Long-distance (magnetic) relationship

Figure. 1 Magnetic order in CrBi(SCN)₆. Left: the magnetic superexchange pathways. Right: Neutron diffraction data showing magnetic order.

Ions with unpaired electrons are magnetic and when those magnetic ions are assembled into a crystalline solid, their magnetic spins interact. At low enough temperatures, these spins therefore typically lock into an ordered state. The temperature at which this ordering happens depends, however, on the strength of the magnetic ’exchange’ interactions between the spins, which is strong for ions next to each other (e.g. in magnetite, Fe₃O₄) but gets rapidly weaker as the magnetic ions are separated by more intervening atoms (e.g. chrome alum, KCr(SO₄)₂.12H₂O). In crystals where the magnetic ions are connected by multiple multi-atomic molecules this means that the temperature at which magnetic ordering occurs can very often be too cold to practically measure (as low as 0.05K, i.e. within a twentieth of a degree of absolute zero).

In this paper we investigated the magnetic properties of chromium bismuth thiocyanate (CrBi(SCN)₆), which has a 3D structure where every magnetic chromium ion is connected by thiocyanate ions to a nonmagnetic bismuth. This means the each chromium ion are separated by eight bonds from the next magnetic ion (or 1.5 nm). This is a long distance for a magnetochemist! Surprisingly, this material orders at the high temperature of 4 K (again, for a magnetochemist). Using neutron diffraction (which can measure the direction and size of the magnetic spins) at the ILL in France, we were able to also work out the ordering pattern of spins in this material, which resembles that of much more conventional magnetic materials like manganese oxide. As well as being a surprising discovery about the potential strength of magnetic long distance relationships, this finding suggests some interesting new directions for the investigation of unusual quantum properties. Firstly, the relative weakness of the exchange interactions compared to other magnetic materials means they are potentially easier to probe by applying magnetic fields, which are typically much weaker than exchange interactions. Secondly, there are many different metal thiocyanate compounds with the same structure which have not had their magnetic properties measured and so by exploring these materials we might therefore be able to find materials that have the right balance of exchange interactions to realise some unusual properties (e.g. complex, ’non-coplanar’ magnetic structures).

This work was carried out in collaboration with the Institut Laue Langevin (Grenoble, France).