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Femtosecond spin-to-charge current conversion in FeCo/graphene nanobilayers excited by femtosecond optical laser pulse

Ivan Komissarov, Assistant Research Professor, Laboratory for Laser Energetics

Wegmans Hall 1400

Spintronics is a type of electronics where not only charge but also spin is used to manipulate electrons. Giant magnetoresistance (GMR), tunnelling magnetoresistance (TMR), Spin-Transfer Torque (STT) are usually referred as classical spintronic phenomena and they find their applications in hard drives, and magnetic and STT memories, respectively. In these classical spintronic phenomena, magnetic materials are used to spin-polarize the current. In early 2000s, it was experimentally demonstrated the possibility of creating pure spin-polarized currents by using spin–orbit coupling (SOC), theoretically predicted in 1971 by Dyakonov and Perel. In non-magnetic materials with the large SOC (usually heavy metals) this phenomenon of conversation of the charge current to spin current is known as the Spin Hall Effect (SHE), while its opposite manifestation, i.e., conversion of the spin current into charge current is known as the Inverse Spin Hall Effect (ISHE) and the latter is implemented in modern spintronic devices. More efficient conversion between spin to charge currents, as compared to heavy metals, can be obtained in two-dimensional electron gas (2DEG) systems, e.g., at the surface/interface of topological insulators (TI) or, at so-called, Rashba interfaces. Such 2DEGs are characterized by the Fermi contour with helical locking between the spin and momentum degrees of freedom (spin texture). In 2D materials, SHE and ISHE effects are analogous to the effects referred as the Rashba-Edelstein Effect (REE) and Inverse Rashba-Edelstein Effect (IREE), respectively. Graphene is a natural 2DEG system with a unique band structure and optical properties, and it has been demonstrated as a very attractive material in a huge number of applications. However, its extremely low, on the order of few μeV, intrinsic spin-orbit coupling (SOC) limits its applicability in spintronic applications, where the manipulation of spin is required. To overcome this drawback several approaches to enhance the strength of SOC have been proposed. They include addition of small amounts of covalently bound hydrogen atoms, fluorine functionalization, or decorating graphene with heavy ion atoms. Additionally, SOC in graphene can be increased by placing graphene in contact with a 3D ferromagnetic material via hybridization between the p states of graphene and 3d states of a ferromagnet. This approach has a special interest from the perspective of using ferromagnet/graphene bilayers as transient THz spintronic emitters. This talk presents generation of THz bandwidth (single picosecond in duration) electromagnetic transients from a FeCo/graphene heterostructure triggered by 100-fs-wide optical laser pulses. It will be shown that the transient THz emission originates form the IREE, which implies the active role of FeCo in SOC enhancement in graphene. The results of circularly polarized light experiments probing the peculiarities of the graphene band structure spin texture and its impact on the ferromagnetic exchange-coupling it will be also discussed.

Ivan Komissarov is an Assistant Research Professor at ECE department at the University of Rochester. His current research focuses on ultrafast carrier dynamics in semiconductors and designing SFQ-based redout system for superconducting single photon detectors. Prior of that he was a Leading Researcher and Assistant Professor (Lecturer) at Belarusian State University Informatics and Radioelectronics, Minsk Belarus. He earned his Ph.D. from Institute of Physics Polish Academy of Science, Warsaw Poland, in 2004. Ivan is co-author of more than 60 peer reviewed articles and 3 book chapters in the field of solid-state physics.

Refreshments will be provided.