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29.08.2007



Radicals in a spin



28 August 2007



Scientists in Canada have observed spontaneous magnetic ordering in two neutral radicals at 18K and 27K.


Most organic magnets reported to date are based on nitroxide or nitronyl nitroxide radicals, which tend to have magnetic ordering temperatures close to absolute zero. A number of thiazyl radicals have shown magnetic properties because the intermolecular sulfur-nitrogen bonds provide a direct pathway for magnetic exchange. 


Now, Richard Oakley at the University of Waterloo, Ontario, and colleagues have swapped the sulphur in thiazyl radicals for selenium, which is heavier. As the researchers had expected, the conductivity of the resulting radical increased; the inclusion of selenium caused a dramatic increase in bandwidth and hence conductivity. 



Structure of selenium-based radical








The selenium-based radicals have an unusually high magnetic ordering temperature



Many radical-base systems display either magnetic activity or conductivity, but not both. So the team were surprised to find that their new radicals also showed spin-canted anti-ferromagnetism, where an ordered array of magnetic moments spontaneously formed, with the electron spins alternately aligned in opposite directions. 


Radicals containing heavy atoms are rare because these elements are larger and the electrons easily form pairs. But widespread delocalization of electrons in Oakley's selenazyl radicals suppresses the electron-pairing. The group believes that radicals such as these, which are magnetic and conductive, represent a major step forward in the development of multi-functional spin-based electronic materials. 


'These results indicate that sulphur-nitrogen and selenium-nitrogen radicals are capable of showing comparatively high magnetic ordering temperatures,' said Jeremy Rawson, an expert in organic magnets from the University of Cambridge, UK. 'Nevertheless the long term objective of a room temperature organic magnet continues to be elusive,' he added. 


In the future, the team plan to continue their work with other heavy atoms in the hope of further improving the conductivity and magnetic performance. 'We believe that materials displaying spin-correlated conductivity and magnetoresistive effects are very real possibilities,' said Oakley. 


Jenna Wilson




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