(published on 05/16/2013)
We've addressed parts of this question before: http://van.physics.illinois.edu/qa/listing.php?id=435, http://van.physics.illinois.edu/qa/listing.php?id=8686. Lots of other materials are magnetic, and many iron compounds aren't. Some simple elemental metals- e.g. cobalt, nickel, and neodynium-do have magnetic domains at room temperature, but most don't.
Magnetic domains are generally not the same as molecules. In fact, most of the crystals we're talking about here, for example pure iron, have no molecules in them. They are just a bunch of atoms stacked up in a regular pattern.
The polar molecules you're thinking of have more positive electrical charge on one side and more negative electrical charge on the other. That makes them electrical dipoles. Unless the electrical dipole moments on many neighboring atoms or molecules line up with each other, there are no domains. If they do line up, they form ferroelectric domains, not ferromagnetic domains.
The magnetic domains we're talking about typically have no electrical dipole moment. They have most of their electrons spinning the same way. That gives them a magnetic dipole moment.
(published on 09/14/2013)