Abstract:We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.* Author to whom correspondence should be addressed: kawakami.15@osu.edu 2 Graphene is a promising spin channel material for next generation spintronic devices due to the experimental demonstration of long spin diffusion lengths at room temperature 1-3 and theoretical predictions of long spin relaxation times 4,5 arising from the weak spin-orbit and hyperfine couplings 5,6 .However, experimentally measured spin relaxation times [1][2][3]7,8 in graphene are orders of magnitude shorter than theoretically predicted 4,5 . In graphene spin valves, the tunnel barrier plays a crucial role for spin injection by circumventing the problem of impedance mismatch 9 between graphene and the ferromagnetic electrodes. As demonstrated by Han et. al. 8 , high quality tunnel barriers are critical for obtaining higher spin relaxation times (τ s ) in graphene because barriers with pinholes or rough surface morphology can cause additional contact-induced spin relaxation, which has received a great deal of interest recently. [10][11][12][13][14] As opposed to growing oxide tunnel barriers on graphene, a thin insulating twodimensional (2D) van der Waals material can also be used as a tunnel barrier. A particular material of interest is single (or few) layer h-BN because of its various suitable properties 15 : large energy band gap ~5.97 eV, high crystallinity, spin filtering 16 , absence of pinholes and dangling bonds, atomic lattice similar to graphene, and chemical stability at ambient conditions. In addition, atomically clean vertical heterostructures of h-BN/graphene can be mechanically assembled using polymer-based transfer techniques 17,18 . The first experimental report demonstrating spin injection into graphene using a monolayer h-BN tunnel barrier showed τ s less than 100 ps 19 . This was followed by the work of Kamalakar et. al. 20,21 and Fu et. al. 22 , which used chemically grown h-BN barriers, yielding τ s ~ 500 ps. Another recent study using an encapsulated geometry 23 with graphene sandwiched between a thick bottom layer of h-BN and a monolayer of h-BN on top showed τ s less than 200 ps. As evident from these studies, graphene spin valve devices ...

We report the experimental demonstration of a magnetologic gate built on graphene at room temperature.This magnetologic gate consists of three ferromagnetic electrodes contacting a single layer graphene spin channel and relies on spin injection and spin transport in the graphene. We utilize electrical bias tuning of spin injection to balance the inputs and achieve "exclusive or" (XOR) logic operation. Furthermore, simulation of the device performance shows that substantial improvement towards spintronic applications can be achieved by optimizing device parameters such as device dimensions. This advance holds promise as a basic building block for spin-based information processing. 1Spintronics is an approach to electronics that utilizes the spin of the electron for information storage and processing [1][2][3]. By providing the ability to integrate logic with nonvolatile storage in ferromagnetic data registers, spintronics could greatly reduce the power consumption in logic circuits and go beyond traditional CMOS architectures. The demonstration of spin injection into semiconductors [4,5] prompted a variety of proposals for spintronic devices taking advantage of the tunable nature of semiconductors [6][7][8][9][10][11]. Among these was a proposal by Dery and Sham [12] for an "exclusive or" (XOR) gate based on spin accumulation in a semiconductor channel contacted by three ferromagnetic (FM) electrodes (see Fig. 1(a)). In this device, the magnetization directions of the first two FM electrodes represent the logic inputs ('0' and '1'), and spin injection from these input electrodes generates a current through the third FM electrode which represents the logic output. Subsequently, a more general proposal was developed that combines two such XOR gates to form a universal reconfigurable magnetologic gate (MLG) [8]. This MLG consists of five FM electrodes and the logic operation is represented by OR(XOR(A, B), XOR(C, D)), where A, B, C and D are the four logical input states and the fifth FM electrode reads the output. This can also be utilized as a universal two-input gate, where B and D define the gate operation (e.g. NAND, OR) and A and C represent the two inputs. The experimental discoveries of room temperature spin transport [13] and efficient spin injection into graphene [14] provided an ideal materials platform to realize such MLG devices. Motivated by these advances, the theoretical performance of graphene-based MLG was analyzed and novel spintronic circuits for rapid parallel searching were developed [15]. However, despite these extensive advances in the device modeling and spintronic circuit design, the experimental demonstration of the proposed three-terminal XOR and fiveterminal universal MLG has been lacking.In this Letter, we experimentally demonstrate the proposed three-terminal XOR magnetologic gate operation in a graphene spintronic device at room temperature. By carefully tuning the bias current between the two input electrodes, and an offset voltage in the detection loop, a clear non-ze...

We report the synthesis and transfer of epitaxial germanane (GeH) onto arbitrary substrates by electrochemical delamination and investigate its optoelectronic properties. GeH films with thickness ranging from 1 to 600 nm (2-1000 layers) and areas up to ∼1 cm 2 have been reliably transferred and characterized by photoluminescence, x-ray diffraction, and energy-dispersive x-ray spectroscopy. Wavelength dependent photoconductivity measurements on few-layer GeH exhibit an absorption edge and provide a sensitive characterization tool for ultrathin germanane materials. The transfer process also enables the possibility of integrating germanane into vertically stacked heterostructures.

We investigate spin relaxation in graphene by systematically comparing the roles of spin absorption, other contact-induced effects (e.g. fringe fields, etc.), and bulk spin relaxation for graphene spin valves with MgO barriers, Al 2 O 3 barriers, and transparent contacts. We obtain effective spin lifetimes by fitting the Hanle spin precession data with two models that include or exclude the effect of spin absorption. Results indicate that additional contact-induced spin relaxation other than spin absorption dominates the contact effect. For tunneling contacts, we find reasonable agreement between the two models with median discrepancy of ~20% for MgO and ~10% for Al 2 O 3 .

