Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes

Abstract

In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (R c A) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large R c A values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20000 cm 2 /(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.

Prospects of spintronics based on 2D materials

“…To enhance the spin‐injection efficiency, it was proposed to use an insulating oxide such as Al 2 O 3 or MgO as a tunnel barrier between the FM electrode and graphene . This is effective in tuning interfacial spin‐dependent resistivity and improving the spin injection .…”

“…This is effective in tuning interfacial spin‐dependent resistivity and improving the spin injection . However, a new challenge is to grow a uniform layer of oxide between graphene and the electrode . Pinholes in the oxide barrier and clumps of the insulating material on graphene were observed which lead to reduction in spin injection efficiency .…”

“…However, a new challenge is to grow a uniform layer of oxide between graphene and the electrode . Pinholes in the oxide barrier and clumps of the insulating material on graphene were observed which lead to reduction in spin injection efficiency . Some improvements were made by using alternative sample growth technique or by incorporating an interfacial TiO 2 layer, and with the latter approach, the measured spin injection efficiency reached ~30% .…”

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

“…Since oxidation of the contacts has a large impact on the measured spin lifetimes and a flat d V /d I characteristic of the contacts is observed even after oxidation, we conclude that the spin injection and detection process is still governed by conducting pinholes. These pinholes can occur either due to an island formation of the MgO during growth or residual particles on the substrate which lead to an uneven Co surface (). If the resulting corrugation is larger than the MgO layer thickness, the side of the corrugation may not be completely covered by the directional MBE growth of MgO which favors metallic contact to graphene, i.e., a pinhole.…”

Measuring anisotropic spin relaxation in graphene

“…To increase the spin lifetime in graphene, it is necessary to understand the limiting mechanisms to be able to design effective countermeasures. Several sources for additional spin relaxation in graphene have been proposed [2]: impurities (adatoms) [14], the substrate [5], polymer residues [15][16][17], ripples [18], resonant magnetic scattering at magnetic impurities [19,20], and contact induced spin relaxation [21][22][23]. To determine which is the dominant effect, experiments focused on finding a correlation between the momentum scattering time τ p of electrons and their spin relaxation time τ s [24][25][26].…”