. M AGNETIC P ROPERTIES OF M ATERIALS The discussion in the previous section helps us to classify materials as diamagnetic, paramagnetic or ferromagnetic. In terms of the susceptibility χ , a material is diamagnetic if χ is negative, para- if χ is positive and small, and ferro- if χ is large and positive. A glance at Table .
gives one a better feeling for these materials. Here ε is a small positive number introduced to quantify paramagnetic materials. Next, we describe these materials in some detail. T ABLE .
M AGNETIC SUSCEPTIBILITY OF SOME ELEMENTS AT K Diamagnetic substance χχ Paramagnetic substance χχ Bismuth – . × – Aluminium . × – Copper – . × – Calcium .
Diamagnetism Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field. In other words, unlike the way a magnet attracts metals like iron, it would repel a diamagnetic substance. Figure . (a) shows a bar of diamagnetic material placed in an external magnetic field.
The field lines are repelled or expelled and the field inside the material is reduced. In most cases, as is evident from Table . , this reduction is slight, being one part in . When placed in a non-uniform magnetic field, the bar will tend to move from high to low field.
The simplest explanation for diamagnetism is as follows. Electrons in an atom orbiting around nucleus possess orbital angular momentum. These orbiting electrons are equivalent to current-carrying loop and thus possess orbital magnetic moment. Diamagnetic substances are the ones in which resultant magnetic moment in an