Spin Transfer Torque in a Graphene Lateral Spin Valve Assisted by an External Magnetic Field Chia-Ching Lin, Ashish Verma Penumatcha, Yunfei Gao, Vinh Quang Diep, Joerg Appenzeller, and Zhihong Chen* School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States * S Supporting Information ABSTRACT: Spin-based devices are widely discussed for post-complementary metal-oxide-semiconductor (CMOS) applications. A number of spin device ideas propose using spin current to carry information coherently through a spin channel and transfering it to an output magnet by spin transfer torque. Graphene is an ideal channel material in this context due to its long spin diffusion length, gate-tunable carrier density, and high carrier mobility. However, spin transfer torque has not been demonstrated in graphene or any other semiconductor material as of yet. Here, we report the first experimental measurement of spin transfer torque in graphene lateral nonlocal spin valve devices. Assisted by an external magnetic field, the magnetization reversal of the ferromagnetic receiving magnet is induced by pure spin diffusion currents from the input magnet. The magnetization switching is reversible between parallel and antiparallel configurations, depending on the polarity of the applied charged current. The presented results are an important step toward developing graphene-based spin logic and understanding spin-transfer torque in systems with tunneling barriers. KEYWORDS: Graphene, spin transfer torque, nonlocal spin valve, all spin logic D ue to its potential for low-power operation, “All Spin Logic” 1 using pure spin currents to communicate information is considered an attractive approach to continue the downscaling of post-complementary metal-oxide-semi- conductor (CMOS). Information in terms of magnetization gets transferred and imprinted onto nanomagnets by means of spin transfer torque. 2,3 While separation of the charge and spin currents has been experimentally realized in lateral nonlocal spin valve (LNLSV) structures, 4,5 it is essential to demonstrate that spin currents can be unambiguously utilized to switch the magnetization of nanomagnets. Yang et al. 6 first showed that, when a sufficiently large spin current diffuses through a Cu channel in a LNLSV structure, the magnetization of the receiving magnet can be altered by spin transfer torque. This result has been confirmed by Zou and Ji, 7 while the same has not been achieved in any other channel material although comparable or even longer spin diffusion lengths have been found in those. Expectations of low power and high performance operation lead to a detailed exploration of graphene for both electronic and spintronic applications. Graphene has been proposed as an ideal channel material. 8 It exhibits a very long spin diffusion length even at room temperature 9-16 due to its weak spin- orbit coupling. Moreover, both the carrier concentration in the graphene channel and contact resistance at the graphene/ ferromagnetic interface can be modulated by a vertical gate field, 17 which offers device designers more flexibility. Most importantly, the high carrier mobility and high current carrying capability in graphene are essential to achieve strong spin transfer torque. Although large spin valve signals have been observed in graphene devices at room temperature, it had not been clear whether spin angular momentum from electrons in graphene can be successfully transferred onto a detector magnet. Here, we demonstrate for the first time magnetic field assisted, reversible magnetization switching through spin transfer in graphene LNLSV devices. These findings are an important step toward developing a graphene based spin logic. A magnetic field dependent critical current is observed and analyzed. In addition to the traditional spin transfer by a polarized spin current that can affect the magnetization of the detector magnet, the so-called “Slonczewski” spin torque 18 that had been observed before in Cu devices, 6 we suggest an extra “effective- field” like spin torque 19,20 to account for the observed switching of magnetization in graphene devices. For our study we have fabricated LNLSV structures on high quality peeled multilayer graphene (seven layers in the presented device) using permalloy (Py) as the ferromagnetic contacts (contact 2 and 3), as shown in Figure 1. Different from conventional LNLSV devices where the thickness of the Py Received: July 10, 2013 Revised: October 15, 2013 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl402547m | Nano Lett. XXXX, XXX, XXX-XXX