6.3 Classification of Magnetism

Magnetic materials can be classified based on the values of magnetic susceptibility. 

Materials with negative susceptibility are diamagnetic (χ_m < 1) . Most diamagnetic materials show very small negative susceptibility except superconductors in superconducting effect when χ_m is equal to 1 which is very useful for applications such as magnetic levitation.

Materials with positive susceptibility are either paramagnetic, ferromagnetic or ferrismagnetic (χ_m > 1). Susceptibility is positive but very small for paramagnetic materials but can be very large for ferro- and ferri-magnetic materials.

Susceptibility values of some of the common materials are provided below.
(Reference:Electronic Properties of Materials, R.E. Hummel, Springer).

Material	χ(SI) unitless	(cgs) Unitless	Unitless	Type of Magnetism
Bi			0.99983	Diamagnetic
Be			0.99998
Ag			0.99997
Au			0.99996
Ge			0.99999
Cu			0.99999
Si			0.99999
Water			0.99999
Superconductors (only in superconducting state)			0
β-Sn			1	Paramagnetic
W			1.00008
Al			1.00002
Pt			1.000026
Low carbon steel				Ferromagnetic
Fe-3%Si (Grain Oriented)
Ni-Fe-Mo superalloy

It should be noted, also as we will see, that except for diamagnetic materials, magnetic susceptibilities are temperature dependent. Sign of susceptibility can also be related (qualitatively) to the penetration of magnetic flux inside the material.

This says that for diamagnetic materials, when an external field is applied, the magnetic moment that is induced is in opposite direction to the field direction i.e. opposite magnetization as shown below. This is an inherent effect present in all materials. It is just that some materials like silver, gold, silicon are only diamagnetic i.e. they don’t have any other effect present in them.

In many other materials, on top of the diamagnetic effect which is inherent to all materials, other effects are present which contribute significantly to the magnetization and all of these tend to have induced magnetization that is in the direction of the applied i.e. positive susceptibility. This means that magnetic flux penetrates into the material as shown below. We will discuss these effects one by one.

In a nutshell, the net susceptibility of a material is the sum of all types of magnetic contributions. For materials having only diamagnetic contribution, this turns out to be negative.

Moreover, it is quantum mechanics which shows that materials are diamagnetic or paramagnetic or ferromagnetic. If taken too far, classical mechanics simply shows that all the magnetic moments in any material cancel out. We won’t go into details of this exercise here. Those who are interested can go through Feynman Lectures on Physics which provide a wonderful explanation of this conundrum (The Feynman lectures on physics, Vol. II, Chapter 34-37, R. P. Feynman, R. B. Leighton, M. L. Sands, Pearson/Addison-Wesley).