Magnetism

o Magnetism is a physical phenomenon where some materials exert unseen force on

another (attractive or repulsive)- Magnetic Force, one of four fundamental forces in
nature.
o Basic source of this force is movement of electrically charged particles. Thus magnetic
behavior of a material can be traced to the structure of atoms.

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o Electrons in an atom have an orbital motion around the nucleus while spinning itself.
This motion of an electron causes an orbital magnetic moments 𝝁𝒍 and spin magnetic
moments 𝝁𝒔 , contributing to the magnetic behavior of materials.

o Total magnetic moment of an atom due to electron motion strongly depends on its
atomic structure and is greatly varied, since the electrons are oppositely paired (Pauli
exclusion law) and cancels out each other.
o Unpaired electrons causes the moments. Some elements such as transition elements,
lanthanides, and actinides have a net magnetic moment since some of their energy
levels have an unpaired electron.
o So, Not all materials are magnets. 2
o It is found that magnetic materials have magnetic dipoles, analogous to electric dipoles,
and is a small magnet composed of north and south pole. Their orientation is random
but they are aligned with external magnetic fields.

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 Magnetization
o Process of converting a non magnetic material into magnetic sample.
o When a material is placed in an external magnetic flux, then it induces both a
magnetic flux and a magnetization inside the material.

If the external magnetic field density 𝐻, the magnetic flux density inside the material 𝐵
and magnetization density of the material 𝑀 , then
𝝁
𝑩 = 𝝁𝑯 , where 𝝁-permeability of the material and 𝝁𝒓 = .
𝝁𝟎

𝑴 = 𝝌 𝑯 , where 𝝌-magnetic susceptibility of the material and 𝝌 = 𝝁𝒓 − 𝟏.

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 Types of Magnetism
o A material is magnetically characterized based on the way it can be magnetized.

o This depends on the material’s magnetic susceptibility – its magnitude and sign.

o Three basic magnetisms:

✓ Dia-magnetism
✓ Para-magnetism
✓ Ferro-magnetism (Anti-ferro-magnetism and ferri-magnetisms are considered
as subclasses of ferro-magnetism)

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 Dia-magnetism
o Very weak magnetization ; exists ONLY in presence of an external field, non-permanent.

o Applied external magnetic field acts on atoms of a material, slightly unbalancing their orbiting
electrons, and creates small magnetic dipoles within atoms which oppose the applied field. This action
produces a negative magnetic effect known as diamagnetism.

o The induced magnetic moment is small, and the magnetization (M) direction is opposite to the
direction of applied field (H).

Weak B-field inside

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Opposing applied magnetic field
o Thus the relative permeability is less than unity i.e. magnetic susceptibility is
negative, and is in order of -10-5.
o Materials such as Cu, Ag, Si, Ag and Alumina (Al2O3) are diamagnetic at room
temperature.

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 Para-magnetism
o Slightly stronger; when an external field is applied dipoles line-up with the field, resulting in a
positive magnetization. However, the dipoles do not interact.

o Materials which exhibit a small positive magnetic susceptibility in the presence of a magnetic field
are called para-magnetic, and the effect is termed as para-magnetism.
o In the absence of an external field, the orientations of atomic magnetic moments are random
leading to no net magnetization. When an external field is applied dipoles line-up with the field,
resulting in a positive magnetization.

Alignment of applied magnetic field Slightly more B-field inside

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than outside applied B-field
 Para-magnetism

o However, because the dipoles do not interact, extremely large magnetic fields are
required to align all of the dipoles. In addition, the effect is lost as soon as the magnetic
field is removed.

o Since thermal agitation randomizes the directions of the magnetic dipoles, an increase
in temperature decreases the paramagnetic effect.

o Para-magnetism is produced in many materials like Aluminium, Calcium, Titanium,

Alloys of Copper.

o Magnetic susceptibility of these materials is slightly positive, and lies in the range +10-5
to +10-2.

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 Ferro-magnetism
o Both dia- and para- magnetic materials are considered as non-magnetic because they exhibit magnetization only in
presence of an external field.

o But, certain materials possess permanent magnetic moments even in the absence of an external field. This is result of
permanent unpaired dipoles formed from unfilled energy levels.

o These dipoles can easily line-up with the imposed magnetic field due to the exchange interaction or mutual
reinforcement of the dipoles. These are chrematistics of ferromagnetism.

Rearrangement of Aligned to B-field direction Binside  Boutside

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o Materials with ferro-magnetism (Examples: Fe, Co, Ni, Gd) possess magnetic
susceptibilities approaching 106.
o Above the Curie temperature, ferro-magnetic materials behave as para-magnetic
materials and their susceptibility is given by the Curie-Weiss law, defined as
𝑪
𝝌 = 𝑻−𝑻
𝑪

where C – material constant, T – temperature, Tc – Curie temperature.

o Ferro Magnets are very strong; dipoles line-up permanently upon application of
external field.
o It has two sub-classes: Anti-ferro-magnetism & Ferri-magnetism:

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 Hysteresis
❑ Hysteresis, in general, is defined as the lag in a variable of a system with
respect to the effect producing it as this effect varies.
i.e.) Response of the state of a system with respect to input variable

❑ In ferromagnetic material the magnetic flux density B lags behind the

changing external magnetizing field intensity H. Hysteresis curve is
drawn by plotting the graph of B vs H by taking material through a
complete cycle of H values as follows.

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 What is Hysteresis loop?
❑When a ferromagnetic material is magnetized in one direction, it
will not relax back to zero magnetization when the imposed
magnetisation field is removed. It must be driven back to zero by a
field in opposite direction . If an alternating magnetic field is applied
to the material, its magnetization will trace out a loop called
Hysteresis loop

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Hysteresis loop

❑ Retentivity is a material’s ability to retain a

certain amount of residual magnetic flux
density, when magnetizing force is removed
after achieving saturation.
❑ Coercivity is the amount of reverse
magnetizing field intensity which must be
applied to a magnetic material to make the
magnetic flux density of material return to
zero after it has reached saturation.

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