Magnetics- Applications

Question 1

Typical values of Ku and K1 for hard and soft ferromagnetic materials

Answer 1

Hard has Ku about 10^6

Soft has Ku and K1 close to 0

Question 2

What are permanent (hard) magnets for?

Answer 2

Delivering high magnetic fields which revisiting externally applied fields and operating at a required temperature

Question 3

Describe B-H loop

Answer 3

Like M-H hysteresis loop but reaches constant gradient rather than horizontal because H still increases after M saturates.
B=μ0(M+H)
Use 2nd (top left) quadrant to characterise permanent magnetic materials

Question 4

Where is ideal working point of permanent magnet?

Answer 4

The point in the 2nd quadrant of B-H loop where the value of BH product is maximum (the energy product). It is used to characterise a magnet’s strength. Balances requirements of delivering high field while resisting external applied magnetic fields.

Question 5

Why is high uniaxial magnetocrystalline anisotropy needed for permanent magnets?

Answer 5

The high Ku value helps to counteract the effects of shale that create a demagnetising field.
Also need to overcome magnetostatic energy to give flexibility of design (maybe not related to flashcard title)

Question 6

Permanent magnet applications

Answer 6

Motors, turbines, medical, automotive, aerospace, fridge magnets, household appliances

Question 7

How do permanent magnets affect motors and turbines?

Answer 7

Key components are coil of wire and permanent magnet. Torque is produced of a motor. Increasing strength of magnet improves efficiency. Same idea for turbines where you want to maximise generated current for any given torque applied.

Question 8

Give one main application of soft magnets

Answer 8

Transformers

Question 9

How do transformers work in terms of magnets?

Answer 9

Transfer energy from one electrical circuit to another by magnetic coupling without moving parts. Is an application of electromagnetic induction. Faraday’s law is V=-Ndφ/dt. Two coils wound onto same core. Current in primary coil means mutual induction which causes voltage to be induced into the secondary coil.

Question 10

Ideal magnet for transformers

Answer 10

Higher μr provided more flux so need soft magnetic material so a small change in field creates big change in B in transformer core. Also want a low coercivity so area enclosed by B-H loop small so low losses.

Question 11

Characteristics of soft magnets including MAZE

Answer 11

Low effective anisotropy (low coercivity). High magnetic saturation (often). High χ (so high μ). Magnetorestriction (interaction between anisotropy and stress (shape change)).
Magnetocrystalline anisotropy very low (small grains or amorphous) so magnetostatic energy often dominates. High sensitivity to applied fields (Zeeman). Are ferromagnetic (exchange) except for ferrites (ferrimagnetic).

Question 12

How can soft magnets be used as electromagnets?

Answer 12

Obtain high magnetic flux for low currents in electrical coil wrapped around a soft magnet. Can achieve up to 2.4T

Question 13

Magnets in magnetic hard drives

Answer 13

Platter with recording media magnetic needs high K to be stable, low K to be rewritable and high remanence to be readable.

Question 14

Describe read heads

Answer 14

Or spin valves. Have one magnetic layer with fixed magnetisation (pinned layer) on pinning layer. On that is non-magnetic spacer. On that is free layer which responds to applied fields. The electrical resistance depends to alignment of free and fixed layer magnetisation. Used to determine direction of magnetic data buts in hard drives.

Question 15

Other applications of magnets

Answer 15

Security tags, RAM, separation of biomolecules, MRI enhancement, curing cancer, compass.