Credit: © 2006 Nature

Materials that have good magnetic and electronic properties are likely to prove useful in a wide range of applications. Gallium arsenide doped with manganese is one such material, but there is a problem — the magnetism associated with the Mn atoms disappears well below room temperature. However, new insights into the origins of ferromagnetism in Mn-doped GaAs could allow the effect to be observed at higher temperatures.

Ali Yazdani and colleagues1 at Princeton University, the University of Illinois at Urbana-Champaign and the University of Iowa, used a scanning tunnelling microscope to replace single Ga atoms with Mn atoms in a wafer of GaAs. They then studied the interactions between the magnetic atoms, which are mediated by the ‘holes’ that are created when the Mn atoms accept electrons from the host semiconductor. It was found that the strength of the ferromagnetic interaction decays rapidly with increasing separation. Moreover, it also varies with direction: for instance, the interaction is much stronger when the Mn atoms are oriented along a particular crystallographic axis.

Therefore, by growing Mn-doped GaAs structures in which the magnetic atoms tend to be oriented in such a direction, it should be possible to increase the ferromagnetic transition temperature above that observed in randomly doped samples. However, so far the experiments have all been carried out at 4 K so there is still plenty of scope for improvement.