No abstract

We report on efficient spin polarized injection and transport in long ͑10 2 nm͒ channels of Alq 3 organic semiconductor. We employ vertical spin valve devices with a direct interface between the bottom manganite electrode and Alq 3 , while the top-electrode geometry consists of an insulating tunnel barrier placed between the "soft" organic semiconductor and the top Co electrode. This solution reduces the ubiquitous problem of the so-called ill-defined layer caused by metal penetration, which extends into the organic layer up to distances of about 50-100 nm and prevents the realization of devices with well-defined geometry. For our devices the thickness is defined with an accuracy of about 2.5 nm, which is near the Alq 3 molecular size. We demonstrate efficient spin injection at both interfaces in devices with 100-and 200-nm-thick channels. We solve one of the most controversial problems of organic spintronics: the temperature limitations for spin transport in Alq 3 -based devices. We clarify this issue by achieving room-temperature spin valve operation through the improvement of spin injection properties of both ferromagnetic/Alq 3 interfaces. In addition, we discuss the nature of the inverse sign of the spin valve effect in such devices proposing a mechanism for spin transport.

Information and communication technology (ICT) is now calling for solutions enabling lower power consumption, further miniaturization, and multifunctionality requiring the development of new device concepts and new materials. [ 1 ] One of the most fertile approaches to meet such demands is spintronics, which is now facing the challenge of evolving from the fi rst generation of devices that led to a revolution in information storage [ 2 ] (giant magnetoresistance readheads), to devices that exhibit multioperation capabilities (logic, communication, and storage) within the same materials technology. [ 3 ] Indeed, the electrical control of the magnetoresistance (MR) can provide such capabilities and is currently being pursued along several lines. In inorganic spintronics the two main approaches are the use of combined ferromagnetic or ferroelectric materials [ 4 ] and spin-polarized-current-induced control of the electrode magnetization. [ 5 ] In this paper we show that an electrically controlled magnetoresistance can be easily achieved in organic devices by combining magnetic bistability (spin valve) and electrical memory effects into an interacting multifunctional implementation.Electrical resistive switching effects in organic-based devices have recently received widespread attention [ 6 ] and represent one of the most promising ICT applications. On the other hand, spintronic devices based on organic semiconductors (OSC) have already provided signifi cant results [ 7 ] and device performances. [ 8 ] An essential device employed in the fi eld is the one commonly known as a spin valve, in which a spin-polarized current is injected from a ferromagnetic electrode into the organic transport medium and collected subsequently by a second ferromagnetic electrode. The latter acts as a spin analyzer due to the different collection probabilities for electrons arriving with different spin orientations, [ 9 ] giving rise to a measurable spin valve magnetoresistance (SVMR). The organic transport medium can be either a single molecule or, as in this communication, a thin fi lm. The former case pertains to the fi eld of molecular spintronics, while the latter belongs to organic spintronics. SVMR in organic semiconductors has been shown in a number of devices featuring several OSC and different ferromagnetic electrodes, giving evidence of spin injection in the conducting levels of OSCs. Recently this was unambiguously confi rmed by muon spin rotation and two-photon photoemission spectroscopy. [ 10 , 11 ] Previously we documented the coexistence of magnetoresistive and non-volatile electrical switchings in tris (8-hydroxyquinolinato)aluminium (Alq 3 )-based vertical spin valves with manganite and cobalt electrodes. [ 12 ] Such a coexistence was recently reported for rubrene-based devices with similar electrodes. [ 13 ] In this paper we go beyond these results by demonstrating a full electrical control of the magnitude of the spin valve effect: the SVMR can be turned "on" and "off" as well as smoothly tuned between a number of no...

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