Magnetism

Magnetism arises from two sources: electric current and the spin magnetic moment. [1]

The magnetic properties of materials largely arise from the nature of the electron pairing within the constituent atoms, and the resultant magnetic moments.

Generally, electrons are paired within orbitals: each orbital will contain a spin up and spin down electron leading to an overall magnetic moment of zero within that orbital. This is due to the Pauli exclusion principle, which in the case of electrons in atomic orbitals, states that two electrons occupying an atomic orbital must have spin values of ½ and –½ respectively. [3]

Usually, even if an atom contains unpaired electron orbitals there is no net magnetic moment, due to the nature of the random alignment of the magnetic moments. Examples of exceptions lie in nickel, iron and cobalt, where on average there will be some net magnetic moment (the magnetic moments align overall) as a result of an applied external magnetic field. This can also happen spontaneously without the effect of a field.

Magnetic domains

A magnetic domain is a region within a magnetic material in which the magnetisation is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and point in the same direction. Within a sample, the magnetisation of different domains may point in different directions.

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What is a magnetic moment?

The magnetic moment is a vector quantity that describes the magnitude and direction of the strength of a magnetic object.

What is the Pauli exclusion principle?

The Pauli exclusion principle states that no two identical fermions (an electron is an example of a fermion) can occupy the same quantum state within a quantum system at the same point in time.

Figure 1: Magnetic domains within a magnetised material where domains are aligned (left) and unmagnetised material where domains are orientated randomly (right)

Types of magnetism

Diamagnetism and paramagnetism

Diamagnetism is the tendency of a material to oppose an applied magnetic field. It occurs in all materials as a result of the paired electrons within the atomic orbitals, which are repelled by a magnetic field.

Paramagnetism is the tendency of a material to reinforce an applied magnetic field. This is due to the tendency of electrons’ intrinsic magnetic moments to align themselves to the magnetic field, a characteristic which is only observed when the electron is not constricted by the Pauli exclusion principle. It only occurs in materials with one or more singly-occupied atomic orbitals.

The effect of paramagnetism is far greater than that of diamagnetism. Therefore, for materials with both paired and unpaired electrons, the paramagnetic behaviour dominates. As such, a diamagnetic material is one with no singly-occupied orbitals, and a paramagnetic material is one with one or more singly-occupied orbitals.

In diamagnetic and paramagnetic materials, the magnetisation, M and strength of the applied magnetic field, H are usually directly proportional, with the constant of proportionality being χ, the magnetic susceptibility of the material.

However, in paramagnetic materials, the proportionality is reduced with temperature. For a fixed field B, the magnetisation is inversely proportional to the temperature:

This relation is known as Curie's law. [1]

Pierre Curie

Pierre Curie (1859 – 1906) was a French physicist whose pioneering work in magnetism, crystallography and radioactivity won him the Nobel Prize in Physics along with his wife, Marie Curie and also Henri Becquerel. [2]

Source: wikipedia.org

Figure 2: graph showing relationship between temperature and magnetic susceptibility

Source: writeopinions.com

Ferromagnetism

Ferromagnetism is the strongest form of magnetism and also the form of magnetism occurring in the typical magnets we know in daily life, such as fridge magnets. A ferromagnetic material also has unpaired electrons, but these are different to the unpaired electrons of paramagnetic materials in they tend to orient their magnetic moments parallel to each other. As a result, there is a net magnetic moment and thus the material has a magnetic field, even in the absence of an applied, external magnetic field.

The Curie temperature is the temperature at which a ferromagnetic material loses its ferromagnetic properties. This is because beyond this temperature, the electrons’ high energy leads to a tendency towards disorder which is greater than the material’s tendency to align its intrinsic magnetic moments.

Antiferromagnetism

Antiferromagnetic materials orient their magnetic moments antiparallel to eachother. As a result, antiferromagnetic materials have a net magnetic moment of zero, and no magnetic field is produced by the material.

Ferrimagnetism

Ferrimagnetism is a combination of ferromagnetism and antiferromagnetism. Like in ferromagnetism, ferrimagnetic materials produce a magnetic field without the influence of an external field, due to the net magnetisation produced by the internal magnetic moments. However, this is not due to the magnetic moments aligning to all be parallel: in fact, the magnetic moments are antiparallel to eachother like in antiferromagnetic materials.

The reason why there is a net magnetisation despite the antiparallel alignment is that the magnetic moment resulting from the electrons pointing one way is greater than the magnetic moment resulting from the electrons pointing the opposite way.

Superparamagnetism

When a ferromagnet or ferrimagnetic is small enough, it is subject to Brownian motion, and acts like paramagnet, but with a much larger response.

References:

[1] Coey, M. 'Magnetism and Magnetic Materials', Cambridge University Press, 2009

[2] https://en.wikipedia.org/wiki/Pierre_Curie [Accessed: 26/02/19]

[3] Housecroft, C. et al, 'Chemistry 4th ed.', Pearson, 2014