Fl Francisco Bloom

A two-site bipolaron model for organic magnetoresistance

The recently proposed bipolaron model for large “organic magnetoresistance” (OMAR) at room temperature is extended to an analytically solvable two-site scheme. It is shown that even this extremely simplified approach reproduces some of the key features of OMAR, viz., the possibility to have both positive and negative magnetoresistance, as well as its universal line shapes. Specific behavior and limiting cases are discussed. Extensions of the model, to guide future experiments and numerical Monte Carlo studies, are suggested.

Correspondence of the sign change in organic magnetoresistance with the onset of bipolar charge transport

In this work we examine the transition between positive and negative organic magnetoresistance in poly[2-methoxy-5-(3′, 7′-dimethyloctyloxy)-p-phenylenevinylene] in order to understand how different regimes of charge transport affect the organic magnetoresistance effect. To characterize the charge transport in these devices we measured the current, low frequency differential capacitance, and electroluminescence efficiency as a function of voltage. These measurements show that the sign change of the magnetoresistance corresponds with a change from a unipolar diffusive transport below the built in voltage (Vbi) to a regime of bipolar drift transport above Vbi.

Temperature dependent sign change of the organic magnetoresistance effect

A sign change of the organic magnetoresistance effect is observed as a function of temperature. There is a large difference in the IV behavior when the sign of the magnetoresistance (MR) is positive compared to when the sign of the MR is negative, pointing to the possibility that the sign change of the MR is due to a change in the charge transport mechanism. The positive and negative MRs show different characteristic field widths B0 in the MR versus magnetic field curves. Also, the traces with positive MR show a clear temperature dependence of B0 while no systematic dependence on temperature is seen in the traces with negative MR. This behavior can be qualitatively explained by the recently proposed bipolaron model.

Spin in organics: a new route to spintronics

New developments in the nascent field of organic spintronics are discussed. Two classes of phenomena can be discerned. In hybrid organic spin valves (OSVs), an organic semiconducting film is sandwiched between two ferromagnetic (FM) thin films, aiming at magnetoresistive effects as a function of the relative alignment of the respective magnetization directions. Alternatively, organic magnetoresistance (OMAR) is achieved without any FM components, and is an intrinsic property of the organic semiconductor material. Some of the exciting characteristics of OMAR, in both electrical conductance and photoconductance, are presented. A systematic, combined experimental–theoretical study of sign changes between positive and negative magnetoresistance is shown to provide important insight about the underlying mechanisms of OMAR. A simple explanation of experimental observations is obtained by combining a ‘spin-blocking’ mechanism, an essential ingredient in the recently proposed bipolaron model, with specific features of the device physics of space charge limited current devices in the bipolar regime. Finally, we discuss possible links between the physics relevant for OMAR and that for OSVs. More specifically, weak hyperfine fields from the hydrogen atoms in organic materials are thought to be crucial for a proper understanding of both types of phenomena.

Impact of interface crystallization on inelastic tunneling in Al/AlOx/CoFeB

We report the change in inelastic electron tunneling spectra (IETS) for Al∕AlOx∕CoFeB∕Al junctions when the structure of CoFeB at its interface with AlOx is intentionally changed from quasiamorphous to highly textured fcc. While for the quasiamorphous interface there are signs of the size quantization of magnons, the spectra for the fcc interface show distinct excitations at bias voltages associated with known surface magnon modes in fcc Co. These results demonstrate that IETS can be used as a tool to probe distinct structural changes of the magnetic electrode in tunnel junctions.