Dilute magnetic semiconductors (DMSs) — semiconductors that also contain a small amount of a magnetic element — may one day allow the realization of electronic devices that merge current functionalities with the possibility of controlling electron spin.

(Ga,Mn)As is undoubtedly the most widely investigated DMS. In this compound, the ferromagnetic order among localized manganese atoms is mediated by the presence of delocalized carriers of positive charge (‘holes’). The magnetic properties can thus be tuned by varying the charge concentration, such as by adjusting an applied gate voltage, which is a very important characteristic for applications.

Yet the exact nature of the interplay between carrier density and magnetism is rather complex. Below a certain carrier density, for example, the holes undergo ‘Anderson–Mott localization’ causing the compound to become an insulator. Maciej Sawicki and colleagues in Japan and Poland1 have now elucidated the relationship between localization and magnetism through a combination of direct magnetic measurements and calculations of the charge distribution in the sample.

Fig. 1: Parallel plate capacitor used for magnetization measurements of (Ga,Mn)As films.

The team studied (Ga,Mn)As films containing about 7% manganese. By inserting the film in a parallel plate capacitor (Fig. 1), they studied the magnetization at different gate voltages, and therefore different hole concentrations. The researchers found that both the Curie temperature (TC), above which the compound is no longer a ferromagnet, and spontaneous magnetization decrease monotonically as the carrier density is reduced. These results give a strong indication that the magnetism is mediated by carriers in the standard valence band of the semiconductor, and not, as many believe, by carriers in an electronic band formed by the manganese impurities.

The calculation used to describe the data also show a non-uniform distribution of holes along the thickness of the (Ga,Mn)As film and a depletion at the surface. This can have serious consequences for applications. “Neither a working spintronics device nor an increase in TC will be possible without a substantial reduction of the surface charge-traps that are responsible for complete depletion of the near-surface 1–2 nm of the channel,” says Sawicki.

Analysis of the magnetization data also showed that when the density of carriers is reduced to the point at which the carriers become localized, the uniform ferromagnetic order does not disappear abruptly. Rather, it gradually leaves space in which superparamagnetic and ferromagnetic domains can form, until, well below the Anderson–Mott localization density, the ferromagnetic order vanishes completely.