Spintronic technology, in which data is processed on the basis of electron “spin” rather than charge, promises to revolutionize the computing industry with smaller, faster and more energy efficient data storage and processing. Materials drawing a lot of attention for spintronic applications are dilute magnetic semiconductors – normal semiconductors to which a small amount of magnetic atoms is added to make them ferromagnetic. Understanding the source of ferromagnetism in dilute magnetic semiconductors has been a major road-block impeding their further development and use in spintronics. Now a significant step to removing this road-block has been taken.
For the semiconductors used in today’s computers, tablets and smart phones, etc., once a device is fabricated it is the electronic structures below the surface, in the bulk of the material or in buried layers, that determine its effectiveness. HARPES, which is based on the photoelectric effect described in 1905 by Albert Einstein, enables scientists to study bulk electronic effects with minimum interference from surface reactions or contamination. It also allows them to probe buried layers and interfaces that are ubiquitous in nanoscale devices, and are key to smaller logic elements in electronics, novel memory architectures in spintronics, and more efficient energy conversion in photovoltaic cells.
“The key to probing the bulk electronic structure is using hard x-rays, which are x-rays with sufficiently high photon energies to eject photoelectrons from deep beneath the surface of a solid material,” says the researcher. “High-energy photons impart high kinetic energies to the ejected photoelectrons, enabling them to travel longer distances within the solid. The result is that more of the signal originating from the bulk will be detected by the analyzer.”
The researchers used HARPES to shed important new light on the electronic bulk structure of gallium manganese arsenide (GaMnAs). As a semiconductor, gallium arsenide is second only to silicon in widespread use and importance. If a few percent of the gallium atoms in this semiconductor are replaced with atoms of manganese the result is a dilute magnetic semiconductor. Such materials would be especially well-suited for further development into spintronic devices if the mechanisms behind their ferromagnetism were better understood.
“Right now the temperature at which gallium manganese arsenide operates as a dilute magnetic semiconductor is 170 Kelvin,” the researcher says. “Understanding the actual mechanism by which the magnetic moments of individual manganese atoms are coupled so as to become ferromagnetic is critical to being able to design future materials that would operate at room temperature.”
The two prevailing theories behind the origin of ferromagnetism in GaMnAs and other dilute magnetic semiconductors are the “p-d exchange model” and the “double exchange model.” According to the p-d exchange model, ferromagnetism is mediated by electrons residing in the valence bands of gallium arsenide whose influence extends through the material to other manganese atoms. The double exchange model holds that the magnetism-mediating electrons reside in a separate impurity band created by doping the gallium arsenide with manganese. These electrons in effect jump back and forth between two manganese atoms so as to lower their energy when their ferromagnetic magnets are parallel.
“Our bulk-sensitive HARPES measurements revealed that the manganese-induced impurity band is located mostly between the gallium arsenide valence-band maximum and the Fermi level, but the manganese states are also merged with the gallium arsenide valence bands,” the researcher says. “This is evidence that the two mechanisms co-exist and both act to give rise to ferromagnetism.”
The researcher finally adds, “We now have a better fundamental understanding of electronic interactions in dilute magnetic semiconductors that can suggest future materials with different parent semiconductors and different magnetic dopants. HARPES should provide an important tool for characterizing these future materials.”
This story is reprinted from material from Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.