Magnetic- Soft Magnets

Question 1

What are soft ferromagnetic materials used in?

Answer 1

Applications where need material to respond strongly to magnetic fields. The cores of the write-head electromagnets, electromagnets, electrical transformers, magnetic shielding, magnetic sensors

Question 2

What do electromagnets do?

Answer 2

Create magnetic fields using electric currents.

Question 3

Arrangement of electromagnets

Answer 3

Current through solenoid creates magnetic field through its centre. If core is soft magnetic material, this becomes magnetised by field and increases the total strength of the field created.

Question 4

Type of material needed for core of electromagnets

Answer 4

Soft magnetic material so it can be easily magnetised by the field flowing through the current loop

Question 5

Formula for magnetic flux density at radial position away from wire with current through it

Answer 5

B=μ0I/2πr
I is current
r is radial distance from wire

Question 6

Formula for flux density through core of electromagnet

Answer 6

B=μ0nI for air core
B=kμ0nI for soft magnet core
k is some scale factor

Question 7

Arrangement of high field electromagnets

Answer 7

Have continuous yoke (core) around edge with a small gap where a useful field is created

Question 8

Arrangement of transformers

Answer 8

Soft magnetic core. Two sets of coils around it. One of which alternately magnetised the core with an AC current/voltage. The second then has a voltage induced in it due to the changing magnetisation of the core through its centre. If two coils have different numbers of windings, voltage of electrical signal is changed

Question 9

Ratio of voltages for transformers

Answer 9

V1/V2=N1/N2

Question 10

Gauss’ law for magnetism

Answer 10

Field lines have to be continuous
Nabla • B =0
Left means how much of a field comes from or goes into a point in space.
This is 0

Question 11

How does magnetic field screening work?

Answer 11

Want to avoid magnetic fields being present in a region of space. Place high susceptibility material in the way of field, field lines appear to be drawn into it. Can have a wall of soft magnet to block field lines passing it. Can have box of soft magnet to stop field getting in

Question 12

What properties do soft magnets need?

Answer 12

Low remanence, low coercivity, high susceptibility.

Transformer cores need high electrical resistance

Question 13

Hysteresis loop shape for soft magnets

Answer 13

Thin enclosed gap and steep lines until saturation

Question 14

Material you want for electromagnets

Answer 14

High magnetisation with soft magnetic properties. Normally use soft iron with low C content (high Ms of 1700kA/m, low coercivity and remanence)

Question 15

Sources of losses from transformers

Answer 15

Hysteresis loss: area enclosed in a B-H loop.
Eddy current loss: field from primary coil indices eddy currents in core in addition to magnetising it, loss due to resistive heating.

Question 16

How to reduce hysteresis losses in transformers

Answer 16

Use softer material for core

Question 17

How to reduce eddy current losses

Answer 17

Need to increase resistivity, alloy other elements into iron. Add 3% Si to Fe gives high Ms, high susceptibility, low Hc, high resistivity. Above 3% resistivity still increases but material becomes more brittle and difficult to use. Can laminate the strips of FeSi to reduce eddy current losses further. Laminae could have epoxy resin in between

Question 18

Problem of increasing frequency in transformer

Answer 18

Hc increases so greater hysteresis losses. dB/dt larger so greater eddy current losses.

Question 19

Solution to high frequency problems with transformers

Answer 19

Need to further increase resistance of magnetic material. Above few tens of kHz, use ferrite materials like MnZn-ferrite. Magnetisation decreases by almost 10x to reduce eddy current losses. Resistance increases by several orders of magnitude to massively decrease eddy current losses.

Question 20

Material wanted for screening

Answer 20

Highest susceptibility possible (particularly initial susceptibility). Need to minimise sources of anisotropy. So low K1 and low magnetostriction constant λs to prevent strain induced anisotropy. Use permalloy (MuMetal) which is Ni80Fe20

Question 21

Why is permalloy good for screening?

Answer 21

It happens that at 80wt% Ni the values of λs and K1 are about 0. So good for shielding Dc magnetic fields

Question 22

Initial effect of decreasing grain size on coercivity

Answer 22

As grain size decreases to just under a μm the coercivity increases because more more domain wall pinning at grain boundaries (more defects in material)

Question 23

Why is there a dramatic decrease of Hc as grain size is decreased further below 100s of nm?

Answer 23

In very small grains that are exchange coupled, the exchange interaction causes moments in neighbouring grains to lie parallel. This can overcome intrinsic magnetocrystalline anisotropy of individual grains. Coupling dominates over length scales shorter than the exchange length for a given exchange stiffness and anisotropy strength. Smaller grain size relative to exchange length means less able to respond to effects of local anisotropy. Anisotropy becomes increasingly averaged out by exchange coupling of grains, effective anisotropy decreases.

Question 24

Formula for exchange length

Answer 24

lex=rt(A/K1)
A is exchange strength
K1 is anisotropy constant

Question 25

Amorphous/nanocrystalline alloys

Answer 25

The averaging out of anisotropy in them means coercivity is very low. Effectively the softest materials you can make. Create in bulk form via rapid solidification processes. Use cooling rates of 10^6K/s