Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields

Abstract

Abstract:Two dimensional (2D) materials provide a unique platform to explore the full potential of magnetic proximity driven phenomena, which can be further used for applications in next generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene/ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the non-local spin signals in graphene, which is due to spin relaxation by thermally-induced transverse fluctuations of the FMI magnetization. 2Use of the spin degree of freedom of electrons is poised to revolutionize next generation devices for logic [1] and memory [2] applications. Manipulation of spin current, using either a small electric or magnetic field, is the essential operation of such a device and is required to exploit the full versatility of spin related phenomena. Spins in graphene are of particular interest because of the fact that spins can propagate over large distances due to small spin-orbit (SO) coupling and negligible hyperfine interaction [3,4]. However, the absence of a strong SO field in graphene also means that spins in graphene cannot be manipulated by an external applied electric field [5]. In general, spins in graphene are manipulated by an out-of-plane magnetic field [6,7], known as Hanle spin precession, requiring large fields which are not viable for applications. An alternative route for efficient spin manipulation is to use the magnetic proximity effect of an adjacent ferromagnetic insulator (FMI). Two dimensional (2D) materials, like graphene, provide a unique platform to explore the proximity-induced phenomena as these effects are expected to be the strongest in 2D materials. There has been a great deal of interest to study the proximity effect induced changes in the electrical [8], optical [9,10] and spin [11] related properties of low dimensional materials. This research direction is further propelled by recent progress in the experimental techniques to assemble clean van der Waals heterostructures of 2D materials or mechanically transfer 2D samples onto arbitrary materials [12,13]. Recently, magnetic proximity effects in graphene/FMI heterostructures has been explored by charge transport measurements: (1) demonstration of ferromagnetism in graphene coupled to yttrium iron garnet (YIG), [14] and (2) large magnetic exchange fields experienced by charge carriers in graphene/EuS heterostructures [15...

Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

“…We emphasize, however, that we cannot measure values this small directly in the spin valve device geometry. It is therefore possible that the actual product is larger than inferred from our model and that some other mechanism, such as proximityinduced magnetism [30][31][32] or enhanced spin-orbit coupling 33 at the graphene/metal interface, is leading to a larger interfacial spin relaxation rate. Our measurement cannot distinguish between interfacial spin relaxation in the presence of a larger product and spin absorption.…”

Electrical spin injection, transport, and detection in graphene-hexagonal boron nitride van der Waals heterostructures: progress and perspectives

“…These include, SrTiO 3 (STO) substrate for an epitaxial growth of highly spin polarized La 0.67 Sr 0.33 MnO 3 (LSMO) contacts on graphene 31 , Y 3 Fe 2 (FeO 4 ) 3 (YIG) substrate as a magnetically proximity coupling ferromagnetic insulator 32,33 , and recently used transition metal dichalcogenide (TMDC) substrates to proximity induce spin-orbit coupling in graphene 34,35 .…”

Electrically tunable exchange splitting in bilayer graphene on monolayer Cr2X2Te6 with X = Ge, Si, and Sn

“…Several ferromagnetic insulators (FMI), like EuO and Yttrium-Iron-Garnet, have been considered in the context of graphene spintronics [22,[31][32][33][34]. However transition-metal trichalcogenides Cr 2 Si 2 Te 6 , Cr 2 Ge 2 Te 6 (CGT), and transition-metal trihalides CrI 3 , and their monolayers are now extensively discussed in literature as promising materials for low-dimensional spintronics because of their FMI ground state properties [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53].…”