Walid Amamou

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.

Large area epitaxial germanane for electronic devices

We report the synthesis and transfer of epitaxial germanane (GeH) onto arbitrary substrates by electrochemical delamination and investigate its optoelectronic properties. GeH films with thickness ranging from 1 to 600 nm (2–1000 layers) and areas up to ~1 cm2 have been reliably transferred and characterized by photoluminescence, x-ray diffraction, and energy-dispersive x-ray spectroscopy. Wavelength dependent photoconductivity measurements on few-layer GeH exhibit an absorption edge and provide a sensitive characterization tool for ultrathin germanane materials. The transfer process also enables the possibility of integrating germanane into vertically stacked heterostructures.

Contact Induced Spin Relaxation in Graphene Spin Valves with Al2O3 and MgO Tunnel Barriers

We investigate spin relaxation in graphene by systematically comparing the roles of spin absorption, other contact-induced effects (e.g. fringe fields, etc.), and bulk spin relaxation for graphene spin valves with MgO barriers, Al2O3 barriers, and transparent contacts. We obtain effective spin lifetimes by fitting the Hanle spin precession data with two models that include or exclude the effect of spin absorption. Results indicate that additional contact-induced spin relaxation other than spin absorption dominates the contact effect. For tunneling contacts, we find reasonable agreement between the two models with median discrepancy of ~20% for MgO and ~10% for Al2O3.

Epitaxial Co-Deposition Growth of CaGe

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy (MBE) and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 {\deg}C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy (AFM) shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.

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The quality of the tunnel barrier at the ferromagnet/graphene interface plays a pivotal role in graphene spin valves by circumventing the impedance mismatch problem, decreasing interfacial spin dephasing mechanisms and decreasing spin absorption back into the ferromagnet. It is thus crucial to integrate superior tunnel barriers to enhance spin transport and spin accumulation in graphene. Here, we employ a novel tunnel barrier, strontium oxide (SrO), onto graphene to realize high quality spin transport as evidenced by room-temperature spin relaxation times exceeding a nanosecond in graphene on silicon dioxide substrates. Furthermore, the smooth and pinhole-free SrO tunnel barrier grown by molecular beam epitaxy (MBE), which can withstand large charge injection current densities, allows us to experimentally realize large spin accumulation in graphene at room temperature. This work puts graphene on the path to achieve efficient manipulation of nanomagnet magnetization using spin currents in graphene for logic and memory applications.

Films by Molecular Beam Epitaxy for Large Area Germanane

Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which has demonstrated promising results in a variety of material systems. This technique utilizes the resonant precession of magnetization in a ferromagnet to inject spin into an adjacent non-magnetic material. Spin pumping into graphene is attractive on account of its exceptional spin transport properties. This article reports on FMR characterization of cobalt grown on chemical vapor deposition graphene and examines the validity of linewidth broadening as an indicator of spin pumping. In comparison to cobalt samples without graphene, direct contact cobalt-on-graphene exhibits increased FMR linewidth—an often used signature of spin pumping. Similar results are obtained in Co/MgO/graphene structures, where a 1 nm MgO layer acts as a tunnel barrier. However, magnetometry, magnetic force microscopy, and Kerr microscopy measurements demonstrate increased magnetic disorder in cobalt grown on graphene, perhaps due to changes in the growth process and an increase in defects. This magnetic disorder may account for the observed linewidth enhancement due to effects such as two-magnon scattering or mosaicity. As such, it is not possible to conclude successful spin injection into graphene from FMR linewidth measurements alone.

Strontium Oxide Tunnel Barriers for High Quality Spin Transport and Large Spin Accumulation in Graphene

Graphene has been proposed for novel spintronic devices due to its robust and efficient spin transport properties at room temperature. Some of the most promising proposals require current-based readout for integration purposes, but the current-based detection of spin accumulation has not yet been developed. In this work, we demonstrate current-based detection of spin transport in graphene using a modified nonlocal geometry. By adding a variable shunt resistor in parallel to the nonlocal voltmeter, we are able to systematically cross over from the conventional voltage-based detection to current-based detection. As the shunt resistor is reduced, the output current from the spin accumulation increases as the shunt resistance drops below a characteristic value R*. We analyze this behavior using a one-dimensional drift-diffusion model, which accounts well for the observed behavior. These results provide the experimental and theoretical foundation for current-based detection of nonlocal spin transport.