Istituto dei materiali per l'elettronica ed il magnetismo     
Hueso L. E., Dediu V., Bergenti I., Riminucci A., Graziosi P., Borgatti F., Casoli F., Zhan Y., Taliani C., De Jong M. P. Magnetoresistance and energy model of Alq3-based spintronic devices. In: Intermag 2008 (Madrid, Spain, 4-8 maggio 2008).
Spin transport in organic semiconductors has been receiving widespread attention since the first experimental demonstration of magnetoresistive effects (change in resistance under an applied magnetic field) in hybrid ferromagnetic/organic/ferromagnetic structures [1]. Continuous effort in the field has led to the realization, for example, of vertical organic spintronic devices with differ- ent organic semiconductor layers [2,3] or organic tunnel barriers [4]. However, there is still a lack of understanding on the mechanism that governs spin injection and transport in organics, leading to general disagreement even on the expected sign of the devices output magnetoresistance. With the aim to clarify the spin transport behaviour in organic semiconductors, we present new results on hybrid inorganic/organic spin valves with the most successful up-to-date combination of materials [2-6]. The highly spin polarized manganite La2/3Sr1/3MnO3 and Cobalt have been used as ferromagnetic electrodes for spin injection into thick layers (up to 200 nm) of tris(8-hydrox- yquinoline)aluminum(III) (Alq3). In a critical design improvement, we have for the first time intro- duced an artificial tunnel barrier (Al2O3 or LiF) between the organic and the Co top electrode to study its influence on spin injection into organic semiconductors and to improve the chemical sta- bility and reproducibility of the devices. In our manuscript we: explore the importance of artificial tunnel barriers for spin injection in organics, record room temperature magnetoresistance, demonstrate that only ferromagnetic elec- trodes and not organic semiconductor limit device output and, finally, sketch an energy diagram able to explain negative magnetoresistance in LSMO/Alq3/Co spin valves. Our work is a new step forward in organic spintronics, as we prove that organic semiconductors do not have a clear limit for room temperature performance with the adequate ferromagnets, and we present a reliable model that could be easily extrapolated to predict the output of different materi- als combinations in hybrid spin valves. [1] Dediu, V., Murgia, M., Matacotta, F.C., Taliani, C. & Barbanera, S. Sol. State Commun. 122, 181-184 (2002). [2] Xiong, Z.H., Wu, D., Vardeny, Z.V. & Shi, J. Nature 427, 821-824 (2004). [3] Majumdar, S., Majumdar, H.S., Laiho, R. & Osterbacka, R. J. Alloy & Compounds 423, 169-171 (2006). [4] Santos, T.S., Lee, J.S., Migdal, P., Lekshmi, I.C., Satpati, B. & Moodera, J.S. Phys. Rev. Lett. 98, 016601 (2007). [5] Xu, W., Szulczewski, G.J., LeClair, P., Navarrete, I., Schad, R., Miao, G., Guo, H. & Gupta, A. Appl. Phys. Lett. 90, 072506 (2007). [6] Hueso, L.E., Riminucci, A., Bergenti, I., Zhan, Y. & Dediu, V. Adv. Mater. 19, 2639-2642 (2007).
Subject Organic Spintronics

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