Lectures 17&18 (Permanent Magnets and Surface Anisotropy)

Question 1

Figure of merit for a permanent magnet?

Answer 1

Energy product

Question 2

What does a hysteresis loop with a shearing correction loop like?

Answer 2

Very square

Question 3

Equation for energy density outside the magnet (i.e., stray field energy)?

Answer 3

epsilon_outside = -1/2B.Hd

Question 4

How to find the limits on the stray field for permanent magnets?

Answer 4

Subs demag field into B = µ0(M+H)

∂Episilon/ ∂D

2nd derivative less than 0

Question 5

Optimal D for permanent magnet?

Answer 5

D = 1/2

Question 6

How do Hc and Hd compare in a permanent magnet? Why?

Answer 6

Hc > Hd

Otherwise M would be easily reversed under Hd

Question 7

How can Hc of a permanent magnet be increased?

Answer 7

1. Shape anisotropy - but long thin have small D
2. Magnetocrystalline anisotropy - make M along shorter axis in an oblate spheroid for D = 1/2

Question 8

Issue with using long, thin magnets in permanent magnets?

Answer 8

They have small D which gives a small energy product

Question 9

Ideal shape of crystal for permanent magnet?

Answer 9

Oblate spheroid

Question 10

Pros and cons between MBE and sputtering?

Answer 10

Sputtering - faster and cheaper

MBE - more control over crystallography

Question 11

Which direction does surface anisotropy make the easy axis in a thin film point?

Answer 11

Perpendicular to the plane