We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance.However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance.

We report on the fabrication and characterization of graphene three-terminal junctions with nanometer dimensions. The devices have been realized in epitaxial graphene on semi-insulating silicon carbide. All current-carrying device parts consist of graphene resulting in all-carbon structures. Pronounced voltage rectification and frequency multiplication have been observed at room temperature.

A fabrication process for vertical organic spin-valve devices has been developed which offers the possibility to achieve active device areas of less than 500x500 nm² and is flexible in terms of material choice for the active layers. Characterization of the resulting devices shows a large magnetoresistance of sometimes more than 100 %, however with equally large variation from device to device. Comparison with large-area spin-valves indicates that the magnetoresistance of both, large and small devices most likely originates from tunneling through pinholes and tunneling magnetoresistance.Organic spin-valve devices are promising candidates for low cost non-volatile electronics, for example for RF-ID tags.1 Nevertheless, a deeper understanding of the underlying device physics is still necessary, especially of the charge transport mechanisms through the organic material and the origin of the magnetoresistance. In vertical organic spin-valves (OSV) an organic semiconductor (OSC) thin film is sandwiched between two ferromagnetic electrodes and the device is operated by a current flow perpendicular through the layers.2 Similar to giant magnetoresistance (GMR) 2-5 and tunneling magnetoresistance (TMR) 6-9 the device resistance depends on the relative magnetization of the two electrodes. However, in contrast to GMR in all-metal structures or TMR in metal/oxide structures it is still an open question whether the origin of the magnetoresistance (MR) effects in OSVs published so far is dominated by spin polarized charge transport through the organic spacer layer or by tunneling processes. 10,11 There is, however, a possible approach to gain deeper insight into this problem which takes into account the scaling of the MR effect with device size. In the case of spin injection and charge transport (GMR) the complete device area is expected to contribute to the current path through the OSC. Reducing the size of the device in this case should keep the resistance area product and the relative magnetoresistance constant. Tunneling processes, however, most likely occur in just a small part of the active area.12 Most probable candidates for tunneling sites would be pinholes at which the thickness of the OSC is reduced to such an extent that tunneling and thus TMR becomes possible. [8][9][10][11][12][13] Pinholes, however, follow certain statistics resulting in a different resistance change upon downscaling. They vary in size and also exhibit a varying thickness of the remaining OSC layer. Each pinhole thus contributes to the total device characteristics by a different resistance and MR contribution. In order to appreciate the necessary density of pinholes we can use results published by Barraud and coworkers.14 They create artificial pinholes in an OSC layer using conducting tip atomic force microscopy (AFM) and study the magnetoresistance after the pinhole is filled up with a ferromagnetic metal. The resistance of the pinholes is typically in the range of 100 MΩ while the magnetoresistance can be as high as 300%. In addition, they ob...

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Nanoscale multifunctional perpendicular organic spin valves have been fabricated. The devices based on an La 0.7 Sr 0.3 MnO 3 /Alq3/Co trilayer show resistive switching of up to 4-5 orders of magnitude and magnetoresistance as high as -70% the latter even changing sign when voltage pulses are applied. This combination of phenomena is typically observed in multiferroic tunnel junctions where it is attributed to magnetoelectric coupling between a ferromagnet and a ferroelectric material. Modeling indicates that here the switching originates from a modification of the La 0.7 Sr 0.3 MnO 3 surface. This modification influences the tunneling of charge carriers and thus both the electrical resistance and the tunneling magnetoresistance which occurs at pinholes in the organic layer.In the past years a number of multiferroic tunnel junctions have been demonstrated in which tunneling magnetoresistance (TMR) and total device resistance can be modulated by a voltage pulse. 1,2 The effects are typically explained by tunneling electroresistance (TER) due to a ferroelectric barrier which changes the total resistance and magnetoelectric coupling at the interface between ferroelectric barrier and ferromagnetic contact which changes the TMR in magnitude and sometimes in sign. We observe the same functionality in organic spin valves (OSVs, Fig. 1), which after applying a voltage pulse may change the device resistance by three orders of magnitude or more and modulate their magnetoresistance (MR) from +26% to -38% which is a much larger effect than observed in Refs. 1 or 2. Nevertheless, the absence of a ferroelectric layer in our devices excludes both TER and magnetoelectric coupling as possible explanation. It should, however, be noted that our devices and those from Refs. 1 and 2 have a La 0.7 Sr 0.3 MnO 3 (LSMO) bottom electrode as a common property.FIG. 1: MR traces of a nanosized OSV (LSMO/Alq3/MgO/Co/Ru with 20/12/3/30/10 nm in thickness) for two different resistance states after different voltage pulses taken at 4.3 K. The resistance changes by approx. three decades and the relative MR exhibit a sign reversal from +26% to -38%. Already in 2011 the simultaneous observation of magnetoresistance and resistive switching (RS) has been reported for organic spin valves by Prezioso et al. 3,4 In this case the devices were LSMO/Alq3/Co-based spin valves showing a relative MR of -20% in the initial state. By applying 2 voltage pulses the overall device resistance was increased while the relative MR decreased without changing shape or sign. The device resistance could be increased by two decades while the MR was completely suppressed. A possible explanation suggested by Prezioso et al. was the blocking of filaments or charge trapping in combination with giant magnetoresistance (GMR) and spin injection as a prevalent transport mechanism, however, no clear identification of the underlying physics was possible. Recent results from our own group in structures with only one ferromagnetic electrode (LSMO) also demonstrated RS. However, in thi...