• Question: What is exchange coupling? How does this explain magnetism and other magnetic properties?

    Asked by sanban to Chris, Dave, David, Fiona, Jack on 24 Jun 2013.
    • Photo: Fiona Coomer

      Fiona Coomer answered on 24 Jun 2013:

      If an atom or ion has unpaired electrons in its atomic orbitals, the motion of the electron(s) about the nucleus gives rise to a magnetic field (in the same way that passing a current through a coil creates an electromagnet). We call this paramagnetism. This means that we can describe the magnetic atom as a tiny bar magnet, and we usually represent this by drawing an arrow through the atom, to show the direction of the magnetism (we call this the magnetic moment).

      Exchange coupling is the way in which two magnetic atoms (or ions) in a material interact with each other. Physicists like to describe this by equations, but as a chemist, we prefer to describe this in terms of concepts. As it’s the electrons that give rise to the magnetism, it’s the way that the electrons interact that we’re interested in when we’re thinking about exchange coupling, and there are many different ways that this can happen, depending on the material.

      When 2 magnetic atoms are situated very close together (so that their atomic orbitals can weakly overlap), their electrons can interact directly, in the same way as when forming a chemical bond. We call this direct exchange. This type of exchange coupling is actually very unusual.

      The more common form of magnetic exchange in an insulating material is superexchange – this is where the electrons on a magnetic atom interact with electrons on another magnetic atom, by way of the electronic orbitals of a non magnetic atom bonded in between. The orbitals on the non magnetic atoms that are involved, determine the way in which the two magnetic atoms interact – this depends on the way that the atoms are arranged within the structure of the material (the distances and angles between the atoms).

      The third way occurs in a conducting material, where the electrons are delocalised throughout the whole material. In this case, the mechanism for magnetic exchange is called the RKKY mechanism, and occurs as the magnetic ion polarises the conduction electrons, which in turn polarise another magnetic ion. The way it polarises the next magnetic ion, depends on how far away the 2 ions are.

      The way that a magnetic atom interacts with its neighbours gives rise to the magnetic properties of the whole material. If a magnetic atom interacts with all its neighbours ferromagnetically (this means all of the magnetic moments point in the same direction), then if we add all these atoms together, we end up with a magnetic material just like a bar magnet that you will be familiar with.