Transcript Para, Dia and Ferromagnetism

Para, Dia and
Ferromagnetism
Magnetization
The presence (or absence) of a significant
magnetic moment for atoms will govern
how they respond to magnetic fields.
Magnetization
The presence (or absence) of a significant
magnetic moment for atoms will govern
how they respond to magnetic fields.

  B

For those that possess a significant
magnetic moment, the application of an
external magnetic field will tend to align
the atomic magnetic moments

Magnetization
Magnetization is a vector related to the
magnetic moment of a material
In fact, the magnitude of the vector is the
magnetic moment per unit volume
Magnetization
The response of a material to the application of a
magnetic field will depend on the level of
magnetization that the material undergoes
B0
Magnetization
B0
The magnetic moments are of course themselves,
B-Field sources and will generate magnetic Fields
of their own that may either strengthen the overall
field or weaken it
Magnetization
 

B  B0  0 M
The total magnetic field equals the external magnetic
field, B0, and the magnetic field generated by the
alignment (or in some cases, creation) of atomic
magnetic moments
Magnetization

1 
H
B0
0
Magnetic Field Strength
The influence of an external field is often expressed in
terms of the magnetic field shown above
Magnetization

 
B  0 ( H  M )
The total magnetic field can then be
expressed as the sum of the field strength
and magnetization vector with the free
space permeability, 0.
Magnetic Susceptibility and Permeability


M  H
Magnetic Susceptibility
The magnetic susceptibility relates the degree (and
sense) of magnetization given an applied field
strength. It is characteristic of the substance and it
is very much temperature dependent.
Magnetic Susceptibility and Permeability
The expression for the total magnetic field can be rewritten as


B   mH
where
 m  0 (1   )
The magnetic permeability
If the magnetic permeability, m exceeds
the free space permeability, 0, ( a positive
magnetic susceptibility) the material in
question will either be paramagnetic or
ferromagnetic
If m is less than 0, (negative magnetic
susceptibility) the material is said to
diamagnetic.
Paramagnetism
Paramagnetic materials possess significant
atomic magnetic moments, often due to
unpaired valence electrons
Diamagnetism
Diamagnetic materials possess little or no atomic
magnetic moments. What magnetization they
undergo is induced by the application of an external
magnetic field.
Ferromagnetism
Like paramagnetic materials, ferromagnetic substances
possess significant magnetic moments. Unlike
paramagnetic materials, they have a significant
attraction to other magnetic materials
Ferromagnetic materials have other interesting features
Ferromagnetism
Unlike the atomic magnetic moments
associated with paramagnetic substances,
the moments in ferromagnetic materials have
strong interactions with their nearest
neighbor moments.
Ferromagnetism
Unlike the atomic magnetic moments
associated with paramagnetic substances,
the moments in ferromagnetic materials have
strong interactions with their nearest
neighbor moments.
This leads to a strong correlation between
large groups of magnetic moments in
ferromagnetic materials
Ferromagnetism
Unlike the atomic magnetic moments
associated with paramagnetic substances,
the moments in ferromagnetic materials have
strong interactions with their nearest
neighbor moments.
This leads to a strong correlation between
large groups of magnetic moments in
ferromagnetic materials
These large groups are known as “domains”
B
B
H
Paramagnetic
H
Ferromagnetic
Comparing the response of the total magnetic field to the
applied Field Strength
The relation for the Ferromagnetic curve is non-linear
B
B
H
Paramagnetic
H
Ferromagnetic
Comparing the response of the total magnetic field to the
applied Field Strength
The relation for the Ferromagnetic curve is non-linear
In addition, if you decrease H for the ferromagnetic sample,
the B field will not decrease in the same way, it increased
B
H
Ferromagnetic
Because of the domains, ferromagnetic substances will
retain a permanent B-field after magnetization.
B
H
Ferromagnetic
Because of the domains, ferromagnetic substances will
retain a permanent B-field after magnetization.
This property, where the response to magnetization depends
on the previous magnetizations is called hysterisis
BB
H H
The area underneath the curves indicate the work
done by H in changing the magnetic field of the
substance in question
Saturation Point
BB
H
Saturation Point
The figure above shows a hysterisis curve between the two
saturation points of a particular ferromagnetic material
The saturation point corresponds to the maximum
magnetization that a material can achieve
To reverse the process of magnetizing a ferromagnetic
material, one would have to follow this hysterisis curve
Magnetization
Temperature
A little Thermodynamics
Magnetization
Phase Diagram of Typical Ferromagnetic Material
Paramagnetic
Ferromagnetic
Temperature
Magnetization
Paramagnetic
Curie Temperature
Ferromagnetic
Temperature
Diamagnetism
Unlike Ferromagnetism and Paramagnetism, the atomic
magnetic moments associated with Diamagnetic behavior
are induced by the application of a magnetic field
B0
Diamagnetism
Unlike Ferromagnetism and Paramagnetism, the atomic
magnetic moments associated with Diamagnetic behavior
are induced by the application of a magnetic field
B0
S
N
In addition, the orientation of the induced magnetic
moment will be such that the moments will be
repelled by the applied magnetic field
Diamagnetism
Many materials that at sufficiently
low temperatures become
superconductors, become perfect
diamagnets
Meissner Effect