Faster, smaller electronics on the way?

The University of California, Davis team has, for the first time, been able to look inside gallium manganese arsenide - a type of material known as a dilute magnetic semiconductor – using a novel technique called hard X-ray angle resolved photoemission spectroscopy, or HARPES. First discovered at the university in 2011 by researcher Charles Fadley, HARPES uses Einstein's photoelectric effect to study materials. "If you bombard atoms with light particles - photons - you knock out electrons, known as photoelectrons, which fly out at precise angles, energies and spins depending on the structure of the material," he explained. For many years, researchers have used 'soft', low energy X-rays as the photon source, but the technique can look only at the top nanometre of a material - about five atoms deep. Fadley and doctoral graduate Alexander Gray therefore developed a method that uses 'hard', high energy X-rays to look much deeper inside a material, to a depth of about 40 to 50 atoms. The researchers selected gallium manganese arsenide, a widely used semiconductor, because of its potential in technology. "Add a few percent of manganese atoms to the mix, and in the right conditions - a temperature below 170 Kelvin – and it becomes ferromagnetic like iron, with all of the individual manganese atomic magnets lined up in the same direction," Fadley noted. "This class of materials is called dilute magnetic semiconductors." While there are two competing ideas to explain how gallium manganese arsenide becomes magnetic at certain temperatures, the HARPES study shows that both mechanisms contribute to the magnetic properties. "We now have a better fundamental understanding of electronic interactions in dilute magnetic semiconductors that can suggest future materials," concluded Fadley. "HARPES should provide an important tool for characterising these and many other materials in the future."