Electronic devices that utilise the spin degree of freedom hold unique prospects for future technology. They promise low-power logic, possibly at the quantum level, and the combination on the same chip of communication, logic and memory elements. When combined with the properties of organic materials, which have low manufacturing costs, are mechanically flexible and are energy efficient, there is considerable potential for extending the already great scope that spintronic devices have. Furthermore, it has recently been shown that organic materials can preserve the spin information for extremely long times – in some cases over a million times longer than their conventional counterparts. This opens up new horizons in the field of spintronics and may lead to an entirely new generation of spin-enabled devices.
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 magnetisation orientation of a magnetic layer. Device operation normally proceeds with measuring a quantity like the electrical current that depends on how the degree of spin alignment is transferred across the device. The so-called “spin valve”, shown in Figure 1, is a prominent example of such a spin enabled device that has already revolutionised hard drive read heads and magnetic memory. The reduction in spin polarization in the yellow spacer layer is governed by the ‘spin penetration length’, which is related to intrinsic spin relaxation mechanisms in the spacer layer.
Figure 1: An archetypical spintronic device: the spin valve. A spin polarised current (arrows) is injected from a magnetic layer (green) into a non-magnetic spacer layer (yellow), where the spin polarisation is reduced (as indicated by the reducing arrow height) as the current flows further from the interface until it finally reaches a second magnetic layer (blue).
Understanding the transfer of spin polarisation in real device structures remains one of the most difficult challenges in spintronics, since it is dependent on more than just the properties of the individual materials that comprise the device, but also on the structural and electronic properties of the interfaces between the different materials. Knowledge of these factors is especially important for organic spintronics, where many of the standard techniques used to measure spin polarisation in conventional semiconductors are not applicable. Furthermore, there is a need for direct and spatially resolved measurements of how spin propagates through organic spin-based devices, since spin phenomena in organic semiconductors is less well understood than for their inorganic counterparts. Measurements of spin penetration could offer clues as to the relevant spin relaxation mechanisms of organic semiconductors.
Figure 2: Main Panel: The correlation between macroscopic magnetotransport measurements (black) and nanoscopic muon measurements of the spin penetration (red) is clear. Inset: As yet, there is no final explanation of the step-like increase in the magnetotransport at 40 K (same data as in the Main Panel, plotted on a linear scale), although recent results by Dr Drew’s group suggest a temperature dependent electron spin relaxation, with an energy scale similar to the position of the step.
Dr Drew and colleagues used the world’s only Low Energy Muon Spin Rotation spectrometer with a user programme to perform a depth-resolved measurement of the spin polarisation of current-injected charge carriers in an organic spin valve . Crucially, the measurements were carried out below buried interfaces of a fully functional technologically realistic device, which enabled the correlation of macroscopic magnetotransport measurements and the measurements of spin penetration on the nanoscopic lengthscale. In Figure 2, there is a clear correlation between the nanoscopic measurements of the spin penetration length (which governs the reduction in spin polarisation, as schematically shown in Figure 1) and magnetoresistance of the device. These results suggests that the spin diffusion length is a key parameter of spin transport in organic materials. As to the origin of the step-like increase of the spin diffusion length below 40K, this is likely related to a decrease in the spin relaxation rate in the spacer material .
Figure 3: A comparison of the device magnetoresistance and spin polarisation close to the top interface. (a):The magnetoresistance and (b) a measure of the spin polarisation with the LiF layer. (c): The magnetoresistance and (d): a measure of the spin polarisation, without the LiF layer. As can be seen from (a)-(d), there is a clear reversal of spin polarisation as a result of the presence of the LiF layer, resulting in a reversed magnetoresistance. (e): There is an increase in magnetisation close to the interface, resulting from an over-population of spin majority carriers. (f): There is a decrease in magnetisation close to the interface, resulting from an over-population of spin minority carriers.
In an extension to this work on spin penetration in the spacer layers, the QMUL team then went on to investigate how it is possible to engineer the magnetic/semiconducting interface to reverse the spin polarisation of the charge carriers in the device . Shown in Figure 3, is the magnetoresistance, spin polarisation in the organic spacer close to the top electrode and a schematic diagram showing the different states for two devices. The only difference between the device on the left and the one on the right is the inclusion of a very thin (1nm) layer of Lithium Flouride between the ferromagnet and organic semiconductor. The inclusion of this layer, which has an intrinsic electric dipole moment, is to reverse the spin polarisation of the charge carriers at that interface, and therefore change the sign of the magnetoresistance. This opens up the possibility of an electrically controllable spin valve, by the inclusion for example of a ferroelectric material at the interface.
This work is a clear demonstration that Low Energy Muon Spin Rotation measurements can provide unique information about the degree of spin polarisation of the injected charge carriers, as well as their sign, within a buried active layer of a functional organic spin-valve device. The results highlight the unique potential of the technique to reveal the role of the various mechanisms that limit the spin coherence in fully functional and realistic devices, especially in those involving organic materials. Specifically, it can enable one to differentiate between bulk and interface related spin phenomena.
A J Drew, W P Gillin & T Kreouzis
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