3.2. Magnetic Properties of different materials

Circular motion of electron in atom can be considered as circular current loop (Fig. 28). Moving elementary charge q₀ creates an elementary current i₀. Elementary circular current i₀ creates an elementary magnetic moment:

, (99)

where S – area of plane of orbit of electron, - unit normal vector to the area of the loop.

Direction of magnetic moment is being defined by corkscrew rule (Fig. 26 or 28) and coincides whith a direction of own magnetic intensity.

The magnetic moment of a current loop is a vector quantity defined as (Fig. 3):

,

where I – is a current, S – area of the loop, - unit normal vector to the area of the loop. Circular motion of charged particle (an electron in atom) can be considered as such current loop and characterized by its orbital magnetic moment. Beside the orbital moment electrons has self magnetic moment called a spin. Spin does not related with any motion, it is a self characteristic of electron, such as charge, and it is only shows that immovable electrons in external magnetic field behave as current loops with moments.Like a charge in electric field experiences a force, the magnetic moment experiences a torque in external magnetic field tend to re-align it in the direction of the field.

Total moment of electron:

Total moment of an atom:

,

where summing provided for valence electrons.

So the magnetic behavior of a substance depends on a presence of magnetic moment in its atoms.

Magnetic Properties of different materials

Materials may be classified by their response to externally applied magnetic fields as diamagnetic, paramagnetic, or ferromagnetic. These magnetic responses differ greatly in strength. Diamagnetism is a property of all materials and opposes applied magnetic fields, but is very weak. Paramagnetism, when present, is stronger than diamagnetism and produces magnetization in the direction of the applied field, and proportional to the applied field. Ferromagnetic effects are very large, producing magnetizations sometimes orders of magnitude greater than the applied field and as such are much larger than either diamagnetic or paramagnetic effects.

Diamagnetism

The orbital motion of electrons creates tiny atomic current loops, which produce magnetic fields. When an external magnetic field is applied to a material, these current loops will tend to align in such a way as to oppose the applied field. Materials in which this effect is the only magnetic response are called diamagnetic. All materials are inherently diamagnetic, but if the atoms have some net magnetic moment as in paramagnetic materials, or if there is long-range ordering of atomic magnetic moments as in ferromagnetic materials, these stronger effects are always dominant. Diamagnetism is the residual magnetic behavior when materials are neither paramagnetic nor ferromagnetic.

Any conductor will show a strong diamagnetic effect in the presence of changing magnetic fields because circulating currents will be generated in the conductor to oppose the magnetic field changes. A superconductor will be a perfect diamagnet since there is no resistance to the forming of the current loops.