DIAMAGNETIC MATERIALS--- When brought into an external field, a weak internal B field is induced inside the material that opposes the external field.


PARAMAGNETIC MATERIALS--- Brought into an external magnetic field, the atoms of the material weakly align with the field. Liquid oxygen provides a classic demonstration, adhering to the pole faces of a large magnet. This is like a very, very weak form of ferromagnetism.


FERROMAGNETIC MATERIALS--- contain "unpaired" electrons whose magnetic moments align with those of neighboring atoms. When a ferromagnetic material is brought into an external field, it develops a strong internal field due to electron moments aligning with the field. When the field is removed, most of the alignment still remains, and the material is "permanently" magnetized.


Magnetic susceptibility is defined by χ = Bmat/Bvac. The magnetic permeability is then μ = μ0[1 + χ]. For ferromagnets, χ is of the order of 1000 or more. For paramagnets (most materials), χ is very small, typically 10-4 or less. For diamagnets, χ is small and also negative.



THE HALL EFFECT

Scanning Hall Effect Microscopes.
The Quantum Hall Effect depends upon creating quantized “edge states” that depend on electron spin. There has been very intensive study of such phenomena in the past decade, in hopes of large numbers of technological applications to follow. An entire field of research involves electrical phenomena depending on electron intrinsic spin rather than electron charge... this field is usually called “spintronics.”

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