Magnetism is an interesting topic. There are many classifications of magnetism-ferromagnetism, ferrimagnetism, antiferromagnetism, diamagnetism, paramagnetism etc. It gets kind of confusing. The first kind of magnetism to be discovered and the one people are most familiar with is that found in permanent magnets-ferromagnetism since it is the strongest type of magnetism.The usage of lodestones as a compass was probably the first application of magnetism.
Ferromagnetism, ferrimagnetism and antiferromagnetism are the permanent magnetic ordering in the material leading to spontaneous magnetisation in the material. Ferromagnetism is when all the magnetic moments are aligned in the same direction (If some of them are not aligned in the same direction but the net is not zero then its called ferrimagnetism) whereas in antiferromagnetism there is equal amount of magnetic moment aligned in both directions. Diamagnetism and paramagnetism are only temporary magnetic moments induced in the presence of external fields- due to interaction of the field with localized electrons in shells/ions (Larmor,Van Vleck and Curie)and conduction electrons (Landau and Pauli) .
We learnt in class that diamagnetism or floating frog magnetism is a universal magnetism exhibited by all materials although would be overshadowed if the material has other kinds of magnetism. It has negative magnetic susceptibility-repels magnetic fields. It is general cause it due to the motion/response of the electrons to the external magnetic field. Ions with a filled shell (J=L=S=0) only exhibit (Larmor) diamagnetism. For comparison, there is also Landau diamagnetism for free electron gas (metals). Superconductors exhibit fundamentally different, perfect diamagnetism-'Meissner Effect' and completely repels external magnetic fields.
In contrast to diamagnetism, paramagnetism has a positive magnetic susceptibility so the material is attracted towards the magnetic field. If the shell is one electron short from filled (J=0) it exhibits Van Vleck paramagnetism. For other unfilled shells (J not 0), it exhibits paramagnetism (no fancy name probably should call it Curie paramagnetism) which unlike diamagnetism is temperature dependent-Curie Law. In contrast to paramagnetic effects due to ions, there is Pauli paramagnetism for the the nonlocalized conduction electons on the Fermi surface for metals which is weaker and independent of temperature.
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So, have permanent magnetism: ferromagnetism, ferrimagnetism and antiferromagnetism
ReplyDeleteTemporary magnetism: diamagnetism and paramagnetism
We're currently working with the temporary magnetism (Paul's lectures).
I'm still finding the distinction between diamagnetism and paramagnetism unclear.
Is it just the susceptibility (as you mention above?)
Sidenote: Andre Geim won the Nobel Prize for Graphene in 2010.
He won the Ignobel prize in 2000 for levitating a frog in a magnetic field.
He's also the only person to have bothe the Ignobel and Nobel prize...
Wow, I had never herad of the Ignobel prize before, but now I have countless hours of trivial pursuit knowledge to learn! :D
ReplyDeleteIn answer to your question Ann, I found this little blurb about the two http://www.physlink.com/education/askexperts/ae595.cfm
From what I can gather, fundamentally it is just the susceptibilities that are different between the two materials, brought about due to paramagnets shells containing non-zero total angular momentum. As a result these materials contain a non-zero electron magnetic dipole moment.
Whilst an applied magnetic field would cause the dipole to tend to align with the field, causing a stronger magnetic force, when this is removed thermal excitations result in the dipoles assuming random distributions again. This has the effect of cancelling out the previously generated field.
Now a ferromagnet obviously still contains a magnetic moment, but not this electron magnetic dipole moment. As a result thermal excitations influence it less (I would have to read up more about why here, but I'd imagine it would have something to do with zero angular momentum) and it takes much longer for any magnetic effects to wear off (Although they do after a period of time).
Thanks Dale.
ReplyDeleteThat last assignment also made it much clearer.