Scientists at the University of California Riverside took an unusual approach to identify the strength of electron spin interactions with optical phonons in nickel oxide (NiO) crystals, according to a new report published in the journal Applied Physics Letters.
The work could have major ramifications for spintronic devices, which use signals transferred by spin waves, propagated disturbances in the ordering of magnetic materials that are transferred like a row of falling dominoes. NiO is an appealing material for the creation of these devices.
According to their report, the study team used ultraviolet Raman spectroscopy to analyse how spin ordering has an effect on the energies of phonons contained in these materials. Phonons are energies related to the vibrations of ions in the crystal lattice of materials. Phonons are capable of interacting with electrons and their spins, which can lead to energy dispersion. The creation of practical spintronic devices requires precisely knowing the strength of the electron spin interactions with phonons.
“Despite the fact that nickel oxide has been studied for many years, mysteries remained,” study author Alexander Balandin, a professor of electrical and computer engineering at UC Riverside, said in a news release. “Our results shed light on some of the long-standing puzzles surrounding this material, reveling unusual spin–phonon coupling.”
The research team said the key to their investigation was the combination of Raman spectroscopy and an ultraviolet laser, rather than a conventional laser.
“The trick worked because relevant phonon peaks can be seen with much better resolution in the spectrum of nickel oxide under ultraviolet laser excitation,” Balandin said.
The analysis of the spin-phonon interaction will have major significance for growth and development of spintronic devices, the study team said. Unlike customary electronic transistors, spintronic devices translate and communicate data with spin currents or waves, rather than electric currents. Because of this, electrically insulating magnetic materials, like nickel oxide, might be used for memory storage and data processing.
Furthermore, by not using electrical currents, spintronic devices possess massive potential for ultra-fast and low-energy-dissipation functionality. Interaction with phonons is one of many energy transference mechanisms in spintronics. The information reported by the study scientists may help in refining the design of spintronic devices by modifying phonon qualities and the way phonons relate to electron spins.
We hope that our results will contribute to better understanding of mechanisms of spin wave interactions with the crystal lattice vibrations, and energy loss channels in nickel oxide devices. The next step will be investigation of the spin–phonon interaction in nanoscale thin films and structures made of this important antiferromagnetic material.
Alexander Balandin, Study Author
The new study comes just after a paper published last month by the journal Nature Communications revealed it is possible to manipulate an electric current using spinning waves of light. According to that report, researchers were able to manipulate a flow of electrons with ‘spinning’ light due to the spins of those electrons. The study’s use of spinning, or ‘circularly polarized’, light resulted in a spin-polarized electric current, which could be used as a current source for spintronic devices.
The study team also found that the reversal of the light path in turn reverses the direction of the current and its spin polarization. The study team said they were able to rule out other potential causes of the detected effect, like heat created by the light.
"Our research bridges two important fields of nanotechnology: spintronics and nanophotonics. It is fully integrated with a silicon photonic circuit that can be manufactured on a large scale and has already been widely used in optical communication in data centers," said researcher Li He in a news release.
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