Shrinking the gap; strengthening the magnet

The forces that cause magnetic spins on atoms to align vary wildly in strength. This means that the temperature at which a given magnetic material orders into its magnetic state can be as high as red hot iron down to fractions of a degree above absolute zero for magnetic refrigerant salts. In most magnetic materials, these interactions between magnetic spins are transmitted through the non-magnetic atoms or molecules that connect them: this is called superexchange. The strength of superexchange depends on many factors, but one key factor is how close in energy the bonding electrons of the linker molecule is to the magnetic electrons: the smaller the gap, the stronger the superexchange. This is often hard to control directly: for examples many ceramic magnets are metal oxides and the oxygen atoms cannot be swapped for other atoms without changing the structure completely.

In this work we make a new metal-organic magnet, chromium dichloride 2,1,3-benzothiadiazole, which we predicted would have a small energy gap, by iterating on a compound we had previously made, chromium dichloride pyrimidine. We found that it does have a smaller energy gap, and this increases the strength of magnetic interactions by a factor of six. We determined the crystal structure using electron diffraction on a nanocrystal, measured the strength of the interactions using neutron scattering experiments at the UK’s neutron source, ISIS, and did quantum chemical calculations to understand the origin of this difference, locating it as we expected in the lower energy of the 2,1,3-benzothiadiazole molecule.

This work was a collaboration between the FIHM group (spanning its move from Nottingham to Cambridge); the NAMI group at the University of Nottingham; Morris Group at the University of Birmingham and scientists based at the ISIS Neutron and Muon Source. It started with experiments carried out by a Pete Speakman (an MSci student) under the wing of Jem Pitcairn.