Lecture 2 - Ferromagnetic Materials Flashcards
Explain the quantised magnetic dipole moment.
Fields generated by ferromagnetic materials can be estimated by considering the field produced by aligned spins.
A quantised magnetic dipole moment:
1 Bohr magneton μ_B = eh/4πm
9.27x10^-24 Am²
What is the permanent magnet field limit? Explain why
Permanent Magnets are limited to 2T by the number of Bohr magnetons of iron, which determine its saturation magnetic flux density.
Iron specifically because has the largest amount of unpaired spins per atom of the known ferromagnets.
What are the room temperature ferromagnets?
Fe (bcc)
Ni (fcc)
Co(hcp)
What principles determine the magnetic properties of metals?
Crystallographic structure and density of unpaired electrons.
This is because a quantum exchange interaction takes place in ferromagnets to overcome the energy required to rotate individual spins.
This exchange interaction is governed by the Pauli exclusion principle and therefore the above mentioned parameters.
What are ferromagnetic domains?
Ferromagnetic domains:
Regions in BCC iron where the spins are aligned in a preferred direction of magnetisation.
As-smelted BCC iron doesn’t exhibit a net spontaneous magnetisation as domains are balanced to cancel eachother out.
How do we magnetise a ferromagnet? Reference magnetic domains.
Applying a magnetising field Bm leads to a Lorentz Force (BxM) to act on surface currents in domain walls.
What affects the ease with which domain walls can be moved?
Pinning centres:
1) grain boundaries
2) inhomogeneities
3) impurities
Pinning is a source of hysteresis and causes loss that varies linearly with frequency (of pinning).
What is Hc?
What benefits do different values give?
It’s the coercive force, the coercivity of the magnet.
It’s the field required to reduce B to zero and it is determined by hysteresis.
Alternatively it’s the field where M flips direction.
Low Hc is desired for permeable materials
High Hc is desired for permanent magnets
What is Ms and what is Bs?
Ms is the saturation quantity of magnetisation. The material has created one large magnetic domain.
Bs is the saturation flux density since Bs=μοΜs
What is (BH)max?
To generate useful energy, a magnet must move into the second quadrant of the B-H curve. At (BH)max the BH product passes through a negative peak. This is the optimum working point for a permanent magnet.
For a linear material:
B=μoΜsat/2 H=Μsat/2
=> BH = μo(Msat/2)²
What are soft magnetic materials?
What are the properties that classify magnetic materials as soft?
Soft magnetic materials have:
1) Large effective susceptibility (and therefore large relative permeability)
2) Respond significantly to an external field
3) Respond rapidly with minimal energy loss to rapidly changing fields => enhance fields
Where do we use soft magnetic materials?
In inductors and transformers as they allow large power densities in components.
Other uses:
In motors => route generated magnetic fields
In tape and hard disks => store information
What are hard magnetic materials?
1) They have large coercivity Hc
2) They have large (BH)max
3) Large fields for a given volume (from 1 and 2)
4) Can be permanent sources of magnetic fields (PM)
5) They have inhibited dislocation movement and are thus also mechanically hard
