Image courtesy of P. M. Koenraad, Eindhoven University of Technology.
Towards the end of the 1960s, scientists had begun exploring the technological potential of magnetism combined with semiconductor physics. Having succeeded in introducing small amounts of magnetic impurities into otherwise non-magnetic semiconductors, Robert Ga
zka and colleagues presented, in 1978, remarkable data on II–VI compounds doped with manganese. In these 'diluted magnetic semiconductors' (DMSs), the low-concentration defects (the manganese ions) did not compromise the quality of the material, meaning that its magneto-optical and magneto-transport properties could be probed. At the same time, pronounced magnetic properties could be observed — such as the spin splitting of electronic or impurity bands.
A further breakthrough came in the early 1990s, with the advent of low-temperature molecular-beam epitaxy. By growing semiconductor films under conditions that were far from thermal equilibrium, it became possible to introduce manganese impurities into III–V materials (which is more difficult under equilibrium conditions owing to the low solubility of manganese). Hideo Ohno and colleagues then demonstrated, in 1992, ferromagnetic order in the DMS (In,Mn)As — indium arsenide containing only 1.3% manganese — by measuring magneto-transport and, in particular, an anomalous Hall effect in the material. The work was followed up, in 1996, with proof of ferromagnetism in doped gallium arsenide — (Ga,Mn)As — at temperatures up to 110 K. As GaAs can be used in electronic devices that operate at room temperature, these studies established the basis for research into 'technologically relevant' DMSs.
The 1990s also brought theoretical work by Tomasz Dietl, in collaboration with Ohno's group, which explained the origin of ferromagnetism in (Ga,Mn)As using a model developed, by Clarence Zener in 1950, for ferromagnetism in transition metals. According to this model, the magnetic order originates from the delocalized holes that mediate the interaction between localized magnetic moments. The importance of the work was twofold. First, the carrier-mediated magnetic order suggested the possibility of controlling the ferromagnetism using electric fields — which was soon demonstrated — and, beyond that, the development of efficient spintronics devices. Second, the Dietl model provided an effective recipe for calculating the Curie temperature of other zinc-blende and wurzite semiconductors, to advance the search for a room-temperature DMS. In particular, Dietl showed that DMSs based on zinc oxide (ZnO) or gallium nitride (GaN) could have Curie temperatures as high as 300 K.
Investigations since then have indeed revealed room-temperature ferromagnetism in oxides and semiconductors that include ZnO or GaN. However, it is yet to be proved that the carrier-mediated mechanism proposed by Dietl is really at work in these systems. The origin of this ferromagnetism — and its potential use in spintronics devices — is still a matter of controversy, yet the search for a carrier-mediated room-temperature DMS continues.
