1.4b — Magnetic Properties

Magnetic susceptibility, χ

The material property describing magnetic response to an applied magnetic field.

Magnetization, M

The material's measurable response to an applied magnetic field.

Applied magnetic field

H.

Basic magnetic-response equation

M = χH.

What does H represent?

The externally applied magnetic field.

What does M represent?

The material's magnetization response.

What does χ represent?

Magnetic susceptibility, a material property.

H-field vs. B-field

H is only the applied field; B is the total field including the applied field and the material's response.

Why does this class use H when discussing material response?

It separates the applied environment from the material's response.

M-H curve

A graph of magnetization M versus applied magnetic field H.

Another name for an M-H loop

Hysteresis curve.

How is an M-H curve measured?

H is varied from positive to negative while the material's magnetization M is measured.

Saturation magnetization

The maximum value of magnetization; M levels off even if H keeps increasing.

Where is saturation magnetization on an M-H curve?

At the upper or lower plateau of the hysteresis curve.

Remanent magnetization

The magnetization remaining when the applied field H is zero.

Where is remanent magnetization on an M-H curve?

The M-value at H = 0; the y-intercept.

Coercive field, Hc

The applied magnetic field at which magnetization switches direction.

Where is coercive field on an M-H curve?

The H-value where M = 0; the x-intercept.

Hard magnet

A magnet with a high coercive field that strongly resists demagnetization.

Soft magnet

A magnet with a low coercive field that is easily demagnetized.

Hard magnet hysteresis loop

A wide loop with a large coercive field.

Soft magnet hysteresis loop

A narrow loop with a small coercive field.

Which type of magnet stays magnetized after H is removed?

A hard magnet.

Which type of magnet is easily demagnetized?

A soft magnet.

What makes a permanent magnet "hard"?

Its high coercive field makes its magnetization difficult to reverse.

What does coercive field influence?

How difficult and energy-intensive it is to switch magnetic direction.

Which hysteresis curve takes more energy to switch magnetic directions?

The wider curve with the larger coercive field.

Energy loss in a hysteresis cycle

A larger hysteresis-loop area corresponds to greater energy loss per cycle.

For a transformer, hard or soft magnetic material?

Soft, because its narrow hysteresis loop reduces energy loss during repeated switching.

Main types of magnetism in the slides

Paramagnetic, diamagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic.

Paramagnetic material response

Weak magnetic response.

Paramagnets with no external field

Magnetic moments are largely disordered/random.

Paramagnets in an external field

Moments tend to align with the field, producing weak attraction.

Example of a paramagnet from the slides

Oxygen.

Diamagnetic material response

Very weak magnetic response opposite the applied magnetic field.

How does a diamagnet interact with a strong magnet?

It experiences very weak repulsion.

Examples of diamagnets from the slides

Nitrogen, wood, glass, water, gold, and superconductors.

Ferromagnetic material response

Strong magnetic response.

Simplified ferromagnetic arrangement

Magnetic moments strongly align.

Examples of ferromagnets from the slides

Iron, cobalt, nickel, and their alloys.

Antiferromagnetic material response

Weak response because neighboring magnetic moments oppose one another.

Example of an antiferromagnet from the slides

Hematite, Fe₂O₃.

Ferrimagnetic material response

Strong response from opposing but unequal magnetic moments.

Example of a ferrimagnet from the slides

Magnetite, Fe₃O₄.

Which magnetic types have a strong response in the class slide?

Ferromagnets and ferrimagnets.

Which magnetic types have a weak or very weak response?

Paramagnets, diamagnets, and antiferromagnets.

What roughly helps predict magnetic behavior?

Electron arrangement; unpaired electrons are an important clue.

Why is oxygen paramagnetic?

Its molecular orbital diagram contains unpaired electrons.

How does oxygen behave in a strong magnetic field?

It is attracted because it is paramagnetic.

Why is nitrogen diamagnetic?

Its molecular orbital diagram has no unpaired electrons.

How does nitrogen behave in a strong magnetic field?

It mostly does not interact and experiences very weak repulsion.

All superconductors are…

Diamagnets.

Are all diamagnets superconductors?

No.

Meissner effect

Magnetic fields are repelled/excluded from a superconductor below its critical temperature.

Below the superconducting critical temperature, what happens to resistivity?

It drops sharply.

Below the superconducting critical temperature, what happens to magnetic fields?

They are repelled from the material.

Superconductor applications from the slides

MRI, maglev transportation, and quantum/low-energy computing.

Critical temperature for a superconductor

The temperature below which superconducting behavior occurs.

Curie temperature

The temperature above which a permanent magnet loses its magnetization.

What happens when a permanent magnet is heated above its Curie temperature?

It loses its magnetization.

What happens when it cools back below the Curie temperature?

It remains demagnetized unless it is remagnetized by an external field.

Curie temperature is the ______ for magnetic materials

Service temperature.

Can cooling alone restore magnetization lost above Curie temperature?

No. It must be remagnetized using an external field.

For a permanent magnet to operate at a certain temperature…

Its Curie temperature must be above the operating temperature.

Why is a refrigerator door classified as soft magnetic?

Its response depends on an external magnetic field; permanent magnets stick to it but paper clips do not.

Refrigerator magnet vs. refrigerator door

The refrigerator magnet is hard magnetic; the door behaves as a soft magnetic material.

Soft magnet design goal

Easy magnetization and demagnetization with low switching energy loss.

Hard magnet design goal

Retain magnetization and resist demagnetization.