29.10: Summary
- Page ID
- 32854
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This TLP has covered the basic points of ferromagnetism:
- 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.
- Ferromagnetism occurs in materials where all the magnetic dipole moments align parallel below the Curie temperature.
- 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.
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The magnitude of the magnetisation is dependent on temperature and modelled by the Curie-Weiss law:
- The formation of domains is driven by the minimisation of energy, with the main driving force often being that of the magnetostatic energy.
- Magnetic hysteresis is seen, due to the defects found in crystals, as these hinder the movement of domain walls.
- Hard magnets have a large coercive field, whereas soft magnets are easily demagnetised and so has a small coercive field.
- Both magnetocrystalline anisotropy and shape anisotropy give directions in a material along which it is easier to magnetise a sample.
Going further
Books and Papers
- Magnetic Materials : Fundamentals and Device Applications by N.A. Spaldin, Cambridge University Press (2003)
A good overall explanation of ferromagnetism which also covers other types of magnetism and some applications. - The Feynman Lectures on Physics by R.P. Feynman, R.B. Leighton and M. Sands, Addison-Wesley Publishing Company (1970)
Provides a complete explanation of magnetism including the mathematics and qualitative explanations. - P.A.M. Dirac, The quantum theory of the electron, Proc. R. Soc. London A 117 610-612 (1928)
- P.A.M. Dirac, The quantum theory of the electron Part II, Proc. R. Soc. London A 118 351-361 (1928)
These are the original papers by Dirac in which the spin of the electron was derived.
Websites
- Get Perpendicular: Hitachi Global Storage Technologies
This website gives a fun summary of one of the latest developments in magnetic memory.