Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation

Abstract

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.R ecently great efforts have been undertaken to use the spin degree of freedom in electronic devices. These activities are fuelled by the potential prospects of spin-electronic (or 'spintronic') devices for example in terms of increased processing speed and integration, non-volatility, reduced power consumption, multifunctionality and their suitability for quantum computing 1 . The most common method for using the spin in devices is based on the alignment of the electron spin ('up' or 'down') relative to either a reference magnetic field or the magnetization orientation of a ferromagnetic layer. Device operation normally proceeds with measuring a quantity such as the electrical current that depends on how the degree of spin alignment is transferred across the device. The so-called 'spin valve' is a prominent example of such a spin-enabled device that has already revolutionized hard-drive read heads and magnetic memory 1 . The efficient transfer of spin polarization in real device structures remains one of the most difficult challenges in spintronics, because it is dependent on more than just the properties of the individual materials that comprise the device.Recently 2,3 , the use of organic materials in spintronics has become of significant interest, primarily owing to their ease and small cost of processing and electronic and structural flexibility. Furthermore, the extremely long spin coherence times found in organic materials offer considerable advantages over other materials 3 . This favourable property is related to two factors, first the weak spin-orbit coupling of light elements such as carbon and second to the small nuclear hyperfine interaction 4,5 . The latter arises because the electron transport in π-conjugated molecules is normally confined to molecular states, delocalized to the carbon rings, the predominant isotope of which, 12 C, has zero nuclear spin 4 .A common way to measure spin diffusion is based on time-resolved optical techniques, where spin-polarized charge carrie...

The role of interfaces in organic spin valves revealed through spectroscopic and transport measurements

“…For the same arguments as used in Section 6 about ''hot spot'' transport, the LE-mSR measurements appear to show the former is true [43,56]. Adapted from Ref.…”

“…They conclude that the local character of this SHIPS-induced polarization inversion means it should not be observed in their point-like experiments, whereas since these SHIPS states are responsible for the majority of the current, it can be observed in large area devices. However, this model may not be generally applicable, as recent large-area device measurements with Low Energy Muon Spin Rotation (LE-mSR) indicate that the spin-polarized current is not dominated by hotspots, but is present over a large area within the device [43]. This is because the LE-mSR technique measures the areal average over the size of the muon beamspot [44], which is shown in Fig.…”

Spin diffusion in fullerene-based devices: Morphology effect

“…Indeed, giant MR (GMR) has been measured in OSV devices based on small molecule and polymer spacers, both as thick films and thin tunnel junctions 4,[6][7][8][9][10][11][12][13] . Clear proof of spin injection into organic spacer was also provided by muon spin rotation 14 and two photon photoemission spectroscopy 15 .…”

Observation of a large spin-dependent transport length in organic spin valves at room temperature

“…The observation of SDT in OSs in the hopping process has also been reported 2,3 in such materials as tris (8-hydroxyquinolinato) aluminium (Alq 3 ) and rubrene. However, most studies have only been carried out at low temperatures because of the evident reduction in the SDT length with increasing measurement temperatures until the MR effect finally disappears at room temperature 2,[13][14][15][16][17][18][19] . Thus, whether a large SDT length can be realized in OSs at room temperature remains unclear.…”

“…The traditional optical pump-probe method used for studying spin dynamics and transport in inorganic semiconductors is not suitable for OSs because of the weak spin-orbit interaction and the absence of a crystalline structure with inversion asymmetry in OSs 16,17 . The use of an organic spin valve device is still the most popular technique for investigating SDT in OSs 2 .…”