We investigate the impact of tunnel barrier thickness on electron spin dynamics in Fe/MgO/GaAs heterostructures using spin-resolved optical pump-probe spectroscopy. Comparison of the Larmor frequency between thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the inhomogeneous dephasing time, <i>T<sub>2</sub><sup>*</sup></i>, argues that inhomogeneity in the local effective hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs at low temperature and in the presence of an externally applied magnetic field. Further, this work lays the foundation for engineering both the exchange coupling and the free carrier spin dynamics in ferromagnet/semiconductor heterostructures, allowing for the exploration of dissipation and transport in the regime of dynamically-driven spin pumping.
Graphene's two dimensional nature and high surface sensitivity have led to fascinating predictions regarding induced spin-based phenomena through careful control of adsorbates on the graphene surface, including the extrinsic spin Hall effect, band gap opening, and induced magnetism. By taking advantage of atomic scale control provided by MBE, we have investigated submonolayer deposition of adsorbates and their interactions with graphene. Spin transport measurements performed in-situ during systematic introduction of atomic hydrogen demonstrated that hydrogen adsorbed on graphene forms magnetic moments that couple via exchange to the injected spin current. The effects of induced magnetic moments are evident in the non-local magnetoresistance and Hanle spin precession. Exchange coupling between the injected spin current and the induced moments impact the Hanle curves through an effective exchange field leading to new interpretations of Hanle spin precession data and analysis. Here we present a simple procedure in which Hanle curves can be reliably interpreted.
Enhanced spin injection efficiency and extended spin lifetimes are achieved in graphene spin valves. Spin injection
efficiency is enhanced via tunneling spin injection into graphene through an MgO barrier. A large nonlocal
magetoresistance of 130 Ω is observed for a single layer graphene (SLG) spin valve at room temperature (RT) with spin
injection efficiency of ~ 26-30%. Extended spin lifetimes are observed using tunneling contact to suppress the contact
induced spin relaxation. In SLG, spin lifetime as long as 771 ps is observed at RT. In bilayer graphene (BLG), we
observe the spin lifetime of 6.2 ns at 20 K, which is the longest value reported in any graphene spin valve. Furthermore,
contrasting spin relaxation behaviors are observed in SLG and BLG, which suggests that Elliot-Yafet spin relaxation
dominates in SLG at low temperatures, while Dyakonov-Perel spin relaxation dominates in BLG at low temperatures.6
This paper presents a novel design concept for spintronic nanoelectronics that emphasizes a seamless integration
of spin-based memory and logic circuits. The building blocks are magneto-logic gates based on a hybrid
graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle
and present a spin-based circuit design with smaller area, faster speed, and lower energy consumption than the
state-of-the-art CMOS counterparts. This design can also be applied in applications such as data compression,
coding and image recognition. In the proposed scheme, over 100 spin-based logic operations are carried
out before any need for a spin-charge conversion. Consequently, supporting CMOS electronics requires little
power consumption. The spintronic-CMOS integrated system can be implemented on a single 3-D chip. These
nonvolatile logic circuits hold potential for a paradigm shift in computing applications.
Spin transport in graphene devices has been investigated in both local and non-local spin valve geometries. In the nonlocal
measurement, spin transport and spin precession in single layer and bilayer graphene have both been achieved with
transparent Co contacts. Gate controllable non-local spin signal was also demonstrated in this system. For the local
graphite spin valve device, we report MR up to 12% for devices with tunneling contacts. We observe a correlation
between the nonlinearity of the I-V curve and the presence of local MR and conclude that tunnel barriers can be
employed to surmount the conductance mismatch problem in this system. These studies indicate that the improvement of
tunnel barriers on graphene, especially to inhibit the formation of pinholes, is an important step to achieve more efficient
spin injection into graphene.