Nanosecond spin lifetimes in bottom-up fabricated bilayer graphene spin-valves with atomic layer deposited Al2O3 spin injection and detection barriers

Abstract

We present spin transport studies on bi‐ and trilayer graphene non‐local spin‐valves which have been fabricated by a bottom‐up fabrication method. By this technique, spin injection electrodes are first deposited onto Si++/SiO2 substrates with subsequent mechanical transfer of a graphene/hBN heterostructure. We showed previously that this technique allows for nanosecond spin lifetimes at room temperature combined with carrier mobilities which exceed 20,000thinmathspacenormalcm2(Vsfalse)−1. Despite strongly enhanced spin and charge transport properties, the MgO injection barriers in these devices exhibit conducting pinholes which still limit the measured spin lifetimes. We demonstrate that these pinholes can be partially diminished by an oxygen treatment of a trilayer graphene device which is seen by a strong increase of the contact resistance area products of the Co/MgO electrodes. At the same time, the spin lifetime increases from 1 to 2 ns. We believe that the pinholes partially result from the directional growth in molecular beam epitaxy. For a second set of devices, we therefore used atomic layer deposition of Al2O3 which offers the possibility to isotropically deposit more homogeneous barriers. While the contacts of the as‐fabricated bilayer graphene devices are non‐conductive, we can partially break the oxide barriers by voltage pulses. Thereafter, the devices also exhibit nanosecond spin lifetimes.

Simulations on the Influence of Spatially Varying Spin Transport Parameters on the Measured Spin Lifetime in Graphene Non‐Local Spin Valves

“…In this case, the actual spin lifetime is underestimated by more than a factor of two. How- ever, the increase in spin lifetime for low gate voltages is similar to what is observed in experiments [10][11][12][17][18][19][20][21][22][23][24] . But the relative change in amplitude in the simulation is significantly smaller.…”

“…A sketch of the device structure is shown in Figure 1a. The electrode configuration is comparable to the layout used in some of our previous spin transport experiments 10,11,24 . The electrode width for the outer contacts reference electrodes is W = 1 µm and is alternating between W = 600 nm and W = 300 nm for the inner electrodes.…”

“…Hence, the assumption of uniform spin transport parameters may not be valid and, therefore, it is interesting to investigate the effect of spatially‐varying transport parameter. In addition, it is typically observed that the spin lifetime increases with increasing gate voltage, that is increasing charge carrier density . Although there are several theories which can explain such a behavior, none of them can unambiguously be proven up to now.…”

Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices

“…[35] With this procedure we are able to reproducibly build devices showing nanosecond spin lifetimes at room temperature even in case of short transport channel lengths of 2 − 3.5 µm. [36,37] In this Letter, we present spin transport studied in bottom-up fabricated graphene devices that pushes the measured room temperature spin lifetimes in single layer graphene by a factor of 6 from around 2 ns to 12.6 ns yielding spin diffusion lengths of 30.5 µm. These values exceeds current models for contact-induced spin dephasing ruling out spin absorption as the the dominant source of spin dephasing.…”

Li-doped graphene for spintronic applications

“…4,10 Among all of the carbon based structures, the hexagonal arrangement of carbon atoms in two dimensions, graphene, is particularly intriguing. The low efficiency of the spin relaxation of up to a nanosecond, [11][12][13] the scalability of the total spin, and graphene's stability up to room temperature make graphene based materials excellent candidates for spintronic devices. [14][15][16][17] Furthermore, it has been shown that magnetic graphene nanoakes (GNFs) have the potential for an extremely long spin relaxation and decoherence time, with weak coupling between electron spins, and long-range magnetic order.…”