Magnetic- Permanent Magnets

Question 1

What are permanent magnets used for?

Answer 1

Electric motors. Need to be as hard as possible

Question 2

Purpose of permanent magnet

Answer 2

Create a magnetic field in the absence of external energy input

Question 3

Problem of demagnetising field

Answer 3

Magnetising configurations that create a strong field outside the magnet also create a strong demagnetising field within the magnet. If Hd exceeds the Hc of the material then the magnet will naturally collapse back into a demagnetised state in the absence of any external field

Question 4

What is needed for a good permanent magnet

Answer 4

Delivers high magnetic fields (high Ms and high Mr/Ms)

While resisting its own demagnetising field (high Hc)

Question 5

Figure of merit for permanent magnets

Answer 5

Best characterised by 2nd (top left) quadrant of a B-H loop. Figure of merit is |BH|max (the energy product). Look for the highest possible value of this in the 2nd quadrant. Measures energy stored in the magnetic field

Question 6

Formula for B

Answer 6

B=μ0(M+H’)

Where H’=Hexternal+Hd

Question 7

What energy germ needs to be high to maximise figure of merit?

Answer 7

Magnetocrystalline anisotropy

Question 8

Types of permanent magnetic materials

Answer 8

Neodymium based, Co-Sm alloys, hard (hexagonal) ferries, Fe-Co alloys, Alnicos

Question 9

What is good about Sm-Co and Nd-Fe-B magnets?

Answer 9

Very high BHmax and magnetocrystalline anisotropy completely overcomes demagnetising effects so don’t need any shape anisotropy

Question 10

Why do rare earth metals exhibit strong anisotropy?

Answer 10

They show large spin orbit coupling. This is interaction between direction of spin moment of electrons and that of their orbital motion around the nucleus. In a solid this couples the magnetisation to the lattice such that energy depends on orientation of M relative to lattice. Strength of spin orbit coupling proportional to Z^4

Question 11

Curie temperature for permanent magnets

Answer 11

Is the effective temperature limit for usage of a permanent magnet. Above Tc is paramagnet.

Question 12

How does 2nd quadrant change with increasing temperature?

Answer 12

Decreases in height and width

Question 13

Compare Nd-Fe-B to Sm-Co

Answer 13

Nd has by far highest BHmax but lower Tc which limits usefulness in some high speed machines and automotive applications. Sm would be used here. Sm-Co more expensive due to low abundance of Co compared to Fe

Question 14

Why is improving permanent magnets of interest?

Answer 14

Lower cost of energy production, efficient use of energy resources, need to cut l’image changing emissions. They can address this by improving efficiency of clean energy generation, energy recovery systems, electric motors for transportation

Question 15

Process for making Nd-Fe-B magnets

Answer 15

Create single crystal powders. Align and press into mould and apply field to orientate the grains. Singer in vacuum (1100°C). Machine then magnetise. Magnetise last as they are dangerous

Question 16

Structure of Nd-Fe-B magnets

Answer 16

Magnetic phase is Nd2Fe14B and has tetragonal structure with strong magnetocrystalline anisotropy along its c-axis. Pre-sinter field aligns c-axis of grains so final magnet has strong anisotropy. The a and b-axis not aligned. Other Nd rich phases form layers at grain boundaries and fill triple points between grains and are non-magnetic. They decouple the magnetic grains and pin domain walls increasing Hc

Question 17

Effect of Dy to Nd-Fe-B

Answer 17

Increase anisotropy (and Hc).
Increase Tc
Reduces Ms due to Dy coupling antiferromagnetically with Fe. Is very expensive

Question 18

Future improvements

Answer 18

Nd-Fe-B manufacture to improve application performance.
SmCo performance for high T applications.
Removal of Dy from Nd to reduce cost.
Discovery of new RE-free permanent magnets to further reduce cost