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.

For those that possess a significant magnetic moment, the application of an external magnetic field will tend to align

the atomic magnetic moments

B

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

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

Magnetization

MBB

00

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

00

1BH

The influence of an external field is often expressed in terms of the magnetic field shown above

Magnetic Field Strength

Magnetization

)(0 MHB

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

HM

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.

)1(0

m

mHB

Magnetic Susceptibility and Permeability

The expression for the total magnetic field can be rewritten as

where

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.

Paramagnetic materials possess significant atomic magnetic moments, often due to

unpaired valence electrons

Paramagnetism

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

H

B

H

Paramagnetic Ferromagnetic

Comparing the response of the total magnetic field to the applied Field Strength

The relation for the Ferromagnetic curve is non-linear

B

H

B

H

Paramagnetic 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

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

B

H

B

H

B

H

The area underneath the curves indicate the work done by H in changing the magnetic field of the

substance in question

B

H

B Saturation Point

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

Temperature

Phase Diagram of Typical Ferromagnetic Material

Ferromagnetic

Paramagnetic

Magnetization

Temperature

Ferromagnetic

Paramagnetic

Curie 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

NS

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