In contrast to bismuth, a rod of a material such as platinum will settle along the same direction as the applied magnetic field. Further, the induced magnetism will be in the same direction as the field. Platinum is an example of a paramagnetic material. The susceptibility, χ, of a paramagnetic substance is very small and positive, + 0.0001 for example, so that its relative permeability , is very slightly greater than 1 from
Atoms contain circulating and spinning electrons. Each electron possesses a resultant magnetic moment on account of its orbital motion and its spin motion. In a diamagnetic atom, all these contributions to the magnetic moment cancel out. In a paramagnetic atom, however, there is a resultant magnetic moment. Generally, the thermal motions of the atoms will cause these magnetic moments to be oriented purely at random and there will be no resultant magnetization. If, however, a field is applied, each atomic moment will try to set in the direction of the field but the thermal motions will prevent complete alignment. In this case there will be overall weak magnetization in the direction of the applied field. This accounts for the phenomenon of paramagnetism or paramagnetization.
It is clear that paramagnetism is temperature dependent. At low temperatures, the thermal motions will be less successful at preventing the alignment of the atomic moments and so the susceptibility will be larger. At higher temperatures thermal motion will make alignment difficult. At very high temperatures, the material may become diamagnetic, for the diamagnetic contribution to x is not affected by temperature whilst the paramagnetic contribution falls.
If a magnetic field is produced in the neighbourhood of a magnetic material, a changing flux occurs in the current loops within the atoms. An e.m.f. or electric field will then be set up which causes the electrons to alter their motions, so that an extra or induced current is produced. By Lenz’s law, this current gives rise to a magnetic field which oppose the applied magnetic field H. Thus the induced magnetization will be in the opposite direction to H, that is, M/H is negative. Hence the susceptibility χ is negative. This phenomenon is called diamagnetism. For a diamagnetic material, χ is generally very small, about -0.000015 for bismuth, for example. The relative permeability, , which is given by , = 1+x, is thus generally slightly less than 1. All substances have diamagnetic contribution to their susceptibility, since the induced currents always oppose the applied field. In many substances, the diamagnetism is completely masked by another magnetic phenomenon.
If a rod of diamagnetic material is placed in a non – uniform magnetic field, it will settle at right angles to the field. The magnetization will oppose the applied field so that the end will now effectively be weak S pole. It will then experience a force, so that a restoring couple turns the specimen back to its position at right angles to the field.
It should be noted that diamagnetism is a natural ‘reaction’ to an applied magnetic field and that it is independent of temperature.