Spin-polarized transport in magnetically assembled carbon nanotube spin valves

R. Thamankar, S. Niyogi, B. Y. Yoo, Y. W. Rheem, N. V. Myung, R. C. Haddon, R. K. Kawakami
2006 Applied Physics Letters  
Spin polarized transport over 5 m long carbon nanotubes ͑CNTs͒ is observed in magnetically assembled spin valves with electrochemically deposited ferromagnetic ͑FM͒ electrodes. An annealing procedure is developed to produce stable devices with highly transmissive FM/CNT contacts. The devices exhibit magnetoresistance ͑MR͒ with magnitudes up to 4%, and the sign of MR is consistently negative. The temperature dependence is investigated and MR is observed up to 14 K. Bias dependence is also
more » ... ence is also investigated, with MR persisting up to ϳ300 mV. Carbon nanotubes ͑CNTs͒ have attracted considerable interest for spin-polarized electronics because their ballistic conductance and low spin-orbit coupling are ideal for coherent spin transport over macroscopic distances. 1-3 Furthermore, the one-dimensional nature of CNT provides a model system to study the behavior of spin in reduced dimensions. 4 Experimental evidence for spin-polarized transport in CNT was first provided by the observation of low-field magnetoresistance ͑MR͒ in lateral spin-valve devices ͓i.e., a CNT bridging two ferromagnetic ͑FM͒ electrodes͔. 5 Recent report of oscillations in the magnitude and sign of MR with gate voltage provides the strongest support that the MR is primarily due to spin-polarized transport. 6 In this letter, we investigate spin-polarized transport in multiwalled-CNT spin-valve devices produced by a magnetic assembly technique. 7 Postassembly annealing improves the electrical contact between the CNT and the FM electrodes. We observe spin transport over 5 m with MR magnitudes up to 4% in Co/CNT/Ni and Co/CNT/Co lateral spin valves, and the sign of MR is consistently negative. This is different from what is expected in the Julliere model, 8 and raises interesting questions. In addition, we observe evidence that suspending the CNT may improve spin transport properties, and identify a possible relation between nonlinear currentvoltage ͑I-V͒ characteristics and MR magnitude. Samples are fabricated by a magnetic assembly technique described in detail elsewhere. 7 Briefly, multiwalled CNTs are grown vertically on SiO 2 / Si substrates by chemical vapor deposition ͑CVD͒ and Ni caps are deposited onto the free end of the CNT by thermal evaporation. The capped CNTs are released from the substrate and dispersed in a solution of dimethylformamide ͑DMF͒ using hexyl amine to exfoliate the van der Waals aggregates. A drop of this CNT solution is placed onto prepatterned Ni, Co, and Au electrodes with geometry shown in Fig. 1͑a͒ . The FM electrodes are fabricated by electrodepositing FM thin films on gold electrodes. The magnetic properties of electrodeposited FM films on gold substrates have been investigated by magnetooptic Kerr effect ͑MOKE͒. Room temperature magnetic hysteresis loops for electrodeposited Co films typically show sharp magnetization switching but with remanence below
doi:10.1063/1.2221910 fatcat:sit6mrzb7rb65ds7j7t6cvglcm