Pressure tuning of magnetism
It has been known for some time that the bulk magnetisation of ordered materials can be switched by the action of light or pressure and that the magnetic phase transition can be electrochemically tuned. The key step in all three processes is internal electron transfer. Now, Spanish scientists at the Universities of Valencia and Complutense, Madrid, with researchers at Tel Aviv University, Israel, have linked pressure-induced changes in the magnetisation of ferromagnetic single crystals with structural rearrangements.
When single crystals of the Prussian blue analogue K0.4Fe[Cr(CN)6]2.8.16H2O, a soft ferromagnet, were pressurised to 320-1200MPa, the magnetisation decreased markedly and saturation occurred at lower temperatures. The process was completely reversed when the pressure was released and the changes occurred immediately after both pressure application and release.
Raman and Mössbauer spectroscopy confirmed the behaviour is caused by pressure-induced isomerisation of the cyanide ligand between the Cr and Fe centres and can be considered as a molecular switch. As a result, the Fe2+ centre changes from high-spin to low-spin and switches off the magnetic interaction with the Cr3+ neighbour, leading to lower critical temperatures. 'For the first time, the magnetic interaction between two metals can be switched via rotation of the bridging molecule,' say senior researchers Francisco Romero and Eugenio Coronado.
The kinetics of the isomerisation after prolonged pressurisation at 1200MPa revealed two different decay mechanisms. 'Since there is a random distribution of defects in these structures, these two different decays might correspond to different environments for the iron(ii) centres' say the scientists. They added that the slow nature of the process is further evidence for the isomerisation, because an electron transfer reaction would be much faster.
The team will investigate this phenomenon at the molecular level using further Prussian blue analogues of metal-CN-Fe(ii) type.