DoITPoMS

Summary

This TLP has covered the basic points of ferromagnetism:

1. In a magnetic atom there are two contributions to the magnetic dipole moment; firstly the spin of the electrons themselves and secondly that of electrons orbiting the nucleus.
2. Ferromagnetism occurs in materials where all the magnetic dipole moments align parallel below the Curie temperature.
3. Ferromagnetic ordering was explained by Weiss via a hypothetical average field which acts to cause the parallel alignment. However, the microscopic explanation for this can be found by looking at the Pauli Exclusion Principle; it is energetically more favourable for electrons be placed in different orbitals as this reduces the Coulomb repulsion energy and allows for the alignment of the electron spins.
4. The magnitude of the magnetisation is dependent on temperature and modelled by the Curie-Weiss law:

        (Equation 2 in text)

5. The formation of domains is driven by the minimisation of energy, with the main driving force often being that of the magnetostatic energy.
6. Magnetic hysteresis is seen, due to the defects found in crystals, as these hinder the movement of domain walls.
7. Hard magnets have a large coercive field, whereas soft magnets are easily demagnetised and so has a small coercive field.
8. Both magnetocrystalline anisotropy and shape anisotropy give directions in a material along which it is easier to magnetise a sample.
   

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