Types of Magnetism

Types of Magnetism

Before discussing the different types of magnetism we need clear up a few definitions which are used to categorise the types of magnetism.

Magnetic Suseptibily

As the magnetic field increases, the magnetic flux increases. We denote magnetic field intensity by H and magnetic flux by B the constant of proportionality is μ₀ this is known as the magnetic permitivity

In a vacuum, μ₀ has a value of 4π x 10^-7 H m^-1 in SI units

For other materials this proportionality is expressed with the relative permitivity, μ_r

The susceptibility χ is defined in terms of the relative permitivity. χ = (μ_r - 1)

Ferromagnetism

  alignment of electron spins

Anti-Ferromagnetism

Ferrimagnetism  

Ferri magentic material have two sets of magnetic dipole moments pointing in opposite directions. The magnetic moments do not cancel each other out becauses the dipole moment in one direction is smaller than the other. On a B-H graph ferrimagnetism is like ferromagnetism.

Diamagnetism

Diamagnetic materials are composed of atoms that have no net magnetic moments. However, when exposed to a field a weak negative magnetisation is produced which causes repulsion instead of attraction. Diamagnetic materials have negative susceptibilty with magnetitude of around -10 to -10^-4

Many common materials are diamagnetic and hence In the presence of very strong magentic fields the repulsion caused by diamagnetism can cause objects to levitate even frogs.

Paramagnetism

Paramagnetic materials such as liquid oxygen and aluminium show a weak magnetic attraction when placed near a magnet. Some atoms or ions in the material have a net magentic moment due to unpaired electrons in partially filled orbitals. In the presence of a field, there is a partial alignment of the atomic magnetic moments in the direction of the field resulting in a net positive magnetisation and positive susceptibility.

In a strong magnetic field, paramagnetic materials become magentic and will stay magnetic while the field is present. When the strong magnetic field is removed the net magnetic alignment is lost and the magnetic dipoles relax to a random motion.