Quantitative analysis of spin relaxation and spin transfer in mesoscopic nonlocal spin valves
| Author(s) | Cai, Yunjiao | |
| Date Accessioned | 2023-08-21T23:07:52Z | |
| Date Available | 2023-08-21T23:07:52Z | |
| Publication Date | 2019 | |
| SWORD Update | 2020-02-03T13:28:02Z | |
| Abstract | Spintronics is a new, emerging, and advancing academic research area in physics. Understanding the spin relaxation mechanism and spin transfer effect is crucial in spintronics. Accordingly, new methods are developed in this dissertation to determine the spin relaxation lengths accurately, and to explore the spin relaxation mechanism in mesoscopic Cu channels. In addition, novel nonlocal structures are designed and fabricated to generate efficient spin transfer switching with pure spin currents. ☐ A large number of mesoscopic nonlocal spin valve (NLSV) devices have been used to determine the Cu spin relaxation length and analyze the spin relaxation mechanism. Two different but related methods are used. In the first method, many NLSVs are fabricated on the same substrate under identical processing conditions, and the average Cu spin relaxation lengths at 10 K and 295 K are accurately determined. This method relies on Cu resistivity values determined directly from the NLSV devices. An iterative approach is used to take into account the dependence of injection/detection spin polarization on the size of the ferromagnetic electrodes. The probabilities of spin-flip for bulk defects and phonons in the mesoscopic Cu channels are shown to be <5 x 10‾4. ☐ However, experimental data suggest that spin relaxation length and resistivity could both vary even for NLSVs fabricated under identical conditions. Therefore, a second method is developed to extract a distinct value of spin relaxation length from each individual NLSV. A dependence of Cu spin relaxation length on the Cu resistivity can then be established from a large number (>100) of NLSVs. Such a dependence is very important because it reflects the underlying mechanism of spin relaxation. By changing the dimensions of the Cu channels and measurement temperatures, the Cu resistivity is tuned by more than one order of magnitude. By analyzing the relationship between Cu spin relaxation length and Cu resistivity, we conclude that the spin relaxation can be described by the Elliott-Yafet model. However, the spin-flip probabilities at surfaces are substantially higher than those in the bulk. Large values of spin relaxation lengths (∼ 2.0 μm at 10 K and ∼ 700 nm at 295 K) can be achieved in Cu channels with lower resistivity. This is encouraging for the prospect of using mesoscopic Cu wires as spin transport channels. ☐ Another theme of this dissertation is nonlocal spin transfer switching with pure spin currents. Spin transfer effects in nonlocal lateral structures are pivotal in realizing spintronic devices such as all-spin logic with built-in memory. Efficient nonlocal spin transfer switching is achieved by using separately tailored polarizer and free-layer interfaces. A low-resistance oxide interface with larger area is used between the ferromagnetic polarizer and the Cu channel to achieve substantial spin polarization with low injection charge current density. An ohmic interface with smaller area is used between the ferromagnetic free-layer and the Cu channel to facilitate the absorption of spin current with high areal density. ☐ A feasibility study is first conducted to demonstrate that the spin polarization provided by a low-resistance oxide interface with relatively large area (330 nm $\times$ 170 nm) can be sufficiently high. Subsequently, nonlocal spin transfer devices are fabricated and characterized. By designing the polarizer and free-layer interfaces separately, we achieve reversible and bistable spin transfer switching with a modest charge current density of ∼ 6 x 10^6 A·cm‾2 between 100 K and 150 K. With potentials for improvements, nonlocal spin transfer structures can be as efficient and robust as the nanopillar spin transfer structures. | |
| Advisor | Ji, Yi | |
| Degree | Ph.D. | |
| Department | University of Delaware, Department of Physics and Astronomy | |
| DOI | https://doi.org/10.58088/670k-aj87 | |
| Unique Identifier | 1440234632 | |
| URL | https://udspace.udel.edu/handle/19716/33181 | |
| Language | en | |
| Publisher | University of Delaware | |
| URI | https://www.proquest.com/dissertations-theses/quantitative-analysis-spin-relaxation-transfer/docview/2318150246/se-2?accountid=10457 | |
| Keywords | Spin relaxation | |
| Keywords | Spin transfer | |
| Keywords | Spin valves | |
| Title | Quantitative analysis of spin relaxation and spin transfer in mesoscopic nonlocal spin valves | |
| Type | Thesis |