Impure thoughts to improve spintronics materials

Researchers at Berkeley Lab have discovered that they can control the Curie temperature, and hence the magnetism of the semiconductor gallium manganese arsenide (GaMnAs). The breakthrough settles a long running controversy over the usefulness of the material in the emerging field of spintronics.

Where traditional electronics exploits the charge carrying properties of electrons, spintronics looks to other, more flexible properties such as an electron’s spin. If you think about an electron as being a bar magnet, and its spin up and down states as being magnetic north and south, you’d still be wrong, but it’ll give you a decent metaphor to work with. (Thanks to Terry Pratchett and friends for the concept of Lies-to-Children). The useful thing about spin is that each electron has at least two potential states, and so can carry twice as much data as when we’re only interested in charge.

To manipulate spin, you need to be able to exert a fine degree of control over the magnetism of a material, which is where the Curie temperature comes in. This is a temperature over which a magnetic material becomes non-magnetic. In GaMnAs, this point is determined by the holes, or positively charged spaces, within the semiconductors.

Now the Berkeley researchers have shown that these holes are located in a so-called impurity energy band, rather than the valence band. This suggests that the semiconductor can be fabricated with the widest possible impurity band, effectively tuning the material to maximise the Curie temperature, thus making the material as perfect as possible for spintronics applications.

The results were published in Nature Materials on Feb 19th.