Abstract
Vertical crossbar devices based on manganite and cobalt injecting electrodes and a metal-quinoline molecular transport layer are known to manifest both magnetoresistance (MR) and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the MR and magnetic modulation of bistability. To explain the device functionality, we identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with the chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrate under voltage application, resulting in a modification of the electronic properties of the organic material and of the oxidation state of the interfacial layer with the ferromagnetic contacts. Variable oxygen doping of the organic molecules represents the key element for correlating bistability and MR, and our measurements provide the first experimental evidence in favor of the impurity-driven model describing the spin transport in organic semiconductors in similar devices.
Highlights
The former hypothesis is associated with charge-transfer processes in donor-acceptor systems and involves the presence of metallic clusters embedded in the organic layer,19 the latter is explained by the formation of locally highly conductive channels and is related to redox effects
From the IRS, the device can be set to a high resistance state (HRS) by the application of a sufficiently negative voltage
In analogy with what was observed in other organic bistable devices 34, our spin valves do not require an electro-forming pulse to enable the bistability
Summary
Resistive switching (RS) behavior in organic devices has excited substantial attention because, in addition to the well-known aspects of high performance and low volatility that are inherent to RS memory, the use of organic components would yield easy-to-process, flexible devices. RS has been reported for a wide variety of organic semiconductors including metal quinoline complexes. Typical device configurations are simple and consist of two metallic electrodes sandwiching a semiconducting organic layer; their current–voltage (IV) characteristics show an. Hysteretic IV characteristics were observed in a large number of organic materials, from polymers to small molecule ones and are explained either by charging related phenomena or by filament formation phenomena The former hypothesis is associated with charge-transfer processes in donor-acceptor systems and involves the presence of metallic clusters embedded in the organic layer, the latter is explained by the formation of locally highly conductive channels (filaments) and is related to redox effects. . The former hypothesis is associated with charge-transfer processes in donor-acceptor systems and involves the presence of metallic clusters embedded in the organic layer, the latter is explained by the formation of locally highly conductive channels (filaments) and is related to redox effects.20, 21 In such a complex scenario, the correlation of electrical bistability with MR has obviously created expectations for an additional way of investigating the transport behavior, both for spin and charge properties. Our investigation provides a key element for the development of a picture of charge-spin effects in organic spintronic devices
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