Impurity-band transport in organic spin valves

Abstract

The central phenomenon in the field of organic spintronics is the large magnetoresistance in thick organic spin valves. A prerequisite for understanding the magnetoresistance is a reliable description of the device resistance, or the I-V characteristics. Here I show that the observed I-V characteristics in the organic spin valves is incompatible with charge injection into the organic's lowest unoccupied molecular orbital or highest occupied molecular orbital but can be explained by electrons tunnelling into a broad impurity band located in the gap between these molecular orbitals. Voltage drop takes place mainly across depletion layers at the two electrode/organic interfaces, giving rise to electrode-limited charge transport. Spin-dependent electron tunnelling into the impurity band from the ferromagnetic electrodes results in spin accumulations inside the organic, which rapidly diffuses through the organic primarily via the exchange between impurity-band electrons. This picture explains the major magnetoresistance features and predicts enhanced capacitance in these devices.

Observation of Skyrmions at Room-temperature in Amorphous Fe/Gd Films

“…These structures can be placed in three categories: type II magnetic bubbles, skyrmions, and biskyrmions, Figure 2 has examples of these three structures. Biskyrmions are a composite state formed by two skyrmions with opposite chirality for which the total topological charge is 2, and they have only been observed in La 2-2x Sr 1+2x Mn2O 7 by Yu et al [8]. As seen in Figures 2c and j [8].…”

“…Biskyrmions are a composite state formed by two skyrmions with opposite chirality for which the total topological charge is 2, and they have only been observed in La 2-2x Sr 1+2x Mn2O 7 by Yu et al [8]. As seen in Figures 2c and j [8]. The presence of biskyrmions and skyrmions with both helicities indicates that in this material these topological spin textures are stabilized by long range dipole interactions, as opposed to the Dzyaloshinskii-Moriya interaction responsible for stabilizing the skyrmion phase in non-centrosymmetric crystals such as MnSi and FeGe [9].…”

Active Morphology Control for Concomitant Long Distance Spin Transport and Photoresponse in a Single Organic Device

“…[ 14 ] From the molecular side, an obvious choice for improving spin charge carrier injection would be a compound whose energy level responsible for electronic transport matches well the Fermi level of the ferromagnetic electrodes used. [17][18][19] From the material point of view, an amorphous fi lm creates a smooth and homogeneous organic layer that prevents the formation of metal fi lament during the deposition of the top electrode. [ 20 ] Unfortunately, many organic semiconductors, especially those high-mobility materials which should be compatible with long-distance spin transport, [ 2,21 ] commonly grow as rough polycrystalline thin fi lms prone to the formation of spurious metallic fi laments.…”

“…Moreover, the F 16 CuPc has shown high electron transport mobility in previous studies, [ 25,26 ] which might be leading to fast hopping rates and therefore longer spin diffusion lengths in spintronic devices. [ 21 ] To briefl y summarize, from the molecular point of view, devices based on F 16 CuPc have a great potential for obtaining long-range spin transport due to the effi cient charge carrier injection [17][18][19] and the high conductivity. [ 2,21 ] Besides, F 16 CuPc is an air-stable organic semiconductor, [ 25 ] being this property crucial for any application of organic spintronic devices.…”

Solution Monolayer Epitaxy for Tunable Atomically Sharp Oxide Interfaces

“…Such a layer has not been observed and other explanations for the charge transfer have been proposed. [30] In order to check if a mobile electron layer is created on the LaAlO 3 surface of our nominally p-type hetero-structure, we fabricated a device with an additional Au electrode on the LaAlO 3 surface. If a mobile electron layer exists, it would simply short the top electrode to the contacts.…”