GaAs forms basis of tunable spintronics
An international team of scientists has for the first time developed an efficient electrically tunable spin-charge converter made of the common semiconductor material GaAs. Comparable efficiencies have only been observed in platinum before.
Spin-charge converters transform electric into magnetic signals and vice versa. As such they are important in the emerging field of spintronics, which is a way of making circuits based on the charge of electrons and their spin (the rotation of the electron around its own axis) and spin-related magnetism. Spin generates a magnetic field like a small magnet. In some materials, electron spins spontaneously align their direction, leading to ferromagnetism. Additionally, "˜spin-up' or "˜spin-down directions can be used to represent two states - 0 and 1 - the basis of digital technology.
The work of Jairo Sinova (pictured above) and colleagues from the Johannes Gutenberg University Mainz in collaboration with researchers from the UK, Prague, and Japan shows that it is possible to precisely control spin using electric fields rather than magnetic ones - crucial for making use of electron spin for information transmission and storage. The results were published in the journal Nature Materials.
The spin Hall effect is the key to generating, detecting and using spin currents. The effect appears when an electric field drives electrons through a semiconductor plate. Taking a look at the classical Hall-effect, the interaction of moving electrons and an external magnetic field forces the electrons to move to one side of the plate, perpendicular to their original direction. This leads to the so-called Hall voltage between both sides of the plate.
For the spin-Hall effect, electron-spins are generated by irradiating the sample with circularly polarised light. The electron spins are then parallel or anti-parallel, and their direction is perpendicular to the plate and the direction of movement. The moving electron spins are now forced to one or the other side of the plate, depending on the spin orientation. The driving force behind this is "˜spin-orbit coupling', a relativistic electromagnetic effect which influences moving electron spins. This leads to the separation of both spin orientations.
To make practical use of this effect, it is essential to get a highly efficient spin separation. Up to now, platinum has been the most efficient spin-charge converter material, as it is a heavy metal, and the spin-orbit coupling of heavy metals is known to be especially strong due to the large amount of protons (positive charge) in their core.
Use of Inter-Valley Transitions
Sinova and his colleagues have shown that GaAs can be an as efficient spin-charge converter as platinum, even at room temperature, which is important for practical applications. Moreover, the physicists have demonstrated for the first time that the efficiency can be tuned continuously by varying the electric field that drives the electrons.
The reason for this lies in the existence of 'valleys' in the conduction band of the semiconductor material. One can think of the conduction band and its valleys as of a motor highway with different lanes, each one requiring a certain minimum velocity. Applying a higher electric field enables a transition from one lane to the other.
Since the spin-orbit coupling is different in each lane, a transition also affects the strength of the spin-hall effect. By varying the electric field, the scientists can distribute the electron spins on the different lanes, thus varying the efficiency of their spin-charge converter.
By taking into account the valleys in the conduction band, Sinova and his colleagues open up new ways to find and engineer highly efficient materials for spintronics. Especially, since current semiconductor growth technologies are capable of engineering the energy levels of the valleys and the strength of spin-orbit coupling, e.g. by substituting Ga or As with other materials like Aluminum.
'Electric control of the spin Hall effect by intervalley transitions' by N. Okamoto et al appears in Nature Materials, 2014; DOI: 10.1038/nmat4059