Scientists at the U.S. Naval Research Laboratory (NRL) have created a new type of room-temperature tunnel device structure in which the tunnel barrier and transport channel are both made of graphene. Such functionalized homoepitaxial structures can be seen as an elegant approach for realization of graphene-based spintronic devices.
The scientists show that hydrogenated graphene acts as a tunnel barrier on another layer of graphene for charge and spin transport. They demonstrate spin-polarized tunnel injection through the hydrogenated graphene, and lateral transport, precession and electrical detection of pure spin current in the graphene channel. The team further reports higher spin polarization values than found using more common oxide tunnel barriers, and spin transport at room temperature.
The scientists used CVD to grow and then sequentially deposit a four-layer graphene stack. They then hydrogenate the top few layers so that they serve as a tunnel barrier for both charge and spin injection into the lower graphene channel. They deposit ohmic (gold) and ferromagnetic permalloy (red) contacts, forming a non-local spin valve structure. When the scientists apply a bias current between the left two contacts, a spin-polarized charge current tunnels from the permalloy into the graphene transport channel, generating a pure spin current that diffuses to the right. This spin current is detected as a voltage on the right permalloy contact that is proportional to the degree of spin polarization and its orientation. The vectorial character of spin (compared to the scalar character of charge) provides additional mechanisms for the control and manipulation needed for advanced information processing.
The NRL team demonstrated the higher spin injection efficiency (16.5%) than most previous graphene spin devices, determined spin lifetimes with the Hanle effect, and observed only a 50% loss in spin valve signal from 10 K to room temperature (left graph).
Hydrogenation of graphene offers an alternative method to achieve a homoepitaxial tunnel barrier on graphene. In contrast with fluorination and plasma treatments, the chemical hydrogenation process provides a rapid, gentle and more stable functionalization with much higher hydrogen coverage. Moreover, hydrogenated graphene could be magnetic, which could be used to control spin relaxation in the graphene.