The development of organic spin valves from unipolar to bipolar operation
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Introduction A report on magnetoresistance (MR) in a lateral organic diode with ferromagnetic (FM) electrodes in 20021 marked the entrance of organics into the well-developed, two-decades old field of spintronics based on the giant MR (GMR) effect in spin valves.2,3 Subsequently, a demonstration2 of the first vertical organic spin valve (OSV) device in 2004 (see Figure 1) boosted the young field of organic spintronics that propelled many follow-up studies.3–7 OSV devices, similar to their inorganic analogs, are devices for which the electrical resistance switches between high and low values with the application of relatively small magnetic field. These devices have been based on unipolar injection of charges and passive operation; namely, only one kind of spin-polarized charge (either holes or electrons) was injected and subsequently collected by FM electrodes. Their operation was limited to the control of device resistance upon application of a relatively small magnetic field, B < 100 mT. It was not until 2012 that a truly bipolar OSV was introduced that showed both GMR and hysteretic variations in the device electroluminescence (EL) upon the application of a magnetic field: magnetoelectroluminescence (MEL).8 The MEL figure of merit is Δ(EL)/EL(%). The operation of such a device, dubbed “spin-organic light-emitting diode” or spin-OLED,9 was limited to temperatures T < 200 K and showed a maximum MEL of ∼1%. Nevertheless, it proved
that spin alignment of both injected electrons and holes can be obtained in an OLED. The operation of a spinOLED is similar to that of an ordinary OLED except that the spin polarization of the injected electrons and holes alters the eventual electron-hole radiative recombination, leading to a MEL response that follows the coercive fields of FM electrodes. The operation of a spin-valve device rests upon the ability of the FM electrodes to efficiently inject and subsequently collect spin-polarized charges and the ability of the interlayer between the electrodes to maintain the injected spin polarization throughout its thickness, d. Therefore, the device interlayer should possess a spin diffusion length, λS∼d. In organics, where charge hopping is the main electrical conduction mechanism, λS = (DτS)½, where D is the charge diffusion coefficient and τS is the spin relaxation time. Composed of light elements, organic semiconductors (OSCs) have relatively small spin– orbit interaction. In the absence of spin–orbit interaction, the main spin diffusion length-limiting mechanism in organics is the spin flip caused by the hyperfine interaction (HFI) between the spins of the charge carrier and adjacent nuclei. The small HFI (∼3 mT) in OSC leads to a large τS ∼1–10 μs. However, OSCs have small carrier mobility (μ∼10–7–10–9 m2/Vs)10 leading to D (= μkBT/e, where μ is the carrier mobility, kB is the Boltzmann constant, and e is the elementary charge) ∼10–9– 10–11 m2/s at room temperature. Thus, λS in OSCs is expected
Tho D. Nguyen, Physics and Astronomy Department, University of Georgia, USA; [email protected]
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