Enhancement, quantitative understanding, and tuning of spin diffusion length in copper
Date
2024
Authors
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Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Spintronics exploits electron’s intrinsic spin degree of freedom along with its fundamental charge for information processing and data storage. Spin current, the flow of electronic spin angular momentum, is integral to the functionalities of spintronics and has been a central subject of fundamental research. When a spin current is injected into a material, it decays exponentially over distance, a phenomenon known as the spin relaxation with the characteristic decay length being the spin diffusion length λ. Therefore, to maintain a robust spin current in spintronic circuity, a long spin diffusion length is highly desirable. ☐ In mesoscopic metallic channels made of Cu, Al, and Ag, λ has been shown to be substantial but is in general limited to 1 µm even at low temperatures. The direction toward further enhancement of λ is unclear. In addition, detailed quantitative understanding of the spin relaxation processes in these metals remains challenging, because phonons, impurities, surfaces, spin-orbital effects, grain boundaries, and Kondo effects all contribute to the spin relaxation and are non-trivial to separate. Furthermore, the electrical tunability of spin relaxation provides an intriguing avenue for achieving electrically tunable spin currents but has remained largely unexplored to date. In this dissertation, progress toward resolving these issues will be presented via three related studies that utilize nonlocal spin valve (NLSV) structures with mesoscopic spin transport channels made of Cu. ☐ In the first study, enhanced spin diffusion lengths up to 5 µm have been demonstrated in mesoscopic Cu channels at low temperatures. With a modest increase of the cross-sectional area of the Cu channels to 0.5 λ 0.5 µm2, the low temperature electrical conductivity values of channels are dramatically enhanced. Based on 14 structures fabricated in a single batch, we found an average spin diffusion length of 𝜆𝐶𝑢 = 3.2 ± 0.7 µm and an average spin relaxation time of 𝜏𝑠 = 120 ± 50 ps at 30 K. Substantial variations are observed among devices, and the spin diffusion length correlates well with the resistivity. The most conductive Cu channel in the batch yields 𝜆𝐶𝑢 = 5.3 ± 0.8 µm and 𝜏𝑠 = 250 ± 80 ps. These superior values exceed expectations for metals and can be attributed to reduced spin relaxation from grain boundaries and surfaces. ☐ In the second study, we have quantitatively established the relation between the Kondo spin relaxation rate 𝜏𝑠𝐾 −1 and the Kondo momentum relaxation rate 𝜏𝑒𝐾 −1 by using Cu channels that contain dilute Fe impurities. A linear relation between 𝜏𝑠𝐾 −1 and 𝜏𝑒𝐾 −1 is established under varying temperatures for any given device. Among 20 devices, however, 𝜏𝑠𝐾 −1 remains nearly constant despite variation of 𝜏𝑒𝐾 −1 by a factor of 10. This surprising relation can be understood by considering spin relaxation through overlapping Kondo screening clouds and supports the physical existence of the elusive Kondo clouds. ☐ In the third study, we demonstrate the electrical tunability of 𝜆𝑐𝑢 by using the ionic gating technique via a Li+ containing solid polymer electrolyte. At 5 K, 𝜆𝑐𝑢 is tuned reversibly between 670 nm and 410 nm and the Cu resistivity 𝜌𝑐𝑢 is tuned by 9% in relatively thin Cu channel with 65 nm thickness. The strength of the Kondo effect due to the Fe impurities is tuned by one order of magnitude. At room temperature, 𝜆𝑐𝑢 is tuned between 380 nm and 300 nm and 𝜌𝑐𝑢 is tuned by 7%. A gradual amplification of the tuning ranges by repeated gate cycling is observed and clearly suggests an electrochemical origin. We hypothesize that reversible diffusion of Li along the Cu grain boundaries leads to the tuning effects. Tunable spin relaxation in simple metals enriches functionalities in metal-based spintronics.
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Keywords
Spintronics, Spin current, Information processing, Nonlocal spin valve, Tuning effects