Magnetic order comes about from the electrons in a compound. Local moment
magnetism is associated with the orbital motion of the electrons as they live
near particular ions. Itinerant magnetism is associated with the intrinsic spin
of the electrons as they wander around the metal in the conduction band. There
are many types of magnetic order, and this list is not meant to be
comprehensive. It should just give you some idea of how I think of the
different types of order. A simple picture is to think of eight sharpened
pencils arrayed on the corners of a cube. The lead points represent the
direction of the magnetic moment.
Ferromagnetism (FM): This order is characterized by all of the
moments pointing in the same direction. Depending on the nature of the
interaction and any mediating particles between the magnetic ions, the moments
can point perpendicular to one of the faces of the cube or along some angle.
The key thing about FM order is that all the moments point in one direction.
Antiferromagnetism (AFM): This order is characterized by
alternating directions of the magnetic moments. If you pick one of the moments
to be pointing up all of its nearest neighbors (those connected along the edges
of the cube, in our crude picture) point down. The next nearest neighbors
(those along the face diagonals) point up. Along a line the moments flip up and
down. AFM order results in no bulk magnetic moment, so these materials have
little practical value. However, it might be possible to envision some use in
the so-called "spintronics" industry.
Ferrimagnetism: Ferrimagnetism is AFM order at its heart. The
key difference is that one direction of the moments (which we arbitrarily pick
as "up") is stronger than the other. Therefore, ferrimagnetic materials do have
a bulk moment and do find commercial applications. Generally ferrimagnetic
materials are electrically insulating (and therefore local moment systems).
Spin Helix: Spin helices are interesting objects. The moments
spiral about an axis giving no bulk magnetic moment. One way to think of these
magnets is to consider a stack of pencils, one pencil wide. It looks like the
pencils just came out of the box. Now, twist them as if there were a rigid rod
connecting them down the center so the stack looks like a spiral staircase.
This is a spin helix, as I understand.
My Interpretations of Some Things
Local Moment vs. Itinerant Systems
Types of Magnetic Order
How a TDR Works
The Normal Skin Effect
Matt's Home
If you have any questions, feel free to email me at vannette@iastate.edu
Updated 27 April 2007 from home.