1. Choreographing a Whirling Dervish
How spinning electrons communicate across interfaces
Spintronics is the study of how to use the fact that electrons spin (similar to the daily rotation of the earth) to design next-generation devices for information technology
A central challenge in this field is to understand how different spins communicate. For example, in a magnetic material like iron more electrons spin clockwise than counterclockwise. In contrast, semiconductors used in computing (like silicon and gallium arsenide) are non-magnetic and have equal numbers of electrons spinning in each direction. Many spintronic devices rely on communicating the choreography of the electrons in magnetic materials (iron) to a non-magnetic material (gallium arsenide).
In this study IRG-2 discovered that this choreography is actually controlled by a third dance partner, the spin of the protons and neutrons of the gallium and arsenic atoms of the semiconductor. This new understanding will play an important role in the design of future spintronic devices.
We demonstrate that electron spin relaxation in GaAs in the proximity of a Fe/MgO layer is dominated by interaction with an exchange-driven hyperfine field at temperatures below 60 K. Temperature-dependent spin-resolved optical pump-probe spectroscopy reveals a strong correlation of the electron spin relaxation with carrier freeze out, in quantitative agreement with a theoretical interpretation that at low temperatures the free carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. As the regime of large nuclear inhomogeneity is accessible in these heterostructures for magnetic fields < 3kG, inferences from this result resolve a long-standing and contentious dispute concerning the origin of spin relaxation in GaAs at low temperature when a magnetic field is present. Further, this improved fundamental understanding clarifies the importance of future experiments probing the time-dependent exchange interaction at a ferromagnet/semiconductor interface and its consequences for spin dissipation and transport during spin pumping.
Full reference: “Exchange-driven spin relaxation in ferromagnet/oxide/semiconductor heterostructures,” Yu-Sheng Ou, Yi-Hsin Chiu, N. J. Harmon, Patrick Odenthal, Matthew Sheffield, Michael Chilcote, R. K. Kawakami, M. E. Flatté and E. Johnston-Halperin, Physical Review Letters 116, (2016).
Figure: Top two panels show a 2D slice of a gallium arsenide layer with electron position shown in yellow and the direction of their spin shown as red arrows for low (left panel) and high (right panel) temperature. Once the electrons are hot enough, they evaporate out of local traps (bottom panels) and homogeneously distribute throughout the material, evenly spreading the spin.
Center for Emergent Materials—an NSF MRSEC
Award Number DMR
Partially supported by DOE DE-FG02-03ER46054
Y.-S. Ou, Y.-H. Chiu, M. Sheffield, M. Chilcote, E. Johnston-Halperin (Ohio State), P. Odenthal, R. K. Kawakami (UCR/Ohio State), N. J. Harmon, M. E. Flatté (U Iowa), Phys. Rev. Lett. 116, (2016)