Large Magnetoresistance Observed in Junctions to Single Organic Radical Molecule–Electrode Separation

—Conductivity of a Single Molecule Successfully Manipulated by the Spin of Electrons. Technique May Be Applicable to Next-Generation Spintronic Materials—

2016.08.18


National Institute for Materials Science(NIMS)

A NIMS–University of Konstanz–University of Hamburg joint research group bridged the gap between two electrodes with a single, purely organic radical molecule that contains no metal element. Then, the group successfully observed large magnetoresistance in molecule–electrode junctions for the first time in the world.

Abstract

  1. Ryoma Hayakawa, a senior researcher at the Quantum Device Engineering Group, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), and researchers at the University of Konstanz and the University of Hamburg, jointly bridged the gap between two electrodes with a single, purely organic radical molecule that contains no metal element. Then, the joint research group successfully observed large magnetoresistance in molecule–electrode junctions for the first time in the world.
  2. Spintronic devices, which operate using the properties of electron charge and electron spin degree of freedom, are expected to be applied as next-generation logic circuits and memory devices. To create solid-state logic circuits that take advantage of the spin of electrons, it is necessary for spin information to propagate in the devices without being scattered and lost. Organic radical molecules consist exclusively of light elements and contain no metal element. Due to this composition, these molecules have a weak spin−orbit coupling, and scattering of conduction electrons rarely occurs in them. Accordingly, these molecules are presumably capable of transporting spin information without any loss. In addition, spins of unpaired electrons are known to contribute to various magnetic properties (ferromagnetism, paramagnetism and antiferromagnetism), and for that reason, organic thin films and organic bulk crystals have been used to evaluate magnetic properties. However, because these organic materials consist of molecules, their electrical conductivity is low in intermolecular junctions. Moreover, taking account of the fact that the spin of electrons in these materials scatters less extensively compared to inorganic materials, it was critical to understand changes in magnetic properties at the single-molecule level.
  3. The research group recently synthesized stable organic radical molecules by coupling oligo (p-phenyleneethynylene) molecules with radical groups (unpaired electrons), and bridged the gap between two gold nanoelectrodes with the synthesized single molecule at a low temperature of 4 K, thereby forming single molecular junctions. The group evaluated electrical resistance in the molecular junctions while applying magnetic fields to them, and observed large magnetoresistance, 287% at maximum (44% on average), at a magnetic field of 4 T. The group also conducted similar experiments using non-radical molecules that contain no unpaired spinning electron, and observed magnetoresistance of only 2 to 4%. Based on these results, the group found that an addition of radical groups increases the electrical resistance of parent molecules by at least one order of magnitude. An analysis of current−voltage characteristics revealed that the observed large magnetoresistance may be caused by a weakening of the coupling between the crosslinking molecule and the electrodes through the application of magnetic fields.
  4. The outcome of the study demonstrated that the electrical resistance of organic radical molecules may be manipulated by the spin of unpaired electrons, and will contribute to the development of new, organic molecule-based spintronic devices. Also, this study produced academically significant results as they provided a clue to understanding the effect of the spin of unpaired electrons on the electrical conductivity of organic radical molecules, which was virtually unknown before. In future studies, we aim to determine how the spin of unpaired electrons weakens the coupling between the molecule and the electrodes and understand in detail the mechanism involved in the occurrence of large magnetoresistance.
  5. This study was published in the “Just Accepted Manuscripts” section of the online version of Nano Letters, a journal issued by the American Chemical Society, on July 26, 2016.

"Figure. (a) Schematic of single molecular junctions formed between an organic radical molecule (TEMPO-OPE), synthesized by the MCBJ technique, and electrodes. (b) Scanning electron micrograph of gold (Au) electrodes used in this study. The distance between the two electrodes can be precisely adjusted using a variable rod attached to the lower surface of the substrate, as shown in (a). The gap between two electrodes can be bridged with a single organic molecule through repeated fracturing and joining of the electrodes." Image

Figure. (a) Schematic of single molecular junctions formed between an organic radical molecule (TEMPO-OPE), synthesized by the MCBJ technique, and electrodes. (b) Scanning electron micrograph of gold (Au) electrodes used in this study. The distance between the two electrodes can be precisely adjusted using a variable rod attached to the lower surface of the substrate, as shown in (a). The gap between two electrodes can be bridged with a single organic molecule through repeated fracturing and joining of the electrodes.



Contacts

(Regarding this research)

Ryoma Hayakawa
Senior researcher, Quantum Device Engineering Group, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science
Tel: +81-29-860-4808
E-Mail: HAYAKAWA.Ryoma=nims.go.jp
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