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Advancing Spintronics With Ferromagnetic GaAs

By doping gallium arsenide with manganese, researchers have unlocked some ferromagnetic secrets of promising materials for computing
Spintronic technology promises to revolutionise the computing industry with smaller, faster and more energy efficient data storage and processing.

In spintronics, data is processed on the basis of electron “spin" rather than charge.

Materials drawing much attention for spintronic applications are dilute magnetic semiconductors. These are normal semiconductors incorporating a small amount of magnetic atoms 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.

Until now.

Now researchers at Berkeley Lab say they have made a significant step forward in removing this road-block.

The multi-institutional collaboration of researchers led by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), have used a new technique called HARPES. This stands  for Hard x-ray Angle-Resolved PhotoEmission Spectroscopy.

The scientists used the technique to investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor gallium manganese arsenide (GaMnAs). Their findings show that the material’s ferromagnetism arises from both of the two different mechanisms that have been proposed to explain it.

With the HARPES technique, a beam of x-rays flashed on a sample causes photoelectrons from within the bulk to be emitted. Measuring the kinetic energy of these photoelectrons and the angles at which they are ejected reveals much about the sample’s electronic structure. Here the manganese (Mn) atoms in GasMnAs are shown to be aligned ferromagnetically, with all their atomic magnets pointing the same way. (Image from Alex Gray)

“This study represents the first application of HARPES to a forefront problem in materials science, uncovering the origin of the ferromagnetism in the so-called dilute magnetic semiconductors," says Charles Fadley, the physicist who led the development of HARPES. “Our results also suggest that the HARPES technique should be broadly applicable to many new classes of materials in the future."

Alexander Gray (left) and Charles Fadley at Beamline 9.3.1 of Berkeley Lab’s Advanced Light Source where they are now carrying out HARPES experiments. (Photo by Roy Kaltschmidt, Berkeley Lab)

Fadley, who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Davis is the senior author of a paper describing this work in the journal Nature Materials.

For the semiconductors used in today’s computers, tablets and smart phones, 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 Gray, who worked with Fadley to develop the HARPES technique.

“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 analyser."

In this new study, Gray and Fadley and their collaborators, used HARPES to shed important new light on the electronic bulk structure of gallium manganese arsenide.

As a semiconductor, GaAs 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 1700 Kelvin," Fadley 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."

HARPES data on GaMnAs indicate that the ferromagnetism of dilute magnetic semiconductors from two distinct mechanisms

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 this model, ferromagnetism is mediated by electrons residing in the valence bands of GaAs 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 GaAs 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 GaAs valence-band maximum and the Fermi level, but the manganese states are also merged with the GaAs valence bands," Gray says. “This is evidence that the two mechanisms co-exist and both act to give rise to ferromagnetism."

Fadley 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 characterising these future materials."

Gray and Fadley conducted this study using a high intensity undulator beamline at the SPring8 synchrotron radiation facility in Hyogo, Japan. The plant is operated by the Japanese National Institute for Materials Sciences.

New HARPES studies are now underway at Berkeley Lab’s Advanced Light Source (ALS) using the Multi-Technique Spectrometer/Diffractometer endstation at the hard x-ray photoemission beamline (9.3.1).

More details of this work have been published in the paper, “Bulk electronic structure of the dilute magnetic semiconductor GaMnAs through hard X-ray angle-resolved photoemission,"  by Alexander Gray et al in Nature Materials, published online on 14th October 2012. DOI :10.1038/nmat3450

This research was primarily supported by the DOE Office of Science.


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