although atomic magnetic moments are present in both
Paramagnetic and Ferromagnetic materials, magnetic forces are much stronger in
ferromagnetic materials. This is
not due to differences in the atomic magnetic moments, but due to the crystal
structure of ferromagnetic materials. In
a ferromagnet coupling interactions cause the magnetic moments of adjacent atoms
to align with one another. This
contrasts sharply with paramagnets, in which the magnetic moments are randomly
distributed in many directions, essentially canceling each other out, except in
the presence of a strong magnetic field. The
alignment of the atomic magnetic moments in ferromagnetic materials results in a
strong permanent internal magnetic field within the material.
It is this strong internal magnetic field that causes iron or other
ferromagnetic materials to be attracted by a magnet.
While coupling forces tend to cause adjacent moments to
align, usually not all of the moments point in the same direction throughout the
material. Instead the material
consists of a number of regions called domains. Within each domain the atomic magnetic moments are aligned,
however, the various domains may or may not be aligned with each other.
For example, the domains in a metal paperclip are not usually aligned
with each other. As a result the magnetic forces from the various domains
cancel each other and two paper clips are not magnetically attracted to each
other. However, if the material is
placed within a magnetic field (for example if a permanent magnet is brought
near a paperclip), the magnetic forces will cause some of the domains to align.
This alignment will then result in a magnetic force drawing the material
to the magnet and causing the material to behave as if it too were a magnet.
If the magnetic field is removed, the domains will often
shift back to their original alignment and the material will no longer act as a
magnet. However, if the material is
subjected to a strong magnetic field for a sufficient length of time the domains
will permanently align and the material will become a permanent magnet.
Superparamagnetism
Superparamagnetism is a phenomena by which magnetic
materials may exhibit a behavior similar to paramagnetism at temperatures below
the Curie or the Neel temperature.
Normally, coupling forces in magnetic materials cause the
magnetic moments of neighboring atoms to align, resulting in very large internal
magnetic fields. At temperatures
above the Curie temperature (or the Neel temperature for antiferromagnetic
materials), the thermal energy is sufficient to overcome the coupling forces,
causing the atomic magnetic moments to fluctuate randomly.
Because there is no longer any magnetic order, the internal magnetic
field no longer exists and the material exhibits paramagnetic behavior.
Superparamagnetism occurs when the material is composed of
very small crystallites (1-10 nm). In
this case even though the temperature is below the Curie or Neel temperature and
the thermal energy is not sufficient to overcome the coupling forces between
neighboring atoms, the thermal energy is sufficient to change the direction of
magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization
cause the magnetic field to average to zero.
The material behaves in a manner similar to paramagnetism, except that
instead of each individual atom being independently influenced by an external
magnetic field, the magnetic moment of the entire crystallite tends to align
with the magnetic field.
The energy required to change the direction of
magnetization of a crystallite is called the Crystalline anisotropy energy and
depends both on the material properties and the crystallite size. As the
crystallite size decreases, so does the Crystalline anisotropy energy, resulting
in a decrease in the temperature at which the material becomes superparamagnetic.
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