Part 5: Magnetism Flashcards
(28 cards)
What is a magnetic moment?
Some charge that possesses some angular momentum
How is angular momentum quantised?
In units of h bar
What do electrons have?
Spin angular momentum and so there is also a spin magnetic moment
What is diamagnetism?
All solids possess it
If the material is placed in a magnetic field then you will generate very small magnetisations that point in the opposite direction
What is Lenz’s law?
Electrons adjust their orbits so as to oppose an applied field. This gives very small negative susceptibility
What is paramagnetism?
When materials are made up of atoms that possess magnetic moments. (These can align under a field, giving a positive contribution to the susceptibility)
The field aligns the moments against thermal fluctuations
What does paramagnetism do to diamagnetism?
It overwhelms any diamagnetic contribution
What does the Langevin function (Curie paramagnetism) do at low fields?
It is linear in a and gives the response that it is proportional to the field and the inverse of temperature (i.e looks like Curie law)
What does the Brillouin function do?
It gives the saturation magnetisation and then a function that depends on the ratio of field H to temperature
When is the orbital moment quenched by crystal fields?
When there are strong electric fields around an ion in a solid
How do 3d ions behave?
As if L = 0
What is ferromagnetism?
The moments interact with one another and the interaction attempts to align themselves (quantum mechanical exchange interaction) , producing something microscopically magnetic.
The material spontaneously magnetises itself
The interactions are strong enough to overcome the thermal fluctuations that try to randomise them and so does not need eternal magnetic field (different to paramagnets)
Which are the three ferromagnetic elements at room temperature?
the 3d transition metals: Fe, Co and Ni (nearly full d orbitals)
These happen when Curie temperature is higher than room temp
Which phase of matter is ferromagnetism?
A thermodynamic phase of matter with a critical temperature Tc (Curie temperature)
There is a sharp distinction between the values above and below the Curie temperature
What happens below the Curie temperature?
There is spontaneous magnetisation (happens without the application of a field)
What happens above the Curie temperature?
The systems goes back to being a paramagnet
What does the Curie-Weiss law describe?
The paramagnetic properties of a ferromagnet when it is above Curie temperature
What happens to the susceptibility as it approaches Tc?
It diverges
Why do the 4f orbitals not form bands and orbital moments are not quenched?
They 4f orbitals are close to the atomic core
When can Hund’s rules be applied?
Atomic-like orbitals
Why is the moment large for 4f orbitals?
They can hold up to 14 electrons
What does weak coupling through electron gas mean for temperature?
Curie temperatures are low
Why does Hund’s rules no longer apply to 3d magnets?
The 3d atomic orbitals have collapsed into bands
3d magnets have non-integer moments per atom
What are itinerant or band ferromagnets?
Ferromagnets with delocalised moments in the electron gas