We observe the magnetic proximity effect (MPE) in Pt/CoFe 2 O 4 bilayers grown by molecular beam epitaxy (MBE). This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalousHall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe 2 O 4 . By demonstrating MPE in Pt/CoFe 2 O 4 , these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields.* equal contributions **email: kawakami.15@osu.edu 2 Spin manipulation inside a nonmagnetic (NM) material using internal effective fields (spin orbit or exchange) is a very promising avenue toward the realization of next generation spintronic devices (spin transistors, magnetic gates, etc.) [1,2]. In particular, magnetic proximity effect (MPE) at the interface of a NM spin channel and a ferromagnetic insulator (FMI) is of great importance for generating exchange fields and induced ferromagnetism in the NM layer. Recently, spin manipulation by MPE has been realized in experiments that modulate spin currents in graphene on YIG (yttrium iron garnet) [3,4]. In addition, proximity exchange fields induced by FMI have been observed for graphene and monolayer transition transition metal dichalcogenides [5,6].Among FMIs, members of the spinel ferrite family (MFe 2 O 4 , with M=Co, Ni, etc.) are attractive because their magnetic properties can be tuned by alloy composition [7,8] as well as epitaxial strain [9][10][11]. In particular, CoFe 2 O 4 (CFO) is a hard ferrimagnetic insulator which exhibits high Curie temperature (728 K), spin-filtering properties [12][13][14], magneto-electric switching [15], and is readily integrated with practical spintronic materials (MgO, Fe, Cr). Unfortunately, all previous experiments have failed to observe MPE using CFO. To test for MPE in NM/FMI systems, Pt is widely used as the NM material due its closeness to fulfilling the Stoner criteria and thus allowing it to become ferromagnetically ordered at the interface with the FMI. Initial studies of Pt/CFO grown by pulsed laser deposition (PLD) utilized magnetotransport measurements and observed no MPE in the Pt layer [16]. Subsequent transport and element-specific magnetization measurements by x-ray magnetic circular dichroism (XMCD) also found no evidence for induced ferromagnetism in the Pt layer [17][18][19][20] contributing to a growing consensus that CFO cannot be used to obtain MPE. However, because the length scale of exchange interactions is on the order of angstroms, it should...

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe 2 films on Ge(111) substrates by molecular beam epitaxy (MBE) and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe 2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 °C. Finally, the CaGe 2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy (AFM) shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate.Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.2

The quality of the tunnel barrier at the ferromagnet/graphene interface plays a pivotal role in graphene spin valves by circumventing the impedance mismatch problem, decreasing interfacial spin dephasing mechanisms and decreasing spin absorption back into the ferromagnet. It is thus crucial to integrate superior tunnel barriers to enhance spin transport and spin accumulation in graphene. Here, we employ a novel tunnel barrier, strontium oxide (SrO), onto graphene to realize high quality spin transport as evidenced by room-temperature spin relaxation times exceeding a nanosecond in graphene on silicon dioxide substrates. Furthermore, the smooth and pinhole-free SrO tunnel barrier grown by molecular beam epitaxy (MBE), which can withstand large charge injection current densities, allows us to experimentally realize large spin accumulation in graphene at room temperature. This work puts graphene on the path to achieve efficient manipulation of nanomagnet magnetization using spin currents in graphene for logic and memory applications.

Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which has demonstrated promising results in a variety of material systems. This technique utilizes the resonant precession of magnetization in a ferromagnet to inject spin into an adjacent non-magnetic material. Spin pumping into graphene is attractive on account of its exceptional spin transport properties. This article reports on FMR characterization of cobalt grown on CVD graphene and examines the validity of linewidth broadening as an indicator of spin pumping. In comparison to cobalt samples without graphene, direct contact cobalt-on-graphene exhibits increased FMR linewidth-an often used signature of spin pumping. Similar results are obtained in Co/MgO/graphene structures, where a 1nm MgO layer acts as a tunnel barrier. However, SQUID, MFM, and Kerr microscopy measurements demonstrate increased magnetic disorder in cobalt grown on graphene, perhaps due to changes in the growth process and an increase in defects. This magnetic disorder may account for the observed linewidth enhancement due to effects such as two-magnon scattering or mosaicity. As such, it is not possible to conclude successful spin injection into graphene from FMR linewidth measurements alone.Microwave-driven FMR has been shown to be a viable means for injecting spins into non-magnetic materials such as Pt, Au, GaAs, Si, and possibly graphene 1-7 . This is accomplished by placing a ferromagnet (FM) in contact with the non-magnetic material (NM), and driving the FM magnetization into precession. The free carriers of the NM absorb some angular momentum from the precessing FM magnetization, thus becoming spin polarized. This angular momentum transfer will enhance the damping of the FM. Spin pumping serves as an alternative to electrical spin injection schemes in which a bias current carries spin-polarized charges across the FM/NM interface 8,9 . The spin pumping effect was first observed by monitoring the FMR linewidth, a measure of the total damping of the FM magnetization by several available mechanisms 10,11 . However, the magnetization dynamics of a thin film FM are also extremely sensitive to sample defects and disorder. Graphene's low surface energy and high surface diffusion cause the deposited metal atoms to form clusters and make growth of smooth layers difficult 8,12 . As a consequence, we anticipate that the ferromagnetic properties of thin film cobalt and possibly other ferromagnets grown on graphene will be affected. In this report, we reproduce the linewidth broadening of cobalt FMR spectra due to underlying graphene 6 . Additional magnetostatic and dynamic characterizations make evident that substantial changes have been made to the cobalt films by growth on graphene.Single layer graphene is synthesized by low pressure chemical vapor deposition (CVD) on Cu foil using a Cu enclosure 13 . The resulting large area graphene (∼ 5 mm × 5 mm) is then transferred to SiO 2 (300 nm)/n-Si substrate by etching the Cu foil while using poly-methyl