5.5 Domains and Hysteresis

Question 1

How does Weiss describe a macroscopic ferromagnet?

Answer 1

As a ferromagnet containing a number of small regions, called domains

Question 2

What is the magnetism of a domain in a ferromagnet?

Answer 2

Not equal to 0

Question 3

Describe how the macroscopic Magnetism can be equal to 0 if the magnetism in the microscopic domains is not equal to 0

Answer 3

If the domains all point in different directions

- Also if B = 0

Question 4

Whatr are the 4 things that the geometry of the domian walls depends on ?

Answer 4

• The model used
• The geometry of the sample (long thin rod, M along axis etc.)
• Relative size of exchange interaction and long range dipolar forces
• Disorder

Question 5

Why do domains form in a ferromagnet?

Answer 5

Because there is an energy saving due to long range magnetic dipolar forces

Question 6

How does the relative magnitude of the dipolar force comapre to the exchange interaction?

Answer 6

Very small

Question 7

If the dipolar force is very small when compared to the exchange interaction, when does it have an effect?

Answer 7

When there are a large number of spins

Question 8

Describe how the sizes of the domain grow/shrink when a B field is applied

Answer 8

The domains aligned with the B field grow in size, and vv

Question 9

How can the domain wall be described in the Ising model and why?

Answer 9

Sharp because the spins are all pointing up or down

Question 10

How do the spins change in the Neél and Bloch domain walls?

Answer 10

Very gradually away from the preferred direction of each neighbour which causes a small increase in energy each time

Question 11

Why can’t the spins of real materials be sharp like the Ising model?

Answer 11

It is energetically unfavourable to have a sudden change

Question 12

Why does magnetisation occur in hysteresis for a small applied field?

Answer 12

Due to small reversible changes in domain walls as an external magnetic field is applied

Question 13

Why does magnetisation occur in hysteresis for a large applied field?

Answer 13

Due to changes in domain walls, however some are irreversible for large magnetic fields as the domains vanish

Question 14

When do you get a hysteresis loop?

Answer 14

When the magnetic field is removed and the magnetisation is non zero due to the loss of domains

Question 15

Describe the hysteresis loop for soft magnetic materials and the properties

Answer 15

The loop has a small area between the two sides.

• Material is easy to magnetise and used in transformer coils
• Kappa small

Question 16

Describe the hysteresis loop for hard magnetic materials and the properties

Answer 16

The loop has a large area between the two sides.

• Material is hard to magnetise but keeps the magnetisation well when the field is turned off
• Useful for permanent magnets e.g. magnetic recording
• Kappa large

Question 17

Describe what Kappa is in the Ising model

Answer 17

An anisotropy term which gives a penalty proportional to Kappa S^2 per spin when its not pointing up or down