Georgia State University
Georgia State University: Hyper Physics: Ferromagnetism
This site from the Georgia State University provides a good overview of ferromagnetic materials, introducing concepts such as magnetic domains, hysteresis, the curie temperature, and magnetostriction.
Georgia State University
Georgia State University: Hyper Physics: Magnetic Domains
A great introduction to magnetic domains, including pictures which illustrate the effect on domains caused by external applied magnetic fields.
Wolfram Research
Wolfram Science World: Magnetic Permeability
This site defines magnetic permeability in terms of magnetic fields, and relates magnetic permeability to magnetic susceptibility and relative permeability.
Oswego City School District
Regents Exam Prep Center: Permeability
A good visual introduction to permeability, comparing the magnetic permeabilities of air and iron. This site provides a good conceptual introduction to this topic.