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Exchange interaction
The exchange or exchange energy reveals much about the amount of energy within an atom and plays a major role in the description of ferromagnetism. Literally, this is not a force that is normally transmitted through an interaction - rather, the exchange interaction describes an effect from quantum mechanics that has similar effects. Because the exchange interaction within the magnetic order deals with the residence probability of electrons, it is often associated with the Pauli principle, but this is not necessarily associated with it.
What is meant by exchange interaction?
In a physical system (a physical object that can be defined in a defined manner from its environment, such as an atom), several particles have a quantum mechanical state.
According to the Pauli principle, two fermions, that is, particles of matter such as electrons or neutrinos, etc., can never assume the same quantum state. The quantum mechanical wave function can be used to indicate where the particles are likely to be located (local component) and how their magnetic moments, ie their spins, are related to each other (spin component). Thus, the wave function must always be antisymmetric, i. with symmetric local component and antisymmetric spin component or vice versa. This unambiguous distinction is called the exchange interaction in physics.
If the atoms in a solid occupy a greater distance from each other, the spatial distribution of the electrons can be more favorable in terms of energy, which leads to a symmetry change in the spatial wave function. As a logical consequence, the spin wave function then changes from an antisymmetric (antiferromagnetic) to a symmetric (ferromagnetic) alignment.
Exchange interaction and ferromagnetism
The exchange interaction is responsible for the emergence of ferromagnetism. The electron spins, ie the elementary magnets, have magnetic moments that align themselves with an external magnetic field. When unpaired electron spins are present in a solid, we magnetize the entire material, since there is a parallel position of the magnetic moments of all the atoms. But here there are differences between paramagnets and ferromagnets.
Paramagnets: The direct exchange interaction between the electron spins is smaller than the thermal energy of the electrons involved. This means that the electron spins do not stay aligned and the magnetization disappears after removal of the external magnetic field.
Ferromagnets: The exchange interaction in ferromagnets is significantly larger than the thermal energy and so the magnetization is retained even after switching off the external magnetic field. The magnetization is lost only by heating above the Curie temperature or more violent impacts. The magnetization of ferromagnets does not happen by chance. Rather, the electron spins align in special areas, the white areas, parallel to each other.