A molecular framework honeycomb magnet

In this paper we investigate a metal thiocyanate compound [Na(OH₂)₃]Mn(NCS)₃ which consists of rods of a honeycomb manganese thiocyanate network threaded by rods consisting of sodium cations and water. This structure has perfect three-fold symmetry, but unusually, the manganese atom do not lie on an inversion centre (i.e. each metal atom knows which way up it is), although the crystal as a whole does have inversion symmetry.

Because of the high symmetry of magnetic honeycombs, they can have some interesting magnetic properties and so we decided to measure them for [Na(OH₂)₃]Mn(NCS)₃. We found by measuring how its magnetic moment varied with temperature that this was an antiferromagnet (the spins line up in opposite directions at low temperature). Through careful control of the humidity (this compound will dissolve in the water in the air, and decomposes if dried out in vacuum!), we were able to grow large single crystals of the compound, large enough for neutron diffraction! The neutron diffraction experiment allow us to establish accurately the magnetically ordered state. This was particularly interesting as depending on the directions of the magnetic spins, we might break the inversion symmetry of the crystal to produce a multiferroic: a material with simultaneous magnetic and electric order. Our neutron measurements suggested that this was not the case for our compound, and this was further confirmed by optical measurements at very low temperatures which are sensitive to the presence of an inversion centre (second harmonic generation).

This work was a collaboration between the University of Nottingham; Institut Laue Langevin, where we carried out neutron measurements, and the University of Leeds where we carried out optical measurements.