On the path toward organic spintronics
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Molecular devices
Organic molecules are inexpensive multifunctional materials that hold profound promise for realizing the goal of sustainable development. Organic molecules have almost unlimited possibilities due to multifarious options of chemical synthesis of tailor-made molecules, in combination with their mechanical flexibility and ease of large-scale development. Most molecules also respond to external stimuli such as electric and magnetic fields, light, and pressure, giving them versatility for multiple functionalities. In particular, organic materials provide a unique platform for exploiting the spin of the electron that can lead to organic spintronics. The main motivation for this directive stems from the weak coupling between the spin and orbital momentum of the electron since, typically, organics are composed of light elements. In addition, the coupling between the electron spin and nuclear magnetic moments is very small in organic matter, and the orientation of the electron spin is typically preserved for a very long time. Hence, the spin information can be maintained and transported over microscopic distances, a feature that has stimulated intensive research activities. Much of the recent research is not limited to just the breakthroughs described in this issue, but rather, these should be considered the tip of the iceberg.
Besides generic interest in exploring and exploiting the spin degree of freedom in organic devices, research activities match the ongoing drive toward truly single molecular devices. The use of a single molecule as a basic functional unit to reduce device dimensions has been the goal and challenge for decades. Why single molecules? Because molecules are well defined (e.g., they always have the same number of atoms, atomic structure, and electronic structure), and they form the smallest possible electronic units that can be controlled with atomic precision. Importantly, it is possible to design their properties in limitless ways that can be precisely tuned by attaching specific functional groups. Four decades ago, for example, Aviram and Ratner envisaged a molecular rectifier based on a single molecule with donor and acceptor groups separated by a barrier.1 In addition to the charge, if one now uses the electron spin degree of freedom and hence incorporate spintronic functions within a single molecular building block, this would be the ultimate versatility for molecular spintronics. Success in achieving spintronic devices based on singleorganic molecules relies on the control of interactions at the organic molecule-metal interfaces. While single molecules are paramagnetic, and hence the spin function cannot be employed, metal-organic molecules adsorbed on a ferromagnetic (FM)
Jagadeesh S. Moodera, Physics Department, Massachusetts Institute of Technology, USA; [email protected] Bert Koopmans, Department of Applied Physics, Eindhoven University of Technology, The Netherlands; [email protected] Peter M. Oppeneer, Department of Physics and Astronomy, Uppsala University, Sweden; pe
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