Organic sulfur-nitrogen radical demonstrates ferromagnetism at low temperatures


The word magnet usually brings to mind a piece of metal. For many years scientists did not believe that (ferro)magnetic materials not containing metal atoms could exist. The first organic ferromagnet was discovered in the early 1990's, and more followed. However, all these materials only become magnetic at very low temperatures, near absolute zero. An international team of researchers working with Jeremy M. Rawson at Cambridge University has now developed an organic substance that becomes ferromagnetic at 1.3 K (-271.85 °C); it is only the second neutral organic compound with a transition temperature (Curie point) above 1 K.

In order to be ferromagnetic, a substance must contain a free unpaired electron. Such compounds are called radicals. The intrinsic angular momentum, or spin, of the electron is the key component. In ferromagnets, the electron spins of neighboring molecules are in close contact. Within small, defined areas, they align themselves parallel. However, because the direction is random, the effect is cancelled out overall. If an external magnetic field is applied, all of the spins align parallel to the direction of the magnetic field lines. This magnetization is partially retained after the magnetic field is turned off.

Several years ago, a team led by Rawson discovered a neutral organic radical that demonstrates weak magnetism at an astonishing 36 K. Starting with this thiazyl molecule, Rawson and his co-workers searched for closely related compounds with strong ferromagnetism -- and they struck gold. The successful candidate is a fluorinated aromatic carbon ring with one nitro group (NO2), as well as another ring. This second ring consists of one carbon, two nitrogen, and two sulfur atoms, and carries the unpaired electron necessary for cooperative magnetic behaviour. Within the crystal, close contacts between sulfur and nitrogen atoms in different molecules allow the magnetic interactions to spread throughout the crystal. The orbitals of the unpaired electrons in neighboring molecules are nearly perpendicular to each other, which is believed to favour the ferromagnetic interaction.

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