Magnetic- Permanent Magnets Flashcards
What are permanent magnets used for?
Electric motors. Need to be as hard as possible
Purpose of permanent magnet
Create a magnetic field in the absence of external energy input
Problem of demagnetising field
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
What is needed for a good permanent magnet
Delivers high magnetic fields (high Ms and high Mr/Ms)
While resisting its own demagnetising field (high Hc)
Figure of merit for permanent magnets
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
Formula for B
B=μ0(M+H’)
Where H’=Hexternal+Hd
What energy germ needs to be high to maximise figure of merit?
Magnetocrystalline anisotropy
Types of permanent magnetic materials
Neodymium based, Co-Sm alloys, hard (hexagonal) ferries, Fe-Co alloys, Alnicos
What is good about Sm-Co and Nd-Fe-B magnets?
Very high BHmax and magnetocrystalline anisotropy completely overcomes demagnetising effects so don’t need any shape anisotropy
Why do rare earth metals exhibit strong anisotropy?
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
Curie temperature for permanent magnets
Is the effective temperature limit for usage of a permanent magnet. Above Tc is paramagnet.
How does 2nd quadrant change with increasing temperature?
Decreases in height and width
Compare Nd-Fe-B to Sm-Co
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
Why is improving permanent magnets of interest?
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
Process for making Nd-Fe-B magnets
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