Novel ways to use graphene and topological insulators in spintronics

03/04/2018 - 14:00
Dr. Olaf M.J. van 't Erve Materials and Sensors Branch, Naval Research Laboratory, Washington, D.C. USA
Sala de Seminários José Roberto Leite - Edifício Alessandro Volta (bloco C)

Graphene’s high carrier mobility and low spin-orbit coupling make it one of the most promising materials for spin transport.

 Graphene has been widely studied for its extraordinarily high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. Theory predicts that the out of plane properties of the graphene-ferromagnet interface have spin filtering effects.

  We have fabricated arrays of ferromagnet-graphene-ferromagnet junctions that exhibit very low resistance and significant magnetoresistance at room temperature.

The junctions exhibit the negative magnetoresistance expected from the minority spin filtering predicted to occur in certain graphene|FM structures.  We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel and a tunnel barrier majority spin channel, and extract spin polarization of at least 80% in the graphene layer.

 We will also demonstrate a homoepitaxial tunnel barrier device in which graphene serves as both the tunnel barrier and the high mobility transport channel. We demonstrate high spin injection efficiency and lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the Hanle effect.  The functionalized graphene tunnel barrier enables us to probe the intrinsic spin properties of graphene, and offers an elegant solution for the conductivity mismatch problem for spin injection in 2-D materials and semiconductors.

 The third topic covers direct electrical detection of current-induced spin in Topological insulator & Rashba 2DEGs.One of the most striking properties of topological insulators is that of spin-momentum locking -- the spin of the TI surface state lies in-plane, and is locked at right angle to the carrier momentum.  An unpolarized charge current should thus create a net spin polarization whose amplitude and orientation are controlled by the charge current. We have shown the first direct electrical detection of spin-momentum locking in a transport measurement, where the bias current induced spin polarization in molecular beam epitaxy grown Bi2Se3 is measured as a voltage on a ferromagnetic (FM) metal tunnel barrier surface contact.